U.S. patent application number 17/677804 was filed with the patent office on 2022-06-09 for -led, -led device, display and method for the same.
The applicant listed for this patent is OSRAM Opto Semiconductors GmbH. Invention is credited to Thorsten BAUMHEINRICH, Peter Brick, Kanishk Chand, Jean-Jacques Drolet, Hubert Halbritter, Patrick Hoerner, Laura Kreiner, Jong Park, Kilian Regau, Jens Richter, Thomas Schwarz, Christopher Soell, Paul Ta, Hoa Vu, Christopher Wiesmann.
Application Number | 20220180805 17/677804 |
Document ID | / |
Family ID | |
Filed Date | 2022-06-09 |
United States Patent
Application |
20220180805 |
Kind Code |
A1 |
BAUMHEINRICH; Thorsten ; et
al. |
June 9, 2022 |
-LED, -LED DEVICE, DISPLAY AND METHOD FOR THE SAME
Abstract
The invention relates to various aspects of a .mu.-LED or a
.mu.-LED array for augmented reality or lighting applications, in
particular in the automotive field. The .mu.-LED is characterized
by particularly small dimensions in the range of a few .mu.m.
Inventors: |
BAUMHEINRICH; Thorsten;
(Altdorf, DE) ; Brick; Peter; (Regensburg, DE)
; Drolet; Jean-Jacques; (Obertraubling, DE) ;
Halbritter; Hubert; (Dietfurt Toeging, DE) ; Kreiner;
Laura; (Regensburg, DE) ; Richter; Jens;
(Hemau, DE) ; Schwarz; Thomas; (Regensburg,
DE) ; Ta; Paul; (Fremont, CA) ; Regau;
Kilian; (Regensburg, DE) ; Soell; Christopher;
(Veitsbronn, DE) ; Vu; Hoa; (Milpitas, CA)
; Wiesmann; Christopher; (Barbing, DE) ; Hoerner;
Patrick; (Regensburg, DE) ; Park; Jong;
(Sunnyvale, CA) ; Chand; Kanishk; (Santa Clara,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OSRAM Opto Semiconductors GmbH |
Regensburg |
|
DE |
|
|
Appl. No.: |
17/677804 |
Filed: |
February 22, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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17039482 |
Sep 30, 2020 |
11302248 |
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17677804 |
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PCT/EP2020/052191 |
Jan 29, 2020 |
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17039482 |
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62937552 |
Nov 19, 2019 |
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International
Class: |
G09G 3/32 20060101
G09G003/32; B60K 35/00 20060101 B60K035/00; G09G 5/10 20060101
G09G005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 29, 2019 |
DK |
PA201970059 |
Jan 31, 2019 |
DE |
10 2019 102 509.5 |
Apr 23, 2019 |
DE |
10 2019 110 497.1 |
Apr 23, 2019 |
DE |
10 2019 110 523.4 |
May 9, 2019 |
DE |
10 2019 112 124.8 |
May 23, 2019 |
DE |
10 2019 113 792.6 |
Jun 7, 2019 |
DE |
10 2019 115 479.0 |
Claims
1. A display arrangement with a .mu.-display comprising a plurality
of pixels arranged in rows and columns, comprising: a first
substrate structure with .mu.-LEDs arranged therein or applied
thereto, the edge length of which is less than 50 .mu.m, in
particular less than 20 .mu.m, and which form a pixel structure
arranged in rows and columns, wherein the .mu.-LEDs are
individually controllable, and a plurality of contacts are arranged
on a surface of the first substrate structure opposite to a light
emission direction; a second substrate structure comprising on a
surface a plurality of contacts corresponding to the contacts of
the first substrate structure and having a plurality of digital
circuits for addressing the .mu.-LEDs; wherein the first and second
substrate structures are connected together and the plurality of
contacts are electrically connected to the corresponding contacts;
and wherein the first substrate structure is formed with a first
material system and the second substrate structure is formed with a
second material system different therefrom.
2. The display arrangement according to claim 1, wherein at least
some contacts of the plurality of contacts have an edge length of
less than 10 .mu.m or an area of less than 20 .mu.m.sup.2.
3. The display arrangement according to claim 1, wherein the
.mu.-LEDs are formed with an edge length of less than 10 .mu.m
and/or have a distance to an adjacent .mu.-LED of less than 7
.mu.m.
4. The display arrangement according to claim 1, further comprising
an adhesive or a form-fitting element partially disposed between
and holding together the first and second substrate structures.
5. The display arrangement according to claim 1, wherein the first
substrate structure is separated from the second substrate
structure by an intermediate structure through which at least
contact lines extend which connect the contacts of the first
substrate structure with the contacts of the second substrate
structure.
6. The display arrangement according to claim 1, wherein the first
material system comprises at least one of the following compounds:
GaN, GaP, GaInP, InAlP, GaAlP, GaAlInP, GaAs, or AlGaAs; and
wherein the second material system comprises at least one of the
following material systems: monocrystalline, polycrystalline,
amorphous silicon, indium-gallium-zinc oxide, GaN, or GaAs.
7. The display arrangement according to claim 1, wherein the first
carrier structure comprises a plurality of switchable current
sources, each of which is connected to a pixel for the supply
thereof, and whose switch inputs are coupled to the contacts for
supplying switching signals from the digital circuits.
8. The display arrangement according to claim 7, wherein the
switchable current sources are arranged in a material system which
is different from a material system used for the .mu.-LEDs or from
the first material system.
9. The display arrangement according to claim 1, wherein the
plurality of digital circuits of the second substrate structure are
adapted to generate a PWM-like signal from a clock signal and a
data word for each pixel.
10. The display arrangement according to claim 9, wherein the
plurality of digital circuits comprises a number of serially
connected shift registers, a respective length of which corresponds
to the data word for one pixel, each shift register being connected
to a buffer for intermediate storage; and/or wherein the plurality
of digital circuits comprise a multiplexer electrically coupled to
a demultiplexer in the first substrate structure for driving
multiple optoelectronic devices.
11. A driver circuit configured for driving a plurality of
optoelectronic elements, comprising: a plurality of first memory
cells, each comprising a set input, a reset input, and an output;
wherein each first memory cell at said output is triggered to a
first state by a set signal at said set input and maintains said
first state until reset to a second state at said reset input; and
wherein the output of each first memory cell is configured to
control a respective one of said optoelectronic elements.
12. The driver circuit according to claim 11, wherein each first
memory cell provides a pulse width modulation ("PWM") signal at the
output, and the PWM signal controls a switch configured to switch a
current through the respective optoelectronic element on and
off.
13. The driver circuit according to claim 11, wherein each first
memory cell comprises two cross-coupled NOR gates or two
cross-coupled NAND gates.
14. The driver circuit according to claim 11, wherein each first
memory cell comprises an NMOS transistor and a PMOS transistor
connected in series, and an inverter having an input connected
between the NMOS transistor and the PMOS transistor and an output
connected to gates of the NMOS and PMOS transistors.
15. The driver circuit according to claim 11, further comprising a
plurality of counters each configured to activate a set signal when
a data value is loaded into a respective counter and to activate a
reset signal when the respective counter reaches the data
value.
16. The driver circuit according to claim 11, further comprising a
common counter configured to generate a common dimming signal for
the plurality of optoelectronic elements.
17. The driver circuit according to claim 11, further comprising a
plurality of second memory cells, each second memory cell being
coupled to a respective one of the first memory cells and
configured to override an output signal of the respective first
memory cell as necessary to leave the respective optoelectronic
element turned off.
18. An optoelectronic device comprising: a plurality of
optoelectronic elements, in particular .mu.-LEDs or .mu.-LED; and
the driving circuit according to claim 11 for driving the plurality
of optoelectronic elements.
19. A method of operating the optoelectronic device according to
claim 18, comprising the following steps performed in the specified
order during a frame: switching off all optoelectronic elements;
controlling the optoelectronic elements that darken during framing
by the second memory cells; and controlling the current through the
optoelectronic elements by the first memory cells.
20. The method according to claim 19, wherein a common dimming of
the optoelectronic elements is carried out before the current
through the optoelectronic elements is controlled by the first
memory cells.
21. Arrangement comprising: a plurality of pixel structure arranged
in rows and columns, which comprise a first substrate structure
with .mu.-LEDs, .mu.-LED arrangements, .mu.-LED modules or
light-emitting devices arranged therein or applied thereto, the
edge length of which is less than 50 .mu.m, in particular less than
20 .mu.m, and which form the pixel structure arranged in rows and
columns, wherein the .mu.-LEDs, .mu.-LED arrays, .mu.-LED modules
or light emitting devices are individually controllable; and a
plurality of contacts are arranged on the surface of the first
substrate structure opposite to a light emission direction; a
second substrate structure comprising on a surface a plurality of
contacts corresponding to the contacts of the first substrate
structure and a plurality of digital circuits for addressing the
optoelectronic components; wherein the first and second substrate
structures are connected together and the plurality of contacts are
electrically connected to the corresponding contacts, and wherein
the first substrate structure is formed with a first material
system and the second substrate structure is formed with a second
material system, in particular different therefrom; and the second
substrate structure comprising: a device for electronically driving
a .mu.-LED pixel cell, in particular created with NMOS technology,
comprising: a data signal line, a threshold line and a selection
signal line; wherein contacting the second substrate structure to
the .mu.-LED, .mu.-LED array, .mu.-LED module or light emitting
device results in it being electrically connected in series to a
dual-gate transistor and together therewith between first and
second potential terminals, the dual-gate transistor being arranged
with its current conduction contacts between one terminal of the
.mu.-LED, .mu.-LED array, .mu.-LED module or light emitting device
and a potential terminal, and a first control gate of the dual-gate
transistor being connected to the threshold line; and a selection
latch circuit with a capacitor coupled to a second control gate of
the dual-gate transistor and to a current conduction contact of the
dual-gate transistor, and with a control transistor having its
control terminal connected to the selection signal line and/or a
supply circuit comprising an error correction detector having a
reference signal input, an error signal input and a correction
signal output; a controllable current source with current output
and a control signal terminal, the control signal terminal being
connected to the correction signal output to form a control loop
for the controllable current source, the current source being
configured to provide a current at the current output in dependence
on a signal at the control signal terminal; a backup source with an
output designed to provide a backup signal; and a switching device
which is configured, depending on a switching signal (VPWM), to
supply either a signal derived from the current at the current
output or the substitute signal to the fault signal input with
additional disconnection of the current output of the current
source; and/or a driver circuit for driving a plurality of
.mu.-LEDs, .mu.-LED arrays, .mu.-LED modules or light emitting
devices, comprising a plurality of first memory cells, each
comprising a set input, a reset input and an output each first
memory cell at the output is triggered to a first state by a set
signal at the set input and holds the first state until it is reset
to a second state at the reset input; and the output of each first
memory cell is configured to drive a respective one of the
.mu.-LEDs, .mu.-LED arrays, .mu.-LED modules or light emitting
devices; and/or a control circuit for adjusting a brightness of at
least one .mu.-LED, comprising a current driving element having a
control terminal, the first terminal of which is connected to a
first potential; a capacitor connected between the control terminal
and the first potential and forming a capacitive voltage divider
with a defined capacitance between the control terminal and the
first terminal; a control element adapted to apply a control signal
to the control terminal during a first time period, on the basis of
which a current flowing through the at least one .mu.-LED is
adjustable during the first time period; wherein during a second
time period subsequent to the first time period, a current flowing
through the .mu.-LED is determined by a reduced control signal
formed by the control signal during the first time period and the
capacitive voltage divider; and the control element is arranged to
provide a first or a second control signal during the first time
period in order to operate the .mu.-LED at at least two different
brightness levels; whereby the arrangement is configured for:
having an IC substrate component with monolithic integrated
circuits and with IC substrate contacts arranged as a matrix; and
having a monolithic pixelated optochip comprising a semiconductor
layer sequence with a first semiconductor layer having a first
doping and a second semiconductor layer having a second doping, the
polarity of the charge carriers in the first semiconductor layer
differing from that of the second semiconductor layer and the
semiconductor layer sequence defining a stacking direction; wherein
.mu.-LEDs arranged as a matrix are present in the monolithic
pixelated optochip; and wherein each .mu.-LED has a .mu.-LED rear
side facing the IC substrate component and a first light source
contact which adjoins the first semiconductor layer in a contacting
manner and is electrically conductively connected to a respective
one of the IC substrate contacts; characterized in that the
projection area of the first light source contact on the .mu.-LED
backside is at most half the area of the .mu.-LED backside; and the
first light source contact in a lateral direction perpendicular to
the stacking direction is surrounded by an absorber on the rear
side.
Description
[0001] This patent application is a continuation of U.S.
application Ser. No. 17/039,482 filed Sep. 30, 2020, which claims
the priorities of German Application Nos. DE 10 2019 112 124.8 of 9
May 2019, DE 10 2019 102 509.5 of 31 Jan. 2019, DE 10 2019 115
479.0 of 7 Jun. 2019, DE 10 2019 110 523.4 of 23 Apr. 2019, DE 10
2019 113 792.6 of 23 May 2019, and DE 10 2019 110 497.1 of 23 Apr.
2019, Danish Application No. DK PA201970059 of 29 Jan. 2019, U.S.
Application No. 62/937,552 of 19 Nov. 2019, and PCT Application No.
PCT/EP2020/052191 of 29 Jan. 2020. The disclosure of each of the
foregoing applications is incorporated herein by reference in the
entirety.
BACKGROUND
[0002] The ongoing current developments within the Internet of
Things and the field of communication have opened the door for
various new applications and concepts. For development, service and
manufacturing purposes, these concepts and applications offer
increased effectiveness and efficiency.
[0003] One aspect of new concepts is based on augmented or virtual
reality. A general definition of "augmented reality" is given by an
"interactive experience of the real environment, whereby the
objects from it, which are in the real world, are augmented by
computer generated perceptible information".
[0004] The information is mostly transported by visualization, but
is not limited to visual perception. Sometimes haptic or other
sensory perceptions can be used to expand reality. In the case of
visualization, the superimposed sensory-visual information can be
constructive, i.e. additional to the natural environment, or it can
be destructive, for example by obscuring parts of the natural
environment. In some applications, it is also possible to interact
with the superimposed sensory information in one way or another. In
this way, augmented reality reinforces the ongoing perception of
the user of the real environment.
[0005] In contrast, "virtual reality" completely replaces the real
environment of the user with an environment that is completely
simulated. In other words, while in an augmented reality
environment the user is able to perceive the real world at least
partially, in a virtual reality the environment is completely
simulated and may differ significantly from reality.
[0006] Augmented Reality can be used to improve natural
environmental situations, enriching the user's experience or
supporting the user in performing certain tasks. For example, a
user may use a display with augmented reality features to assist
him in performing certain tasks. Because information about a real
object is superimposed to provide clues to the user, the user is
supported with additional information, allowing the user to act
more quickly, safely and effectively during manufacturing, repair
or other services. In the medical field, augmented reality can be
used to guide and support the doctor in diagnosing and treating the
patient. In development, an engineer may experience the results of
his experiments directly and can therefore evaluate the results
more easily. In the tourism or event industry, augmented reality
can provide a user with additional information about sights,
history, and the like. Augmented Reality can support the learning
of activities or tasks.
SUMMARY
[0007] In the following summary different aspects for .mu.-displays
in the automotive and augmented reality applications are explained.
This includes devices, displays, controls, process engineering
methods and other aspects suitable for augmented reality and
automotive applications. This includes aspects which are directed
to light generation by means of displays, indicators or similar. In
addition, control circuits, power supplies and aspects of light
extraction, light guidance and focusing as well as applications of
such devices are listed and explained by means of various
examples.
[0008] Because of the various limitations and challenges posed by
the small size of the light-generating components, a combination of
the various aspects is not only advantageous, but often necessary.
For ease of reference, this disclosure is divided into several
sections with similar topics. However, this should explicitly not
be understood to mean that features from one topic cannot be
combined with others. Rather, aspects from different topics should
be combined to create a display for augmented reality or other
applications or even in the automotive sector.
[0009] For considerations of the following solutions, some terms
and expressions should be explained in order to define a common and
equal understanding. The terms listed are generally used with this
understanding in this document. In individual cases, however, there
may be deviations from the interpretation, whereby such deviation
will be specifically referred to.
[0010] "Active Matrix Display"
[0011] The term "active matrix display" was originally used for
liquid crystal displays containing a matrix of thin film
transistors that drive LCD pixels. Each individual pixel has a
circuit with active components (usually transistors) and power
supply connections. At present, however, this technology should not
be limited to liquid crystals, but should also be used in
particular for driving .mu.-LEDs or .mu.-displays.
[0012] "Active Matrix Carrier Substrate"
[0013] "Active matrix carrier substrate" or "active matrix
backplane" means a drive for light emitting diodes of a display
with thin-film transistor circuits. The circuits may be integrated
into the backplane or mounted on it. The "active matrix carrier
substrate" has one or more interface contacts, which form an
electrical connection to a .mu.-LED display structure. An
"active-matrix carrier substrate" can thus be part of an
active-matrix display or support it.
[0014] "Active Layer"
[0015] The active layer is referred to as the layer in an
optoelectronic component or light emitting diode in which charge
carriers recombine. In its simplest form, the active layer can be
characterized by a region of two adjacent semiconductor layers of
different conductivity type. More complex active layers comprise
quantum wells (see there), multi-quantum wells or other structures
that have additional properties. Similarly, the structure and
material systems can be used to adjust the band gap (see there) in
the active layer, which determines the wavelength and thus the
color of the light.
[0016] "Alvarez Lens Array"
[0017] With the use of Alvarez lens pairs, a beam path can be
adapted to video eyewear. An adjustment optic comprises an Alvarez
lens arrangement, in particular a rotatable version with a Moire
lens arrangement. Here, the beam deflection is determined by the
first derivative of the respective phase plate relief, which is
approximated, for example, by z=ax2+by2+cx+dy+e for the
transmission direction z and the transverse directions x and y, and
by the offset of the two phase plates arranged in pairs in the
transverse directions x and y. For further design alternatives,
swivelling prisms are provided in the adjustment optics.
[0018] "Augmented Reality (AR)"
[0019] This is an interactive experience of the real environment,
where the subject of the picking up is located in the real world
and is enhanced by computer-generated perceptible information.
Extended reality is the computer-aided extension of the perception
of reality by means of this computer-generated perceptible
information. The information can address all human sensory
modalities. Often, however, augmented reality is only understood to
be the visual representation of information, i.e. the
supplementation of images or videos with computer-generated
additional information or virtual objects by means of
fade-in/overlay. Applications and explanations of the mode of
operation of Augmented Reality can be found in the introduction and
in the following in execution examples.
[0020] "Automotive."
[0021] Automotive generally refers to the motor vehicle or
automobile industry. This term should therefore cover this branch,
but also all other branches of industry which include .mu.-displays
or generally light displays--with very high resolution and
.mu.-LEDs.
[0022] "Bandgap"
[0023] Bandgap, also known as band gap or forbidden zone, is the
energetic distance between the valence band and conduction band of
a solid-state body. Its electrical and optical properties are
largely determined by the size of the band gap. The size of the
band gap is usually specified in electron volts (eV). The band gap
is thus also used to differentiate between metals, semiconductors
and insulators. The band gap can be adapted, i.e. changed, by
various measures such as spatial doping, deforming of the crystal
lattice structure or by changing the material systems. Material
systems with so-called direct band gap, i.e. where the maximum of
the valence band and a minimum of the conduction band in the pulse
space are superimposed, allow a recombination of electron-hole
pairs under emission of light.
[0024] "Bragg Grid"
[0025] Fibre Bragg gratings are special optical interference
filters inscribed in optical fibres. Wavelengths that lie within
the filter bandwidth around AB are reflected. In the fiber core of
an optical waveguide, a periodic modulation of the refractive index
is generated by means of various methods. This creates areas with
high and low refractive indexes that reflect light of a certain
wavelength (bandstop). The center wavelength of the filter
bandwidth in single-mode fibers results from the Bragg
condition.
[0026] "Directionality"
[0027] Directionality is the term used to describe the radiation
pattern of a .mu.-LED or other light-emitting device. A high
directionality corresponds to a high directional radiation, or a
small radiation cone. In general, the aim should be to obtain a
high directional radiation so that crosstalk of light into adjacent
pixels is avoided as far as possible. Accordingly, the
light-emitting component has a different brightness depending on
the viewing angle and thus differs from a Lambert emitter.
[0028] The directionality can be changed by mechanical measures or
other measures, for example on the side intended for the emission.
In addition to lenses and the like, this includes photonic crystals
or pillar structures (columnar structures) arranged on the emitting
surface of a pixelated array or on an arrangement of, in
particular, .mu.-LEDs. These generate a virtual band gap that
reduces or prevents the propagation of a light vector along the
emitting surface.
[0029] "Far Field"
[0030] The terms near field and far field describe spatial areas
around a component emitting an electromagnetic wave, which differ
in their characterization. Usually the space regions are divided
into three areas: reactive near field, transition field and far
field. In the far field, the electromagnetic wave propagates as a
plane wave independent of the radiating element.
[0031] "Fly Screen Effect"
[0032] The Screen Door Effect (SDE) is a permanently visible image
artefact in digital video projectors. The term fly screen effect
describes the unwanted black space between the individual pixels or
their projected information, which is caused by technical reasons,
and takes the form of a fly screen. This distance is due to the
construction, because between the individual LCD segments run the
conductor paths for control, where light is swallowed and therefore
cannot hit the screen. If small optoelectronic lighting devices and
especially .mu.-LEDs are used or if the distance between individual
light emitting diodes is too great, the resulting low packing
density leads to possibly visible differences between pointy
illuminated and dark areas when viewing a single pixel area. This
so-called fly screen effect (screen door effect) is particularly
noticeable at a short viewing distance and thus especially in
applications such as VR glasses. Sub-pixel structures are usually
perceived and perceived as disturbing when the illumination
difference within a pixel continues periodically across the matrix
arrangement. Accordingly, the fly screen effect in automotive and
augmented reality applications should be avoided as far as
possible.
[0033] "Flip Chip"
[0034] Flip-chip assembly is a process of assembly and connection
technology for contacting unpackaged semiconductor chips by means
of contact bumps, or short "bumps". In flip-chip mounting, the chip
is mounted directly, without any further connecting wires, with the
active contacting side down--towards the substrate/circuit
carrier--via the bumps. This results in particularly small package
dimensions and short conductor lengths. A flip-chip is thus in
particular an electronic semiconductor component contacted on its
rear side. The mounting may also require special transfer
techniques, for example using an auxiliary carrier. The radiation
direction of a flip chip is then usually the side opposite the
contact surfaces.
[0035] "Flip-Flop"
[0036] A flip-flop, often called a bi-stable flip-flop or bi-stable
flip-flop element, is an electronic circuit that has two stable
states of the output signal. The current state depends not only on
the input signals present at the moment, but also on the state that
existed prior to the time under consideration. A dependence on time
does not exist, but only on events. Due to the bi-stability, the
flip-flop can store a data quantity of a single bit for an
unlimited time. In contrast to other types of storage, however,
power supply must be permanently guaranteed. The flip-flop, as the
basic component of sequential circuits, is an indispensable
component of digital technology and thus a fundamental component of
many electronic circuits, from quartz watches to microprocessors.
In particular, as an elementary one-bit memory, it is the basic
element of static memory components for computers. Some designs can
use different types of flip-flops or other buffer circuits to store
state information. Their respective input and output signals are
digital, i.e. they alternate between logical "false" and logical
"true". These values are also known as "low" 0 and "high" 1.
[0037] "Head-Up Display"
[0038] The head-up display is a display system or projection device
that allows users to maintain their head position or viewing
direction by projecting information into their field of vision.
[0039] The Head-up Display is an augmented reality system. In some
cases, a Head-Up Display has a sensor to determine the direction of
vision or orientation in space.
[0040] "Horizontal Light Emitting Diode"
[0041] With horizontal LEDs, the electrical connections are on a
common side of the LED. This is often the back of the LED facing
away from the light emission surface. Horizontal LEDs therefore
have contacts that are only formed on one surface side.
[0042] "Interference Filter"
[0043] Interference filters are optical components that use the
effect of interference to filter light according to frequency, i.e.
color for visible light.
[0044] "Collimation"
[0045] In optics, collimation refers to the parallel direction of
divergent light beams. The corresponding lens is called collimator
or convergent lens. A collimated light beam contains a large
proportion of parallel rays and is therefore minimally spread when
it spreads. A use in this sense refers to the spreading of light
emitted by a source. A collimated beam emitted from a surface has a
strong dependence on the angle of radiation. In other words, the
radiance (power per unit of a fixed angle per unit of projected
source area) of a collimated light source changes with increasing
angle. Light can be collimated by a number of methods, for example
by using a special lens placed in front of the light source.
Consequently, collimated light can also be considered as light with
a very high directional dependence.
[0046] "Converter Material"
[0047] Converter material is a material, which is suitable for
converting light of a first wavelength into a second wavelength.
The first wavelength is shorter than the second wavelength. This
includes various stable inorganic as well as organic dyes and
quantum dots. The converter material can be applied and structured
in various processes.
[0048] "Lambert Lamps"
[0049] For many applications, a so-called Lambertian radiation
pattern is required. This means that a light-emitting surface
ideally has a uniform radiation density over its area, resulting in
a vertically circular distribution of radiant intensity. Since the
human eye only evaluates the luminance (luminance is the
photometric equivalent of radiance), such a Lambertian material
appears to be equally bright regardless of the direction of
observation. Especially for curved and flexible display surfaces,
this uniform, angle-independent brightness can be an important
quality factor that is sometimes difficult to achieve with
currently available displays due to their design and LED
technology.
[0050] LEDs and .mu.-LEDs resemble a Lambert spotlight and emit
light in a large spatial angle. Depending on the application,
further measures are taken to improve the radiation characteristics
or to achieve greater directionality (see there).
[0051] "Conductivity Type"
[0052] The term "conductivity type" refers to the majority of (n-
or p-) charge carriers in a given semiconductor material. In other
words, a semiconductor material that is n-doped is considered to be
of n-type conductivity. Accordingly, if a semiconductor material is
n-type, then it is n-doped. The term "active" region in a
semiconductor refers to a border region in a semiconductor between
an n-doped layer and a p-doped layer. In this region, a radiative
recombination of p- and n-type charge carriers takes place. In some
designs, the active region is still structured and includes, for
example, quantum well or quantum dot structures.
[0053] "Light Field Display"
[0054] Virtual retinal display (VNA) or light field display is
referred to a display technology that draws a raster image directly
onto the retina of the eye. The user gets the impression of a
screen floating in front of him. A light field display can be
provided in the form of glasses, whereby a raster image is
projected directly onto the retina of a user's eye. In the virtual
retina display, a direct retinal projection creates an image within
the user's eye. The light field display is an augmented reality
system.
[0055] "Lithography" or "Photolithography"
[0056] Photolithography is one of the central methods of
semiconductor and microsystem technology for the production of
integrated circuits and other products. The image of a photomask is
transferred onto a photosensitive photoresist by means of exposure.
Afterwards, the exposed areas of the photoresist are dissolved
(alternatively, the unexposed areas can be dissolved if the
photoresist is cured under light). This creates a lithographic mask
that allows further processing by chemical and physical processes,
such as applying material to the open areas or etching depressions
in the open areas. Later, the remaining photoresist can also be
removed.
[0057] ".mu.-LED"
[0058] A .mu.-LED is an optoelectronic component whose edge lengths
are less than 70 .mu.m, especially down to less than 20 .mu.m,
especially in the range of 1 .mu.m to 10 .mu.m. Another range is
between 10 to 30 .mu.m. This results in an area of a few hundred
.mu.m.sup.2 down to several tens of .mu.m.sup.2. For example, a
.mu.-LED can comprise an area of about 60 .mu.m.sup.2 with an edge
length of about 8 .mu.m. In some cases, a .mu.-LED has an edge
length of 5 .mu.m or less, resulting in a size of less than 30
.mu.m.sup.2. Typical heights of such .mu.-LEDs are, for example, in
the range of 1.5 .mu.m to 10 .mu.m.
[0059] In addition to classic lighting applications, displays are
the main applications for .mu.-LEDs. The .mu.-LEDs form pixels or
subpixels and emit light of a defined color. Due to their small
pixel size and high density with a small pitch, .mu.-LEDs are
suitable for small monolithic displays for AR applications, among
other things.
[0060] Due to the above-mentioned very small size of a .mu.-LED,
the production and processing is significantly more difficult
compared to previous larger LEDs. The same applies to additional
elements such as contacts, package, lenses etc. Some aspects that
can be realized with larger optoelectronic components cannot be
produced with .mu.-LEDs or only in a different way. In this
respect, a .mu.-LED is therefore significantly different from a
conventional LED, i.e. a light emitting device with an edge length
of 200 .mu.m or more.
[0061] ".mu.-LED Array"
[0062] See at .mu.-Display
[0063] ".mu.-Display"
[0064] A .mu.-display or .mu.-LED array is a matrix with a
plurality of pixels arranged in defined rows and columns. With
regard to its functionality, a .mu.-LED array often forms a matrix
of .mu.-LEDs of the same type and color. Therefore, it rather
provides a lighting surface. The purpose of a .mu.-display, on the
other hand, is to transmit information, which often results in the
demand for different colors or an addressable control for each
individual pixel or subpixel. A .mu.-display can be made up of
several .mu.-LED arrays, which are arranged together on a backplane
or other carrier. Likewise, a .mu.-LED array can also form a
.mu.-Display.
[0065] The size of each pixel is in the order of a few .mu.m,
similar to .mu.-LEDs. Consequently, the overall dimension of a p
display with 1920*1080 pixels with a .mu.-LED size of 5 .mu.m per
pixel and directly adjacent pixels is in the order of a few 10
mm.sup.2. In other words, a .mu.-display or .mu.-LED array is a
small-sized arrangement, which is realized by means of
.mu.-LEDs.
[0066] .mu.-displays or .mu.-LED arrays can be formed from the
same, i.e. from one work piece. The .mu.-LEDs of the .mu.-LED array
can be monolithic. Such .mu.-displays or .mu.-LED arrays are called
monolithic .mu.-LED arrays or .mu.-displays.
[0067] Alternatively, both assemblies can be formed by growing
.mu.-LEDs individually on a substrate and then arranging them
individually or in groups on a carrier at a desired distance from
each other using a so-called Pick & Place process. Such
.mu.-displays or .mu.-LED arrays are called non-monolithic. For
non-monolithic .mu.-displays or .mu.-LED arrays, other distances
between individual .mu.-LEDs are also possible. These distances can
be chosen flexibly depending on the application and design. Thus,
such .mu.-displays or .mu.-LED arrays can also be called
pitch-expanded. In the case of pitch-expanded .mu.-displays or
.mu.-LED arrays, this means that the .mu.-LEDs are arranged at a
greater distance than on the growth substrate when transferred to a
carrier. In a non-monolithic .mu.-display or .mu.-LED array, each
individual pixel can comprise a blue light-emitting .mu.-LED and a
green light-emitting .mu.-LED as well as a red light-emitting
.mu.-LED.
[0068] To take advantage of different advantages of monolithic
.mu.-LED arrays and non-monolithic .mu.-LED arrays in a single
module, monolithic .mu.-LED arrays can be combined with
non-monolithic .mu.-LED arrays in a .mu.-display. Thus,
.mu.-displays can be used to realize different functions or
applications. Such a display is called a hybrid display.
[0069] ".mu.-LED Nano Column"
[0070] A .mu.-LED nano column is generally a stack of semiconductor
layers with an active layer, thus forming a .mu.-LED. The .mu.-LED
nano column has an edge length smaller than the height of the
column. For example, the edge length of a .mu.-LED nanopillar is
approximately nm to 300 nm, while the height of the device can be
in the range of 200 nm to 1 .mu.m or more.
[0071] ".mu.-Rod"
[0072] .mu.-rod or Rod designates in particular a geometric
structure, in particular a rod or bar or generally a longitudinally
extending, for example cylindrical, structure. .mu.-rods are
produced with spatial dimensions in the .mu.m to nanometer range.
Thus, nanorods are also included here.
[0073] "Nanorods"
[0074] In nanotechnology, nanorods are a design of nanoscale
objects. Each of their dimensions is in the range of about 10 nm to
500 nm. They may be synthesized from metal or semiconducting
materials. Aspect ratios (length divided by width) are 3 to 5.
Nanorods are produced by direct chemical synthesis. A combination
of ligands acts as a shape control agent and attaches to different
facets of the nanorod with different strengths. This allows
different shapes of the nanorod with different growth rates to
produce an elongated object. .mu.LED nanopillars are such nano
rods.
[0075] "Miniature LED"
[0076] Their dimensions range from 100 .mu.m to 750 .mu.m,
especially in the range larger than 150 .mu.m.
[0077] "Moire Effect" and "Moire Lens Arrangement"
[0078] The moire effect refers to an apparent coarse raster that is
created by overlaying regular, finer rasters. The resulting
pattern, whose appearance is similar to patterns resulting from
interference, is a special case of the aliasing effect by
subsampling. In the field of signal analysis, aliasing effects are
errors that occur when the signal to be sampled contains frequency
components that are higher than half the sampling frequency. In
image processing and computer graphics, aliasing effects occur when
images are scanned and result in patterns that are not included in
the original image. A moire lens array is a special case of an
Alvarez lens array.
[0079] "Monolithic Construction Element"
[0080] A monolithic construction element is a construction element
made of one piece. A typical such device is for example a
monolithic pixel array, where the array is made of one piece and
the .mu.-LEDs of the array are manufactured together on one
carrier.
[0081] "Optical Mode"
[0082] A mode is the description of certain temporally stationary
properties of a wave. The wave is described as the sum of different
modes. The modes differ in the spatial distribution of the
intensity. The shape of the modes is determined by the boundary
conditions under which the wave propagates. The analysis according
to vibration modes can be applied to both standing and continuous
waves. For electromagnetic waves, such as light, laser and radio
waves, the following types of modes are distinguished: TEM or
transverse electromagnetic mode, TE or H modes, TM or E modes. TEM
or transverse electromagnetic mode: Both the electric and the
magnetic field components are always perpendicular to the direction
of propagation. This mode is only propagation-capable if either two
conductors (equipotential surfaces) insulated from each other are
available, for example in a coaxial cable, or no electrical
conductor is available, for example in gas lasers or optical
fibers. TE or H modes: Only the electric field component is
perpendicular to the direction of propagation, while the magnetic
field component is in the direction of propagation. TM or E modes:
Only the magnetic field component is perpendicular to the
propagation direction, while the electric field component points in
the propagation direction.
[0083] "Optoelectronic Device"
[0084] An optoelectronic component is a semiconductor body that
generates light by recombination of charge carriers during
operation and emits it. The light generated can range from the
infrared to the ultraviolet range, with the wavelength depending on
various parameters, including the material system used and doping.
An optoelectronic component is also called a light emitting
diode.
[0085] For the purpose of this disclosure, the term optoelectronic
device or also light-emitting device is used synonymously. A
.mu.-LED (see there) is thus a special optoelectronic device with
regard to its geometry. In displays, optoelectronic components are
usually monolithic or as individual components placed on a
matrix.
[0086] "Passive Matrix Backplane" or "Passive Matrix Carrier
Substrate"
[0087] A passive matrix display is a matrix display, in which the
individual pixels are driven passively (without additional
electronic components in the individual pixels). A light emitting
diode of a display can be controlled by means of IC circuits. In
contrast, displays with active pixels driven by transistors are
referred to as active matrix displays. A passive matrix carrier
substrate is part of a passive matrix display and carries it.
[0088] "Photonic Crystal" or "Photonic Structure"
[0089] A photonic structure can be a photonic crystal, a
quasi-periodic or deterministically aperiodic photonic structure.
The photonic structure generates a band structure for photons by a
periodic variation of the optical refractive index. This band
structure can comprise a band gap in a certain frequency range. As
a result, photons cannot propagate through the photonic structure
in all spatial directions. In particular, propagation parallel to a
surface is often blocked, but perpendicular to it is possible. In
this way, the photonic structure or the photonic crystal determines
a propagation in a certain direction. It blocks or reduces this in
one direction and thus generates a beam or a bundle of rays of
radiation directed as required into the room or radiation area
provided for this purpose.
[0090] Photonic crystals are photonic structures occurring or
created in transparent solids. Photonic crystals are not
necessarily crystalline--their name derives from analogous
diffraction and reflection effects of X-rays in crystals due to
their lattice constants. The structure dimensions are equal to or
greater than a quarter of the corresponding wavelength of the
photons, i.e. they are in the range of fractions of a .mu.m to
several .mu.m. They are produced by classical lithography or also
by self-organizing processes.
[0091] Similar or the same property of a photonic crystal can
alternatively be produced with non-periodic but nevertheless
ordered structures. Such structures are especially quasiperiodic
structures or deterministically aperiodic structures. These can be
for example spiral photonic arrangements.
[0092] In particular, so-called two-dimensional photonic crystals
are mentioned here as examples, which exhibit a periodic variation
of the optical refractive index in two mutually perpendicular
spatial directions, especially in two spatial directions parallel
to the light-emitting surface and perpendicular to each other.
[0093] However, there are also one-dimensional photonic structures,
especially one-dimensional photonic crystals. A one-dimensional
photonic crystal exhibits a periodic variation of the refractive
index along one direction. This direction can be parallel to the
light exit plane. Due to the one-dimensional structure, a beam can
be formed in a first spatial direction. Thereby a photonic effect
can be achieved already with a few periods in the photonic
structure. For example, the photonic structure can be designed in
such a way that the electromagnetic radiation is at least
approximately collimated with respect to the first spatial
direction. Thus, a collimated beam can be generated at least with
respect to the first direction in space.
[0094] "Pixel"
[0095] Pixel, pixel, image cell or picture element refers to the
individual color values of a digital raster graphic as well as the
area elements required to capture or display a color value in an
image sensor or screen with raster control. A pixel is thus an
addressable element in a display device and comprises at least one
light-emitting device. A pixel has a certain size and adjacent
pixels are separated by a defined distance or pixel space. In
displays, especially .mu.-displays, often three (or in case of
additional redundancy several) subpixels of different color are
combined to one pixel.
[0096] "Planar Array"
[0097] A planar array is an essentially flat surface. It is often
smooth and without protruding structures. Roughness of the surface
is usually not desired and does not have the desired functionality.
A planar array is for example a monolithic, planar array with
several optoelectronic components.
[0098] "Pulse Width Modulation"
[0099] Pulse width modulation or PWM is a type of modulation for
driving a component, in particular a .mu.-LED. Here the PWM signal
controls a switch that is configured to switch a current through
the respective .mu.-LED on and off so that the .mu.-LED either
emits light or does not emit light. With the PWM, the output
provides a square wave signal with a fixed frequency f. The
relative quantity of the switch-on time compared to the switch-off
time during each period T (=1/f) determines the brightness of the
light emitted by the .mu.-LED. The longer the switch-on time, the
brighter the light.
[0100] "Quantum Well"
[0101] A quantum well or quantum well refers to a potential in a
band structure in one or more semiconductor materials that
restricts the freedom of movement of a particle in a spatial
dimension (usually in the z-direction). As a result, only one
planar region (x, y plane) can be occupied by charge carriers. The
width of the quantum well significantly determines the quantum
mechanical states that the particles can assume and leads to the
formation of energy levels (sub-bands), i.e. the particle can only
assume discrete (potential) energy values.
[0102] "Recombination"
[0103] In general, a distinction is made between radiative and
non-radiative recombination. In the latter case, a photon is
generated which can leave a component. A non-radiative
recombination leads to the generation of phonons, which heat a
component. The ratio of radiative to non-radiative recombination is
a relevant parameter and depends, among other things, on the size
of the component. In general, the smaller the component, the
smaller the ratio and non-radiative recombination increases in
relation to radiative recombination.
[0104] "Refresh Time"
[0105] Refresh time is the time after which a cell of a display or
similar must be rewritten so that it either does not lose the
information or the refresh is predetermined by external
circumstances.
[0106] "Die" or "Light-Emitting Body"
[0107] A light-emitting body or also a die is a semiconductor
structure which is separated from a wafer after production on a
wafer and which is suitable for generating light after an
electrical contact during operation. In this context, a die is a
semiconductor structure, which contains an active layer for light
generation. The die is usually separated after contacting, but can
also be processed further in the form of arrays.
[0108] "Slot Antenna"
[0109] A slot antenna is a special type of antenna in which instead
of surrounding a metallic structure in space with air (as a
nonconductor), an interruption of a metallic structure (e.g. a
metal plate, a waveguide, etc.) is provided. This interruption
causes an emission of an electromagnetic wave whose wavelength
depends on the geometry of the interruption. The interruption often
follows the principle of the dipole, but can theoretically have any
other geometry. A slot antenna thus comprises a metallic structure
with a cavity resonator having a length of the order of magnitude
of wavelengths of visible light. The metallic structure can be
located in or surrounded by an insulating material. Usually, the
metallic structure is earthed to set a certain potential.
[0110] "Field of Vision"
[0111] Field of view (FOV) refers to the area in the field of view
of an optical device, a sun sensor, the image area of a camera
(film or picking up sensor) or a transparent display within which
events or changes can be perceived and recorded. In particular, a
field of view is an area that can be seen by a human being without
movement of the eyes. With reference to augmented reality and an
apparent object placed in front of the eye, the field of view
comprises the area indicated as a number of degrees of the angle of
vision during stable fixation of the eye.
[0112] "Subpixels"
[0113] A subpixel (approximately "subpixel") describes the inner
structure of a pixel. In general, the term subpixel is associated
with a higher resolution than can be expected from a single pixel.
A pixel can also consist of several smaller subpixels, each of
which radiates a single color. The overall color impression of a
pixel is created by mixing the individual subpixels. A subpixel is
thus the smallest addressable unit in a display device. A subpixel
also comprises a certain size that is smaller than the size of the
pixel to which the subpixel is assigned.
[0114] "Vertical Light Emitting Diode"
[0115] In contrast to the horizontal LED, a vertical LED comprises
one electrical connection on the front and one on the back of the
LED. One of the two sides also forms the light emission surface.
Vertical LEDs thus comprise contacts that are formed towards two
opposite main surface sides. Accordingly, it is necessary to
deposit an electrically conductive but transparent material so that
on the one hand, electrical contact is ensured and on the other
hand, light can pass through.
[0116] "Virtual Reality"
[0117] Virtual reality, or VR for short, is the representation and
simultaneous perception of reality and its physical properties in a
real-time computer-generated, interactive virtual environment. A
virtual reality can completely replace the real environment of an
operator with a fully simulated environment.
[0118] Several aspects disclosed herein relate to the control of
light emitting elements in a .mu.-LED display. The limited space
available among the current matrix element pixels requires further
consideration of how to address and control the individual pixels.
Conventional approaches and techniques may not be usable due to the
limited space. This may also apply to concepts where the current is
controlled by each pixel. Since the required space for a .mu.-LED
as subpixel is much smaller than for normal pixels, newer concepts
are necessary.
[0119] In addition, driver circuits should be capable of providing
the current frame rates of 60 Hz to 240 Hz. In this context it is
also necessary or at least expedient to achieve a wide dynamic
brightness range (1:100,000) or 100 dB per individual pixel. This
range is necessary to achieve sufficient contrast and brightness of
the image even under different external light influences in
automotive or augmented reality applications.
[0120] Due to the already mentioned size of the individual
.mu.-LEDs in both pixelated displays and monolithic arrays, a
digitally generated pulse width modulation, PWM, seems to be
appropriate. Accordingly, the technology should be scalable in
terms of both pixel array size and CMOS technology process nodes. A
digitally generated PWM also allows calibration for non-uniformity
of both pixel array and pixel current.
[0121] A digital nonlinear PWM can process digital codes so that
the pulse width can be generated by a nonlinear transfer function
of the codes to pulse width. In the following different concepts
are presented, which are suitable for implementation in monolithic
displays or pixelated arrays with .mu.-LEDs due to their special
size or scalability.
[0122] Typically, in a pulse width modulation (PWM) implementation,
a standard pixel cell circuit is very quickly switched alternately
to "off" and "rated current". For this purpose, a so-called 2T1C
circuit is used in conventional circuits. However, especially in
displays with a high number of lines and columns, the programming
frequency is very high in order to achieve a sufficient "refresh
rate" of the display. In the past, this problem was solved by a
second transistor, which, however, consumes additional space.
Especially with the .mu.-displays shown here, or even the space
"under" the .mu.-LEDs, the space may no longer be sufficient. In
addition, depending on the wiring (i.e. position of the .mu.-LED
within the current path), a higher inaccuracy and thus intensity
fluctuations can occur. Accordingly, a current driver for .mu.-LEDs
with backgate, which reduces these problems.
[0123] According to an aspect described here, a device for
electronic control and power supply of a .mu.-LED is proposed,
which has a data signal line, a threshold line and a selection
signal line. Furthermore, a .mu.-LED is proposed, which is
electrically connected in series to a dual-gate transistor and
together with it between a first and second potential connection. A
first control gate of the dual-gate transistor is connected to the
threshold line. The device also comprises a select-hold circuit
with a charge accumulator connected to a second control gate of the
dual-gate transistor and to a current conduction contact of the
dual-gate transistor, and with a control transistor whose control
terminal is connected to the selection signal line.
[0124] Instead of an additional transistor for pulse width
modulation (PWM), the additional control gate of a dual-gate
transistor can now be modulated with a PWM signal as an existing
driver transistor. In some aspects, the dual-gate transistor also
acts as a current driver transistor.
[0125] According to a second aspect, a device is also proposed,
where a .mu.-LED and a dual-gate transistor are arranged in series
in a current path. An analogeue data drive signal is applied to one
side of the dual-gate transistor via a selection hold circuit to
control the color of the .mu.-LED by means of the selection signal.
A pulse width modulation signal coupled to the other side of the
dual-gate transistor controls the brightness of the .mu.-LED.
[0126] Advantageously, a backgate transistor is used as a dual-gate
transistor.
[0127] The modulation of the backgate of the driver transistor can
also be used as an actuator for the current control loop in order
to feed back a feedback signal, for example the forward voltage of
the LED, thus achieving a current feedback to a LED temperature
drift. By modulating the voltage at the backgate of the driver
transistor, a light emitting diode current can be easily and, above
all, space-savingly pulse-width modulated, especially in the TFT
(Thin Film Transistor) pixel cell. With RGB cells, this results in
a saving of three power transistors.
[0128] A weak modulation of the voltage at the backgate can be used
to make the current in the .mu.-LED substantially independent of
the .mu.-LED temperature. This is especially useful when using an
NMOS cell with the .mu.-LED on the low side of the driver
transistor, because of the common cathode. Such cells have
intrinsically poor current accuracy, so that the idea of the
present invention can be used to improve such cells
significantly.
[0129] On the one hand, this allows pulse width modulation via the
backgate of the main transistor instead of via an additional
transistor, in addition to the main transistor. On the other hand,
the use of a backgate transistor in displays allows temperature
stabilization by "not digitally" operating the backgate with pulse
width modulation, but with an analogeue voltage. This is derived
from the forward voltage Vf of the LED, which is used as a feedback
loop of a control system. Such temperature stabilization improves
the color accuracy and stability of the .mu.-LED.
[0130] In some aspects, the dual-gate transistor may include a
backgate transistor where the backgate is the first control gate.
This is a compact design. The dual-gate transistor can be designed
as a thin-film transistor with two opposite control gates. This
allows a reliable and compact manufacturing. The first control gate
of the dual-gate transistor can be configured to set a threshold
voltage. In this way, a modulation can be carried out.
Alternatively, a switching signal (PWM signal) can be applied to
the first control gate during operation. This allows a simple
brightness control to be carried out.
[0131] In further aspects, the .mu.-LED can be connected with its
first terminal to the first potential connection, the dual-gate
transistor can be arranged with its current line contacts between a
second terminal of the .mu.-LED and the second potential
connection. The selector hold circuit may have the charge storage
connected to the second control gate of the dual-gate transistor
and to the second terminal of the .mu.-LED. This version can be
easily manufactured in NMOS technology.
[0132] In further aspects, the .mu.-LED can be connected with its
first connection to a second current line contact of the dual-gate
transistor and with its second connection to the second potential
connection. The Dual-Gate Transistor is connected with its current
conducting contacts between a first terminal of the .mu.-LED and
the first potential connection. The charge memory of the selector
holding circuit is connected to the second control gate of the
dual-gate transistor and to the first potential connection. Thus,
the forward voltage of the LED does not affect a gate-source
voltage of the dual-gate transistor.
[0133] Another aspect deals with the realization in p-MOS
technology. There the .mu.-LED is connected with its first terminal
to the first potential terminal and the dual-gate transistor with
its current line contacts is connected between a second terminal of
the .mu.-LED and the second potential terminal. The selector hold
circuit can be connected with the charge storage to the second
control gate of the dual-gate transistor as well as to the second
potential connection.
[0134] In a further aspect, the selection hold circuit comprises a
further control transistor, which is connected in parallel to the
.mu.-LED and whose control terminal can be connected to the
selection signal line.
[0135] According to a further configuration, the charge accumulator
can be connected to the second control gate of the dual-gate
transistor as well as to the first potential terminal, and further
comprises a temperature compensation circuit with a negative
feedback based on the detection of a forward voltage by the
.mu.-LED, whereby the temperature compensation circuit can form the
threshold line on the output side. This allows additional weak
modulation to be impressed on the backgate transistor.
[0136] In some aspects, the temperature compensation circuit may
include a control path, which may be in parallel with the dual-gate
transistor, and may have two paths connected in series. This is a
simple design. In another embodiment, from a node between the two
controlled paths provided by a third control transistor and a
fourth control transistor, the threshold line may be connected to
the first control gate of the dual-gate transistor. By means of the
node the back gate can be effectively controlled. According to a
further embodiment, the control terminal of the fourth control
transistor can be connected to the second potential terminal. In
this way, the gate of the transistor is set stable to the high
potential of the second potential connection.
[0137] Another aspect is that the temperature compensation circuit
may include a second charge storage device, which may be connected
to a control terminal of a control transistor providing one of the
two paths and to the first potential terminal. This allows the gate
voltage of the third transistor to be buffered.
[0138] A second data signal line is coupled to the second charge
storage and the third control transistor. A signal on this line is
used to program a negative feedback factor. The second data signal
line can also be used for fine adjustment of the temperature
compensation. Depending on the application, this fine adjustment
can be switched on or off by means of a further control
transistor
[0139] According to another advantageous embodiment, in the
temperature compensation circuit the control terminal of the third
control transistor can be connected to the second potential
terminal. In this way, the gate voltage of the third control
transistor is advantageously fixed in a clear and stable
manner.
[0140] In accordance with another advantageous embodiment, a fifth
control transistor can be connected in parallel to the .mu.-LED. A
switching signal (PWM signal) is applied to its control terminal
during operation. In this way, the LED can be switched on and off
directly and without charge storage, in particular by means of
pulse width modulation. The dual-gate transistor can then operate
as a temperature-stabilized current source.
[0141] Also a control for a brightness adjustment or a dimming of
pixels of importance. Such dimming is not only required in the
automotive sector, for example to switch between day and night
vision, but also in AR applications. Basically, such dimming can be
useful and advantageous when contrasts have to be adjusted or when
external light makes it necessary to control the brightness of a
display in order to avoid dazzling a user or to show information
reliably.
[0142] For the reasons mentioned above, different technical
solutions are known for the control of lighting units with LEDs, in
particular to operate displays at different brightness levels. For
example, control circuits for controlling matrix displays are
known, with which the individual pixels of the rows formed by
several rows and columns are specifically controlled. Likewise,
control circuits are known with which the LED current can be
reduced or dimmed. This so-called current dimming is used, for
example, in displays with liquid crystal displays or OLEDs. Due to
the limited space available, solutions with a large number of
components are difficult to implement. This can sometimes make the
circuits very complex. Based on this, the following aspects should
further develop the control of a lighting unit with LEDs to vary
the brightness in such a way that a comparatively simple, accurate
and reliable variation of the brightness of the light emitted by
the LEDs is achieved. In particular, the above-mentioned dimming
and operation in different brightness and contrast levels should be
made possible.
[0143] Thus, a control circuit for changing the brightness of a
lighting unit is proposed, which has a voltage source for supplying
the lighting unit with electrical energy and at least one energy
storage device. The latter sets a current for the illuminants of
the lighting unit. Furthermore, a control element is provided which
temporarily changes a voltage of a voltage signal generated by the
voltage source, on the basis of which a LED current flowing through
the at least one LED can be adjusted. According to the proposed
principle, the control circuit has been developed further in such a
way that the control element is set up to operate the lighting unit
at at least two different brightness levels by transmitting a first
and a second voltage signal having different voltages to the
lighting unit during a period, i.e. in a repeating time period, and
the brightness level can be adjusted as a function of the voltage
of the first voltage signal.
[0144] Essential for this concept is therefore that a pulsed
voltage signal is applied to the lighting unit, whereby a current
flows through the at least one .mu.-LED of the lighting unit as a
function of the voltage signal, which causes the LED to light up.
During a period, a first voltage signal, in particular a switch-on
voltage signal, and a second voltage signal, in particular a
switch-off voltage signal, are advantageously provided, wherein the
at least one LED provided in the lighting unit is supplied with a
current proportional to the voltage during the application of the
first voltage signal or a current proportional to the voltage flows
through it. It is basically irrelevant here whether the lighting
unit has one or a plurality of LEDs. One aspect of the switching
element has a transistor, via which the at least one LED of the
lighting unit is supplied with electrical energy depending on the
respective voltage signal and an LED current flows through it so
that it preferably emits visible light.
[0145] According to the proposed concept, the lighting unit is
controlled in such a way that within a period, firstly a first
voltage signal is transmitted to the lighting unit in a first phase
of the period and subsequently a second voltage signal is
transmitted to the lighting unit in a second phase of the period,
whereby a current flow through the at least one LED of the lighting
unit is effected depending on the voltage of the respective voltage
signal. It is important here that the voltage or the voltage value
of the second voltage signal is significantly lower than the
voltage of the first voltage signal. Preferably, the voltage of the
second voltage signal is at least nearly zero.
[0146] In the first phase of the period, in which the first voltage
signal is transmitted to the lighting unit, the energy storage of
the lighting unit is charged. At the same time, a current
proportional to the voltage of the voltage signal flows through the
LED, which emits visible light. While in the second phase of the
period the second voltage signal is transmitted to the lighting
unit, the potential in the energy storage, preferably a capacitor,
is maintained so that until the beginning of the following period a
current caused by this flows through the LED, which thus continues
to emit light. Although the intensity of the current flowing
through the LED during the first phase of the period should
theoretically be equal to the intensity of the current flowing
through the LED during the second phase of the period, this is not
the case in practice. This is due to the fact that the control
circuit usually has a second capacitance in addition to the
capacitance of the energy storage device, in particular a
capacitor, thus creating a capacitive voltage divider so that the
voltage at the energy storage device is lower during the second
phase of the period compared to the voltage during the first phase
of the period. Such a second capacitance is provided, for example,
by the capacitance of the transistor, in particular the so-called
gate-source capacitance.
[0147] In this context, it is significant that the intensity of the
current flowing through the LED during the first phase of the
period in which the first voltage signal is transmitted to the
lighting unit is different from the intensity of the current
flowing through the LED during the second phase of the period in
which the second voltage signal is transmitted to the lighting
unit, namely smaller. However, an observer will not recognize this
difference, which leads to a difference in the maximum brightness
of the LED during a period, but will only perceive the light output
averaged over the period.
[0148] In order to use this effect in a suitable way for the
control of lighting units used in displays, for example, it is
advantageous if the first and second voltage signals are repeated
at a frequency of 60 Hz, which corresponds to the usual refresh
rate of displays. This means that within one second, a first and a
second voltage signal are transmitted to the lighting unit sixty
times each, whereby a LED current flows through at least one LED of
the lighting unit depending on the voltage of the respective
voltage signal.
[0149] In further aspects, it is planned that the .mu.-LED, while
the second voltage signal is transmitted to the lighting unit, is
supplied with the electrical energy required to excite light
emission from an energy storage device designed as a capacitor.
Since the voltage of the capacitor is lower than that of the first
phase of the period, the LED is passed through by a current of
lower intensity in this operating state compared to the first phase
of the period, so that the .mu.-LED shines less brightly.
[0150] Furthermore, it is conceivable in this way that the control
element is set up to generate the first voltage signal with a duty
cycle of 0.0025 to 0.003, the duty cycle corresponding to the ratio
of the duration of the first voltage signal to the duration of the
period. The duty cycle thus indicates the ratio of the duration of
the first voltage signal to the duration of the period. At a
repetition frequency for the first and second voltage signals of 60
Hz, this means that the control element is arranged according to
this embodiment of the invention such that a period within which
the first and second voltage signals are transmitted to the
lighting unit is 0.0166 s or 16.6 ms long. In a preferred
embodiment, the first voltage signal is transmitted to the lighting
unit for a period not exceeding 0.050 ms, which corresponds to a
duty cycle of about 0.003 or 1:333. In this case, the second
voltage signal is transmitted to the lighting unit for a period of
16.6 ms. The duty cycle in relation to this signal is therefore
approximately 1.
[0151] Since the brightness of an LED perceived by an observer
depends on the average brightness or light output emitted during a
period, a current flow in the LED during the second phase of a
period and thus the proportion of light emitted by the at least one
LED in the second, comparatively long phase of the period has a
considerable, disproportionately strong influence on the average
light output of an LED of the lighting unit.
[0152] In some aspects it is conceivable that the control circuit
is arranged to operate the lighting unit at a first, darker
brightness level by setting the voltage of the first voltage signal
to a voltage value within a first voltage interval and to operate
the lighting unit at at least a second, brighter brightness level
by setting the voltage of the first voltage signal to a voltage
value within at least a second voltage interval whose voltages are
higher than those of the first voltage interval. In accordance with
this embodiment, two voltage intervals or voltage ranges are thus
provided for the control of a lighting unit, which each have
different voltages with which the first voltage signal is generated
and which are at different voltage levels. Depending on the voltage
level of the first voltage signal, the lighting unit is thus
operated either at a first, darker brightness level or at a second,
brighter brightness level. If the lighting unit is to be operated
at the brighter brightness level, the lighting unit is controlled
on the basis of a first voltage signal whose voltage lies in the
second voltage interval and thus in the voltage interval which has
the higher value.
[0153] In another aspect, the control element is set up to operate
the lighting unit at the same brightness level by selectively
varying the voltage of the first voltage signal within one of at
least two defined voltage intervals. This means that in a
beneficial manner the first voltage signal, in particular its
voltage, is only varied between two successive periods to such an
extent that the corresponding voltage is still within the same
voltage interval and it is ensured that the lighting unit is still
operated at the same brightness level despite a slight change in
brightness. It is thus possible to dim the lighting unit, in
particular the at least one LED provided within the lighting unit,
to at least two different brightness levels, i.e. to provide an at
least largely stepless range at least at two different brightness
levels in each case, in which the brightness of the at least one
LED of a lighting unit is deliberately changed.
[0154] According to a further embodiment, it is intended that the
first voltage interval or the first voltage range should have
voltage values at least in the range of 1.3 V to 3.0 V.
Furthermore, it is preferably provided that the second voltage
interval or the second voltage range has voltage values at least in
the range from 4.0 V to 10.0 V. In this way, two ranges are
realized at different brightness levels, within which the
brightness of the lighting unit can again be specifically and
continuously changed or dimmed.
[0155] With regard to the embodiment described above, the idea can
again be considered that--as soon as a comparatively small first
voltage signal is applied to the lighting unit--the total current
flowing through the LED during a period is significantly determined
by the current flowing through the LED during the first phase of
the period in which the first voltage signal is applied to the
lighting unit. In this case, the lighting unit is operated at a
comparatively low brightness and the emission of light due to a
current flow through the LED caused by the second voltage signal
applied to the lighting unit during the second phase of the period
can be neglected in this operating state.
[0156] If, on the other hand, a first voltage signal with a
comparatively high voltage is transmitted to the lighting unit, the
total current flowing through the LED during a period is determined
to a large extent by the current flowing through the LED during the
second phase, i.e. while the second voltage signal is applied to
the lighting unit. In this case, the lighting unit is operated at a
high brightness level and can be dimmed in this range by selective
variation of the first voltage signal. The presented control
circuit can be used in a display or monitor for image generation.
These can be part of a larger screen or display device, for example
in a motor vehicle. Also a realization in AR or VR glasses or any
other device is conceivable. Again, it is essential that a control
circuit is used which allows a display or monitor to be operated at
at least two different brightness levels.
[0157] In addition to a specially designed control circuit, some
aspects also relate to a method for selectively changing the
brightness of a lighting unit, in which a voltage source supplies
the lighting unit with electrical energy and at least one LED as
the illuminant of the lighting unit is supplied with electrical
energy at least temporarily from an energy storage device of the
lighting unit. Furthermore, in this method a voltage signal is
transmitted to the lighting unit at least intermittently and the
current flowing through the at least one LED is adjusted on the
basis of the voltage signal.
[0158] The method is characterized in that the lighting unit is
operated at at least two different brightness levels by
transmitting a first and a second voltage signal having different
voltages to the lighting unit during one period and adjusting the
brightness level depending on the voltage of the first voltage
signal. It is again substantial to the invention that the
brightness of an LED, which is decisively determined by the total
current flowing through at least one LED during a period, can be
selectively changed by transmitting a first voltage signal which is
transmitted to the lighting unit in a first phase of the period. To
drive the lighting unit, a first voltage signal is applied to the
lighting unit in a first phase of the period, so that initially,
while the first voltage signal is applied to the lighting unit, the
energy storage device of the lighting unit is charged and a current
proportional to the voltage of the voltage signal flows through the
at least one LED of the lighting unit. In a second phase of the
period, a second voltage signal is transmitted to the lighting unit
with a voltage, which is significantly lower than the voltage of
the first voltage signal, preferably close to zero. This initially
lowers the potential of the energy storage device, in particular a
capacitor, which also reduces the strength of the current flowing
through the LED accordingly.
[0159] Compared to the first phase of the period, the LED therefore
shines less brightly in the second phase, but this over a
comparatively long period. Depending on the level of the voltage
value of the first voltage signal, the lighting unit can be
operated at a high brightness level with comparatively high average
light output or at a low brightness level with comparatively low
average light output. In this context, it should be noted that for
a first voltage signal with a comparatively low voltage, the
influence of the first phase of the period on the average light
output of the LED is comparatively high, whereas for a first
voltage signal with a high voltage value, the second phase of the
period in which the second voltage signal is applied to the
lighting unit is of decisive importance for the average light
output of the LED.
[0160] In this way, it is intended that the LED of the lighting
unit, while the second voltage signal is applied to the lighting
unit, is supplied with electrical energy from an energy storage
device designed as a capacitor. Furthermore, it is advantageous if
the lighting unit is at least temporarily operated at a first,
darker brightness level by setting the voltage of the first voltage
signal to a voltage value lying within a first voltage interval and
the lighting unit is at least temporarily operated at at least a
second, brighter brightness level by setting the voltage of the
first voltage signal to a voltage value lying within at least a
second voltage interval, the voltages of which are higher than
those of the first voltage interval.
[0161] In one embodiment it is provided that between two
consecutive periods the voltage of the first voltage signal is
varied without changing the brightness level at which the lighting
unit is operated. This means that the average light output of an
LED is varied while it is operated at a constant brightness level.
The voltage of the first voltage signal is thus varied between two
successive periods within the voltage interval or voltage range
provided for the corresponding brightness level.
[0162] In addition to the question of temperature stability and
drift of an input voltage or current through the diode due to
process fluctuations, the pulse modulation used is also an aspect
to be considered. In current displays, light emitting diodes are
usually operated in pulse width modulation, i.e. they are switched
on and off in rapid succession for contrast and brightness
adjustment. The frequency is several 100 kHz up to the MHz range.
The switching operations act back on the current source. This can
affect the precision and stability of the power source. In the case
of control loops within the power source, the switching processes
lead to spikes or other behaviour, which can bring the control loop
out of its control range.
[0163] Following these considerations, a regulated current source
for .mu.-LEDs which controls a current source in such a way that
its output current remains in its control state and follows a
setpoint value even during PWM modulation and in particular during
switching operations. The current source and in particular the
feedback loop is suitable
[0164] For this purpose the output current or a signal derived from
it is fed to the control loop, which compares it with the setpoint.
If the current source is now switched off or operated in an on/off
mode (intermittent operation), a substitute signal is fed to the
control loop while the output current is switched off. The
substitute signal keeps the control loop in its control range. The
substitute signal corresponds or is similar to an expected output
current or the signal derived from it. All in all, continuous
control in the output range is achieved in this way, independent of
the switching state of a current source. The precision and
stability of the supply circuit is maintained.
[0165] In an embodiment, a supply circuit is proposed which
comprises an error correction detector with a reference signal
input, an error signal input and a correction signal output.
Furthermore, a controllable current source with current output and
a control signal connection is provided. The control signal
terminal is connected to the correction signal output to form a
control loop for the controllable current source. In other words,
the error correction detector controls the output current of the
current source within certain limits. The current source is thus
configured to provide a current at the current output depending on
a signal at the control signal terminal.
[0166] According to the proposed principle, the supply circuit
comprises a substitute source with one output, which is configured
to provide a substitute signal. Finally, a switching device is
arranged in operative connection with the controllable current
source and the error correction detector, so that the switching
device, depending on a switching signal, supplies the error signal
input with either a signal derived from the current at the current
output or the substitute signal with additional separation of the
current output of the current source. In other words, the switching
device is coupled to the controllable current source and the error
correction detector and is configured to supply either a signal
derived from the current at the current output or the substitute
signal to the error signal input.
[0167] In addition, the switching device is configured to
disconnect the current output in the latter case.
[0168] This creates an arrangement that keeps the control loop in a
control range independent of the operating state of the power
source. The current source can thus be operated in a PWM or other
intermittent mode in addition to being controlled by the control
loop and the error correction detector.
[0169] It is useful if the substitute signal correspond
substantially to the signal derived from the current signal. In
this way, the control loop, and especially the error correction
detector, will provide a signal that is hardly different from that
of the current source, so that the control and the modulation
remain intact.
[0170] In one aspect, the adjustable current source has a current
mirror with a switchable output branch. This is connected to the
current output or forms it. The output branch may comprise one or
more output transistors whose control terminals or gates are
connected to a control terminal of a current mirror transistor
arranged on the input side.
[0171] In another aspect, the output transistor of the output
branch is connected with its control terminal to the switching
device. The switching device is configured, depending on the
switching signal of the output transistor, to connect to a fixed
potential for opening the output transistor or to connect the
control terminal to the control terminal of the current mirror
transistor arranged on the input side. When the control terminal is
at the fixed potential, the output transistor opens or closes, i.e.
it no longer conducts current and the load and the output of the
supply circuit are disconnected.
[0172] In another aspect, the switching device is located in the
output branch and is configured to disconnect the current output or
output transistors from the load. The tap for the error signal
input of the error correction detector is located between the
switching device and the load.
[0173] In another aspect, the adjustable current source has an
input branch. A reference current signal can be fed to the input
branch so that the current source supplies an output current
dependent on it. The input branch of the adjustable current source
also includes a node, which is connected to the reference signal
input of the error correction detector. Thus, for example, the
reference current, which is fed to the current source to derive the
output current, can also serve as a reference signal for the error
correction detector.
[0174] The adjustable current source may also include a current
mirror, with the control signal terminal connected to the control
terminal of an output transistor of the current mirror. This allows
a current through the output transistor to be changed with a
control signal, thus providing regulation. The control terminal of
the output transistor of the current mirror is coupled to the
current mirror transistor of the current mirror via a capacitor in
positive feedback. The capacitor is used for frequency compensation
and thus improves the stability of the control.
[0175] Another aspect concerns the differential amplifier. This can
include a differential amplifier whose two branches are connected
to a supply potential via a current mirror. Optionally, the two
branches of the differential amplifier can each include an input
transistor, which have different geometrical parameters. Together
with the current mirror, different fixed factors between reference
and error signal can be taken into account.
[0176] In a further aspect, the substitute source comprises an
element coupled to the output for voltage generation, so that the
substitute signal essentially corresponds to the signal derived
from the current signal. This allows the substitute signal to
simulate the current flowing through the load during regular
operation, thus keeping the control loop in the control range.
[0177] The replacement source may comprise a series connection of a
current-generating element and a voltage-generating element, with
the output located between the two elements. Similarly, in a
further aspect, the replacement source may comprise a transistor
whose control terminal is connected to the control terminal of the
current mirror transistor of the current source.
[0178] Another aspect concerns the switching device, which
comprises one or more transmission gates. The supply circuit may
comprise a reference current mirror configured to supply a current
defined on the input side to the error correction detector and to
the current source on the output side.
[0179] Another aspect concerns the use of a supply circuit for a
power supply of a .mu.-LED. This is powered by the power supply
circuit, which is an on/off operation. This means that the .mu.-LED
is driven by a pulse-width modulating signal from the power supply.
This operation is not unusual for optoelectronic devices, but the
supply circuit generates a stable and precise output current during
this pulse-width modulated operation.
[0180] Another aspect relates to a method for supplying a .mu.-LED.
Here, a supply current through the load is detected. This can be
done by detecting the current through the .mu.-LED. Alternatively,
a signal can be derived from the current, which has a known
relationship to the current through the load. The supply current or
the signal derived from it is compared with a reference signal and
a correction signal is generated from this comparison. With the
help of the correction signal, the supply current through the
consumer is controlled to a reference value, if necessary.
[0181] It is now intended that the consumer is switched off at
certain intervals, i.e. disconnected from the supply current. In
such a case, a substitute signal is generated instead of the signal
derived from the supply current and used for the comparison step.
In other words, instead of the supply current or a signal derived
from it, the substitute signal is compared with the reference
signal and a correction signal is generated from this comparison.
This makes the control independent of whether the load is supplied
with power or not. The substitute signal can substantially
correspond to a supply current through the consumer or a signal
derived from it.
[0182] Another aspect lies in the realisation of a driver circuit
with low own power consumption, which can nevertheless drive a
large number of optoelectronic elements and especially
.mu.-LEDs.
[0183] In a first aspect of the present application, a driver
circuit is intended to drive or control a large number of
optoelectronic elements. The optoelectronic elements are configured
as .mu.-LEDs and are arranged in an array of rows and columns. Each
.mu.-LED can represent one pixel. Alternatively, if each pixel
includes several, for example three, sub-pixels, each .mu.-LED can
form one of the sub-pixels.
[0184] The driver circuit comprises a plurality of first memory
cells, each of the first memory cells being associated with a
respective one of the .mu.-LEDs. In addition, each memory cell
includes two inputs, referred to as a set input and reset input,
and an output. The first memory cells may be latches and may be
configured as 1-bit memories. Each first memory cell can have two
different states at the output, a first state and a second state,
where the first state can be a high state and the second state can
be a low state.
[0185] A set signal received from one of the first memory cells at
the set input triggers the first memory cell at the output to the
first state. The first memory cell holds the first state until it
is reset to the second state by a reset signal received at the
reset input. The output, especially the output signal provided at
the output, of each first memory cell is configured to control or
drive a respective one of the .mu.-LEDs. In particular, the output
signal determines whether the .mu.-LED is switched on and emits
light or is switched off and does not emit light.
[0186] CMOS technology would be particularly suitable for the
production of the driver circuit and also the first memory cells
and their associated circuits. The driver circuit according to the
first aspect is a digital driver circuit and requires less power
and space compared to conventional driver circuits. Furthermore,
the driver circuit according to the first aspect provides better
linearity. Each first memory cell can provide a pulse width
modulation signal, PWM signal, at its output.
[0187] In an embodiment, each first memory cell comprises two
cross-coupled NOR gates or two cross-coupled NAND gates. Each of
the NOR or NAND gates has two inputs and one output. The output of
each of the NOR or NAND gates is coupled to one of the inputs of
the other NOR or NAND gate. The other input of one of the NOR or
NAND gates receives the set signal, and the other input of the
other of the NOR or NAND gates receives the reset signal.
[0188] In an alternative embodiment, each first memory cell
comprises an N-type metal oxide semiconductor transistor, NMOS
transistor, and a P-type metal oxide semiconductor transistor, PMOS
transistor, which are connected in series, meaning that the
channels of the two transistors are connected in series.
Furthermore, an input of an inverter is connected between the NMOS
transistor and the PMOS transistor, and an output of the inverter
is connected to the gates of the NMOS and PMOS transistors. The
driver circuit may include a plurality of loadable counters, each
configured to activate a set signal to turn on a current through
the respective .mu.-LED when data, such as a pulse width value, is
loaded into the respective counter. The counter counts until the
current value reaches the loaded data value. Then the counter
activates a reset signal to turn off the current through the
respective .mu.-LED.
[0189] When an array of .mu.-LEDs arranges them in N columns of
pixels, the driver circuit can include N counters that generate PWM
signals for N columns of pixels simultaneously per selected row.
The driver circuit may also include a single common counter
configured to generate a common or global dimming signal for the
plurality of .mu.-LEDs.
[0190] To disconnect dark pixels, the driver circuit can include a
large number of second memory cells. Each second memory cell may be
coupled to a respective one of the first memory cells and may be
configured to override an output signal of the respective first
memory cell when required so that the respective .mu.-LED remains
off. In other words, the second memory cells prevent the respective
first memory cells from turning on the respective .mu.-LED when
these optoelectronic elements represent dark pixels during a
frame.
[0191] An optoelectronic device or .mu.-display according to a
second aspect of the present application comprises a plurality of
.mu.-LEDs and a driver circuit for driving the plurality of
.mu.-LEDs according to the first aspect as described above. The
.mu.-LEDs may be arranged in an array and may form a display or a
portion of a display. Each of the .mu.-LEDs can form one pixel of
the array. Alternatively, each .mu.-LED can also form a sub-pixel.
For example, in an RGB pixel array, a pixel can contain three
optoelectronic elements or .mu.-LEDs that emit red, green and blue
light respectively. Alternatively, converter materials may be
provided so that at least two of the three .mu.-LEDs emit light of
the same color, which is converted by the converter material.
[0192] The .mu.-LED can be arranged above an integrated circuit,
IC, which is located below the .mu.-LED. The circuit can be formed
in a different material system.
[0193] In a third aspect, a method for operating an optoelectronic
device or .mu.-display according to the second aspect is provided.
At the beginning of a frame, a global reset is performed and the
pixel current is switched off so that all optoelectronic elements
are switched off. Next, dark pixels are loaded line by line. Thus,
the optoelectronic elements that are dark during the frame are
controlled by the second memory cells. Afterwards, a line by line
content-dependent PWM, for example grayscale PWM, is performed.
Thus, the current through the optoelectronic elements is controlled
by means of the first memory cells.
[0194] In addition, after a global reset at the beginning of a
frame, the pixel stream can remain switched off until a common or
global dimming starts. The common dimming of the optoelectronic
elements can be performed before the current through the
optoelectronic elements is controlled by the first memory cells.
The global dimming data can be combined with the grayscale data in
the video/image signal processor IC or by the .mu.-LED driver IC so
that no separate global dimming pulse is required and then only the
grayscale data is updated line by line. The optoelectronic device
according to the second aspect and the method according to the
third aspect may include the embodiments disclosed above in
connection with the driver circuit according to the first
aspect.
[0195] A novel concept for the control of .mu.-LEDs, which are
intended as pixels, is based on a analogeue ramp for lighting
control.
[0196] For a control circuit for a display matrix comprising a
plurality of optoelectronic devices arranged in rows and columns,
pulse width modulation can be used to adjust the on/off behavior of
each pixel. Although the principle seems to be similar to
conventional pulse width modulation schemes, the implementation is
different and takes into account the small space available.
[0197] A control circuit for a matrix display, in particular a
.mu.-LED matrix display comprises a row selection input for a row
selection signal, a column data input for a data signal, a ramp
signal input for a ramp signal and a trigger input for a trigger
signal. For the purpose of explanation, a ramp signal is a signal
that varies over time from a first value to a second value.
Usually, a ramp signal is periodic. The circuit includes a column
data buffer configured to buffer the data signal in response to the
row select signal. In some variants, the level of the column data
signal may correspond to the brightness of the light-emitting
device. A pulse generator is coupled to the column data buffer and
the ramp signal input and configured to provide a buffered output
signal to control the on/off ratio of at least one of the plurality
of light emitting devices in response to the trigger signal, the
data signal and the ramp signal.
[0198] The proposed principle implements an analogeue pulse
generator that requires only a small space. Since the ramp signal
can be multiplexed in space and time, artefacts caused by
activation of different pixels can be suppressed. Furthermore, time
multiplexing when using the ramp signal leads to different
switching behavior of the pixels. This means that the .mu.-LED
associated with the pixels is switched at different times, which
causes a more even power distribution and prevents current
peaks.
[0199] In some variants, the pulse generator has a comparator
device to compare the buffered data signal with the ramp signal.
The result is delivered to an output buffer, which is coupled to an
output of the comparator and the trigger input, the column data
buffer can act as an input buffer in such embodiment. Together with
the output buffer of the pulse generator a double buffering is
realized, which allows to implement the circuit in displays using a
longer duty cycle, thus reducing update rates and the like. In
general, this concept will further reduce power consumption, which
is preferred in advanced reality applications.
[0200] The output buffer can have a single memory stage, such as a
flip-flop. In some variants, the buffer may contain an RS
flip-flop, whose inputs are coupled to the output of the comparator
device and, accordingly, to the trigger input. In this respect it
should be noted that depending on the current implementation and
the sign of the corresponding data and trigger signals (positive or
negative), inverted inputs of the corresponding flip-flops can also
be used. The column data buffer in some variants includes a
capacitor to store the data signal and a switch located between the
capacitor and the column data input.
[0201] The capacitor may comprise a small capacitance, just as the
input buffer can only apply a voltage signal of the order of a few
volts and the comparator device has a very high input impedance.
The comparator can be implemented using a differential amplifier.
For example, an inverting input of the comparator can be coupled to
the data column buffer and its non-inverting input can be coupled
to the ramp signal input.
[0202] Depending on the implementation, the .mu.-LED coupled to the
control circuit can only be active for a short period of time. In
some variants, the .mu.-LED can only be active for about 50% of a
normal cycle. In such cases, it is useful to be able to disable
unneeded parts of the control circuit. For this purpose, the
comparator device may have a power control input coupled to the
trigger input for adjusting its power consumption based on the
trigger signal. Alternatively, the comparator device may be coupled
to the output buffer to control its power consumption based on an
output state of the output buffer. In this respect, the output
buffer may be configured to maintain its output state independently
of its input coupled to the comparator device until it is reset or
triggered by the trigger signal.
[0203] Another aspect concerns the generation of the ramp signal.
In some variations, the control circuitry includes a ramp generator
to provide the ramp signal to the ramp signal input, the ramp
generator being configured to generate a varying signal between a
start value and an end value in response to a trigger signal. The
ramp generator can be implemented as a global ramp generator that
sends a common ramp signal to various other control circuits.
Alternatively, some ramp generators can be provided, where each
individual ramp generator drives a number of lines and their
respective pixels. Such an implementation allows to multiplex the
ramp signals temporarily and thus reduce the artefact. Furthermore,
a ramp signal supplied by a ramp generator can also be multiplexed
before being applied to the ramp signal input.
[0204] Another aspect relates to a method of controlling the
illumination of a light emitting device in a matrix display having
a plurality of light emitting devices arranged in addressable rows
and columns. In accordance with the proposed principle, the method
comprises providing a trigger signal and a data signal for a
selected row and at least one light emitting device. A level of the
data signal is then converted to a pulse with respect to the
trigger signal. More precisely, in some variants the level of the
data signal is converted to a pulse width with respect to a trigger
signal. The pulse is used to control the on/off ratio of the light
emitting device with a pulse.
[0205] In some aspects, converting a level of the data signal
involves generating a ramp signal between a first value and a
second value. The data signal is compared with the ramp signal to
generate a state signal. The state signal can be a digital signal.
The pulse signal is then based on the trigger signal and a change
in the state signal. Essentially, the pulse signal is set from LOW
to HIGH or reset from HIGH to LOW in response to the change in the
state signal. Of course, this principle of setting the value and
resetting the value can be changed.
[0206] The ramp signal can be generated or initiated in response to
the trigger signal. In some variants, both signals can be derived
from a common signal. Delivering a data signal can also include
pre-buffering of the data signal in some variants. For example, the
data signal could be pre-buffered in a storage device such as a
capacitor or the like.
[0207] Another aspect deals with the correction of errors in
.mu.-LEDs of a .mu.-Display or .mu.-Display module that occur
during their production by means of redundant .mu.-LED branches
with selection fuse. Several concepts are presented in this
application to create redundant .mu.-LEDs in production.
[0208] With .mu.-displays a .mu.-LED can fail during production.
This can be caused, for example, by faulty assembly or, in the case
of monolithic display modules, by a fault in one of the layers.
There are two main variants of such an error. One is an open
contact, known as "Open", or a short circuit between the anode and
cathode, known as "Short". Both lead to the failure of the cell's
LED.
[0209] Redundant .mu.-LEDs are provided for each subpixel to reduce
the failure probability of a subpixel or a pixel. In the event of a
defect, appropriate circuitry measures are taken to ensure that the
cell does not fail, i.e. the defective LED can be decoupled from
the current source. In some variants, however, this means that in a
fault-free case both .mu.-LEDs are supplied by the same current
source, namely the typical and the redundant one. This in turn
leads to a color shift resulting from a dependency between cross
current and dominant wavelength. In addition, due to the process
technology of .mu.-displays or modules, often only one common
cathode can be used for all LEDs. Depending on the further
construction of the backplane (e.g. TFT backplanes), this can lead
to the fact that only NMOS transistors (N-type metal oxide
semiconductor transistors) can be used to construct the pixel cell.
In a conventional 2T1C (2 transistors, 1 capacitor) cell, this
leads to a clear dependence between the cross-current of the LED
and its forward voltage.
[0210] There are various approaches to solving these difficulties,
most of which, however, require additional work or space. According
to the principle proposed here, a solution is given in which, on
the one hand, redundancy is provided, but halving an electric
current flowing through a light emitting diode is avoided. In
addition, PMOS transistors can be used, which increases
flexibility. The space consumption does not increase significantly,
so that the solutions are just suitable for .mu.-displays with low
space per pixel or subpixel.
[0211] This involves the creation of a device for electronically
driving a plurality of .mu.-LEDs of a pixel cell or sub-pixel, in
particular as a 2T1C cell. By means of a first transistor and an
electronic imprinting component associated with the .mu.-LED, a
current flow is generated which triggers the fuse connected in
series to this .mu.-LED.
[0212] A device for electronically driving a plurality of .mu.-LEDs
of a pixel cell or subpixel thus comprises a first and at least one
second path each having a .mu.-LED connected therein and an
electronic fuse arranged in series with the .mu.-LED. The first and
the at least one second path are connected to one side with a
potential terminal. Furthermore, a driver circuit with a data
signal input, a selection signal input and a driver output is
provided. The driver output is connected to the other side of the
first and the at least one second path. Finally, the device
comprises an imprinting component associated with the at least one
second path, which is configured to generate a current flow
triggering the electronic fuse arranged in series.
[0213] A characterising feature thus consists in the introduction
of an additional imprint signal line in combination with an
additional electronic imprint component, which can be adapted in
particular as a transistor or as a diode. This ensures that after
an end-of-line (EOL) test, only one LED per color and pixel is
active, even in the case of an error-free pixel. In other words, in
the event of an error, the .mu.-LED that is still functioning is
selected. If, on the other hand, there is no error, i.e. both
.mu.-LEDs of a path are working, one of the two will still be
switched off permanently.
[0214] In a method for the electronic configuration of a plurality
of .mu.-LEDs, a test of a function of the .mu.-LED of the first
path and the second path is thus carried out first. If both
.mu.-LEDs of the first and second path are functioning, an
imprinting signal is applied to the electronic imprinting
component. A current flow is then impressed into the second path of
a fuse, which triggers the fuse connected in series with the
.mu.-LED of the second path. For this purpose, the fuse is usually
configured as a fuse link.
[0215] Depending on its embodiment, the imprinting component may
comprise an imprinting transistor whose current line contacts are
electrically parallel to the .mu.-LED to which the imprinting
component is assigned and whose control contact is connected to an
imprinting signal line. Alternatively, the imprinting component can
also comprise an imprinting diode, which is connected with one
terminal to the second terminal of the .mu.-LED to which the
imprinting component is assigned. The other terminal of the
impress-in diode is connected to the impress-in signal line.
[0216] The proposed arrangement makes it possible to design the
.mu.-LED as a so-called common anode or common cathode. This means
that, depending on the embodiment, the .mu.-LED of each path is
either switched between supply potential and current source or
between current source and reference potential connection. Thus, in
one case the .mu.-LED is connected to the supply potential
connection and the electronic fuse. In the other case, the .mu.-LED
is connected between the fuse and the reference potential
connection. The current source is always connected to the
electronic fuse of the respective path. The charge storage of the
2T1C cell is connected to the gate of the current source transistor
and the fixed potential, i.e. to the potential terminal to which
the current source transistor is also connected.
[0217] In a further aspect, a .mu.-display or .mu.-display module
with a variety of the devices described above is presented, in
which pixel cells of the .mu.-display are electrically connected
along a line and/or along a column to a common imprint signal line.
Each pixel cell of a column is electrically connected to the supply
potential terminal by means of a common supply line to a switching
transistor arranged on a common carrier outside the
.mu.-display.
[0218] Small-scale display arrangements with a high resolution are
particularly desirable for AR systems, such as head-up displays or
glasses with a light field display that projects a raster image
directly onto the retina.
[0219] Micro OLEDs have been proposed for .mu.-displays with active
pixel-sized light sources. Their disadvantage is their insufficient
luminance and limited lifetime. An alternative for self-luminous
light sources, which promises a long lifetime and a high efficiency
as well as additionally a fast reaction time, is the use of
.mu.-LEDs arranged in matrix form, for example based on GaN or
InGaN. These are particularly suitable for display arrangements
with a high packing density to form a high-resolution
.mu.-display.
[0220] The starting point for the consideration is a display device
comprising an IC substrate component and a monolithic pixelated
optochip mounted thereon. In the present case, a monolithic
pixelated optochip is understood to be a matrix-shaped arrangement
of light-emitting optoelectronic light sources, which are created
on a continuous chip substrate by a common manufacturing process.
Some of the structures presented here can be produced in a matrix.
These include, for example, the antenna structure, vertical or
horizontal .mu.-rods, the paired bar-shaped configuration with
converter material between the .mu.-LEDs or the .mu.-LEDs along
special crystal directions, to name a few non-limiting examples.
These light sources are adapted as .mu.-LEDs.
[0221] The IC substrate component features monolithic integrated
circuits, which in turn result from a common manufacturing process.
In addition, IC substrate contacts are arranged as a matrix on a
top side of the IC substrate component facing the monolithic
pixelated optochip.
[0222] The monolithic pixelated optochip comprises a semiconductor
layer sequence with a first semiconductor layer having a first
doping and a second semiconductor layer having a second doping,
wherein the polarity of the charge carriers in the first
semiconductor layer differs from that of the second semiconductor
layer. Preferably, the first semiconductor layer and the second
semiconductor layer extend laterally over the entire monolithic
pixelated optochip. For an embodiment, the first semiconductor
layer may have a p-type doping and the second semiconductor layer
may have an n-type doping. A reverse doping is possible as well as
the use of several sublayers of the same doping for at least one of
the semiconductor layers, which differ in the doping strength
and/or with respect to the semiconductor material. In particular,
the semiconductor layer sequence can form a double heterostructure.
Between the first semiconductor layer and the second semiconductor
layer there is a region with a transition in which light-emitting
active zones are formed during operation of the display. For a
possible embodiment, the active zone is located in a doped or
undoped active layer, which is placed between the first and the
second semiconductor layer and has, for example, one or more
quantum well structures.
[0223] The individual light-emitting, optoelectronic light sources
of the pixelated optochip each represent .mu.-LEDs arranged as a
matrix, each .mu.-LED having a .mu.-LED rear side facing the IC
substrate component and a first light source contact which adjoins
the first semiconductor layer in a contacting manner and is
electrically conductively connected to one of the IC substrate
contacts in each case. In other words, each .mu.-LED in the
pixelated optochip is formed so as to include a region of one of
the above-mentioned active layers. Between adjacent .mu.-LEDs, the
active layer or another of the above-mentioned layers may be
interrupted, so that crosstalk is avoided.
[0224] The inventors recognized that a display arrangement with
high packing density, which is simplified in terms of production
technology, can be realized if the projection area of the first
light source contact on the .mu.-LED rear side corresponds to at
most half the area of the .mu.-LED rear side and the first light
source contact is surrounded in lateral direction by an absorber on
the rear side. In the present case, the lateral direction is
understood to be a direction perpendicular to a stacking direction
determined by averaging the surface normals of the semiconductor
layer sequence.
[0225] Due to a small area first light source contact, which is
significantly smaller than the pixel area of the assigned .mu.-LED,
a lateral narrowing of the current path in the semiconductor layer
stack results. Consequently, the lateral extension of an active
zone is limited to [.mu.m] dimensions, so that individually
controllable .mu.-LEDs are separated from each other due to the
localized recombination zone within the semiconductor layer stack.
The pixel size of each .mu.-LED, which is defined as the maximum
diagonal of the .mu.-LED backside, is <70 .mu.m and preferably
<20 .mu.m and especially preferred <7 .mu.m. Again, the
preferred first light source contact is significantly smaller,
whereby for advantageous embodiments the projection area of the
first light source contact on the .mu.-LED backside occupies at
most 25% and preferably at most 10% of the area of the .mu.-LED
backside.
[0226] To limit the lateral expansion of the active zone, the first
semiconductor layer and the second semiconductor layer are
preferably configured with a p or n conductivity of less than
10.sup.4 Sm-1, preferably less than 3*10.sup.3 Sm-1, more
preferably less than 103 Sm-1, so that the lateral expansion of the
current path is limited. In addition, it is advantageous if the
layer thickness of the first semiconductor layer in the stacking
direction is at most ten times and preferably at most five times
the maximum diagonal of the first light source contact in the
lateral direction.
[0227] For further embodiment, a first light source contact on the
monolithic pixelated optochip does not directly abut the associated
IC substrate contact. Instead, the actual optochip contact element,
whose cross-sectional area is larger than that of the first light
source contact, lies below the first light source contact in
relation to the stacking direction. This measure simplifies the
positioning of the monolithic pixelated optochip on the IC
substrate component and the mutual contacting without worsening the
lateral limitation of the current path.
[0228] According to the invention, the area around the small first
light source contact is used for the arrangement of a rear
absorber, which reduces the optical crosstalk between adjacent
.mu.-LEDs. In particular, the downwardly directed electromagnetic
radiation emanating from the active zone at an angle is absorbed as
long as a limit angle to the stacking direction is exceeded.
[0229] Preferred materials for the rear absorber are structured
layers with silicon, germanium and gallium arsenide. It is also
possible to incorporate graphene or soot particles into the rear
absorber.
[0230] The rear absorber laterally surrounds the first light source
contact and extends laterally from it. Rear absorbers of adjacent
.mu.-LEDs are adjacent to each other and are preferably made in one
piece. For an embodiment, the rear absorber extends in stacking
direction at least up to the first semiconductor layer. For a
further embodiment, a partial section of the rear absorber runs
within the correspondingly structured first semiconductor layer and
shields the border region between adjacent .mu.-LEDs. For this
purpose, additionally or alternatively reflective radiation
blockers can be used, such as structured elements made of reflector
materials, such as aluminum, gold or silver, or of dielectric
materials whose refractive index is lower than that of the first
semiconductor layer. For further embodiment, the rear absorber not
only fulfils an optical function, but also serves as an electrical
insulator to limit the current path laterally.
[0231] The display arrangement has a second light source contact
for each .mu.-LED in the stacking direction above the second
semiconductor layer. This contact is made of a transparent material
such as indium tin oxide (ITO) and is electrically connected to a
transparent, flat contact layer on the front side of the pixelated
optochip. For an advantageous embodiment, the second light source
contact is formed by the large-area contact layer itself, so that
the entirety of the second light source contacts of the .mu.-LEDs
arranged in matrix form can be applied as one common area contact.
For an alternative embodiment which further reduces optical
crosstalk, the second light source contact adjoins the contact
layer in each case in a contacting manner, second light source
contacts of adjacent .mu.-LEDs being separated from one another by
an absorber on the front side in a lateral direction perpendicular
to the stacking direction. The front absorber may consist of a
material absorbing the electromagnetic radiation emitted by the
active zone or of a material reflecting this radiation. In addition
or alternatively, the front absorber can act as an electrical
insulator and contribute to the lateral restriction of the current
path for the localization of the recombination zone to an area with
[.mu.m] dimensions.
[0232] For a possible further embodiment, the front absorber
extends against the stacking direction at least in a part of the
second semiconductor layer. Furthermore, the lower and/or the upper
sides of the second light source contact and/or the contact layer
and/or the upper side of the second semiconductor layer may have an
optically effective structuring to improve light extraction.
[0233] For a proposed method of manufacturing a display
arrangement, an IC substrate component with monolithic integrated
circuits and with IC substrate contacts arranged as a matrix is
electrically conductively connected to a monolithic pixelated
optochip. For the preceding manufacturing of the monolithic
pixelated optochip, a semiconductor layer sequence with a first
semiconductor layer having a first doping and a second
semiconductor layer having a second doping is grown preferably
epitaxially, the polarity of the charge carriers in the first
semiconductor layer differing from that of the second semiconductor
layer and the semiconductor layer sequence defining a stacking
direction. Furthermore, .mu.-LEDs arranged in the pixelated
optochip as a matrix are applied, each .mu.-LED having a rear side
facing the IC substrate component and a first light source contact
which adjoins the first semiconductor layer in a contacting manner
and is electrically conductively connected to a respective one of
the IC substrate contacts. In accordance with the invention, the
first light source contact is formed with such a size that its
projection surface with a surface normal perpendicular to the
stacking direction occupies at most half the surface of the rear
side of the .mu.-LED. In addition, the first light source contact
is surrounded by an absorber on the rear side in a lateral
direction perpendicular to the stacking direction.
[0234] Besides the different concepts for driving and providing a
redundancy circuit, another aspect is to connect the carrier with
the .mu.-LEDs or the monolithic array with a carrier that contains
the driving. There are concepts that try to realize both .mu.-LEDs
and the IC circuits in the same material system. This is to be
advocated per se and can be realized at least in parts. However,
the material systems for .mu.-LEDs have disadvantages, so that they
are only partially suitable for IC circuits.
[0235] Another aspect is to create different material systems for
the generation of the driving circuits on one side and the
.mu.-LEDs in a matrix arrangement on the other side. There are
substantially two possibilities for this. Firstly, one material
system can be started with and the components can be manufactured,
then a transition to the other material system is created and in
this the further components are provided. Supply lines through the
material systems and connect the components. One difficulty with
this approach is to select and set the different process parameters
in such a way that it is possible to manufacture one "side" without
damaging the other "side". For example, the process temperature
(e.g. for diffusion or implantation processes) is very different,
so that depending on the temperature, no or undesired diffusion
occurs. In this way, components can be damaged. In some aspects it
is proposed to manufacture the control in one technology, for
example on silicon basis, and then to grow different material
systems as .mu.-rods or similar.
[0236] Another approach proposes to manufacture the control and
pixel array separately and then connect them electrically and
mechanically. In this way, the needs and requirements of the
respective situation can be adapted and production can be
optimized. Due to the small size of .mu.-LEDs, precise orientation
for contacting is essential. The above example already illustrates
this problem and suggests a solution. On the other hand, the use of
digital control techniques allows reducing the number of necessary
contact pads between the carriers without limiting the
functionality. For the production of .mu.-displays or even display
devices and matrices, novel digital and analogeue concepts
developed and jointly implemented.
[0237] One aspect of the design of a .mu.-LED display concerns the
control of the light emission elements or .mu.-LEDs in a
.mu.-display. The .mu.-Display thus has a plurality of .mu.-LEDs
arranged in rows and columns. In some aspects, the .mu.-LEDs can be
combined into subunits. This makes them easier to manufacture, test
and process.
[0238] The limited space available under the actual matrix elements
and pixels requires further considerations regarding addressing and
control of the individual pixels. Conventional approaches and
techniques may not be applicable due to the limited space. This may
also apply to concepts where the current is controlled by each
pixel.
[0239] In one embodiment a .mu.-display is provided, which has a
plurality of pixel structure arranged in rows and columns. A first
substrate structure is manufactured in a first material system and
has a plurality of .mu.-LEDs whose edge length is 70 .mu.m or less,
in particular less than 20 .mu.m. The .mu.-LEDs are individually
addressable by lines in and/or on the first substrate structure. A
large number of contacts are arranged on a surface of the first
substrate structure facing away from the main radiation
direction.
[0240] Furthermore, the .mu.-display has a second substrate
structure, which comprises a plurality of digital circuits for
addressing the .mu.-LEDs. The second substrate structure is
manufactured in a different material system than the first
substrate structure. The second substrate structure comprises on
one surface a plurality of contacts corresponding to the contacts
of the first substrate structure. According to the proposed
principle, the first and second substrate structures are now
mechanically and electrically connected to each other so that the
contact areas correspond to each other. In accordance with this
concept, it is proposed to manufacture digital and analogeue
elements of a display separately in different material systems and
then to connect them with each other. This allows the optimal
technology to be used in each case.
[0241] In this context, the first substrate structure with
.mu.-LEDs can be configured as a monolithic module. In addition,
the modular design revealed here can be used. Thus, the first
substrate structure itself would be a carrier for the modules
comprising the different .mu.-LEDs. The first substrate structure
includes in some aspects the analogeue circuits, for example a
current source for each pixel. The redundancy circuits and driver
circuits provided here are also conceivable. A design of these
circuits in thin-film technology is possible, as long as the
requirements for a current carrying capacity are not too high. If
possible, it may be appropriate in some aspects to provide
multiplexers or other circuits in the first substrate structure.
This can reduce the number of contact areas between the first and
second substrate structure. Simple switches, each selecting one of
two .mu.-LEDs, reduce the number of necessary contact areas by
about half. In other aspects, contacts may be grouped together, for
example when using a common cathode layer for the .mu.-LEDs.
[0242] The .mu.-LEDs have an edge length of 20 .mu.m or less. For
particularly small .mu.-displays the edge length can be 2 .mu.m to
5 .mu.m. Depending on the embodiment, the contacts can be the same
size as the .mu.-LEDs, but also smaller.
[0243] As far as material systems are concerned, the choice is
flexible, with each technology and material system bringing its own
advantages and challenges. The second substrate structure is based,
among others, on monocrystalline, polycrystalline or amorphous
silicon. To realize digital circuits in these material systems is
well understood and can be scaled to small sizes. Likewise,
indium-gallium-zinc-oxide, GaN or GaAs are suitable as material
systems for the second substrate structure. As material system for
the first substrate structure, at least one of the following
compounds can be used: GaN, GaP, GaInP, InAlP, GaAlP, GaAlInP, GaAs
or AlGaAs. One aspect can be the different thermal expansions and
crystallographic parameters depending on the material systems used.
Therefore, both substrate structures are often not bonded together
directly, but via several intermediate layers.
[0244] The second substrate structure with the digital circuits, in
addition to the supply lines, can also contain a variety of digital
circuits to generate a PWM-like signal from a clock signal and a
data word for each pixel. Furthermore, it is possible to implement
series-connected shift registers whose respective length
corresponds to the data word for one pixel, each shift register
being connected to a buffer for intermediate storage.
[0245] For the already mentioned reduction of contact areas, the
second substrate structure can comprise one or more multiplexers,
which are electrically coupled to a demultiplexer in the first
substrate structure for driving several .mu.-LEDs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0246] In the following section, some of the above-mentioned and
summarized aspects are explained in more detail using various
explanations and examples.
[0247] FIG. 1A shows a diagram illustrating some requirements for
so-called .mu.-displays or micro-displays of different sizes with
respect to the field of view and pixel pitch of the
.mu.-display;
[0248] FIG. 1B shows a diagram of the spatial distribution of rods
and cones in the human eye;
[0249] FIG. 1C shows a diagram of the perceptual capacity of the
human eye with assigned projection areas;
[0250] FIG. 1D is a figure showing the sensitivity of the rods and
cones over the wavelength;
[0251] FIG. 2A is a diagram illustrating some requirements for
micro-displays of different sizes in terms of the field of view and
the angle of collimation of a pixel of the .mu.-display;
[0252] FIG. 2B illustrates an exemplary execution of a pixel
arrangement to illustrate the parameters used in FIGS. 1A and
2A;
[0253] FIG. 3A shows a diagram illustrating the number of pixels
required depending on the field of view for a specific
resolution;
[0254] FIGS. 3B-1 and 3B-2 include a table of preferred
applications for .mu.-LED arrays;
[0255] FIG. 4A shows a principle representation of a .mu.-LED
display with essential elements for light generation and light
guidance;
[0256] FIG. 4B shows a schematic representation of a .mu.-LED array
with similar .mu.-LEDs;
[0257] FIG. 4C is a schematic representation of a .mu.-LED array
with .mu.-LEDs of different light colors;
[0258] FIG. 5A illustrates an embodiment of a dual-gate transistor
in a cross-section;
[0259] FIG. 5B shows two top views of the dual-gate transistor;
[0260] FIG. 5C illustrates a plot of the dependence of a threshold
voltage on a top-gate voltage;
[0261] FIG. 6 shows a first embodiment of a control circuit for a
.mu.-LED with some aspects according to the presented concept;
[0262] FIG. 7 shows a second embodiment of a control circuit for a
.mu.-LED with further aspects;
[0263] FIG. 8 is a third embodiment of a control circuit for a
.mu.-LED according to some aspects of the proposed concept;
[0264] FIG. 9 shows embodiment of a control circuit for a .mu.-LED
with further aspects;
[0265] FIG. 10 illustrates a further embodiment of a control
circuit for a .mu.-LED according to some aspects of the proposed
concept;
[0266] FIG. 11 shows a further embodiment in addition to the
previous figure;
[0267] FIG. 12 shows a fifth embodiment of a control circuit for a
.mu.-LED according to some aspects;
[0268] FIG. 13 shows a circuit diagram of an SRAM-6-T cell to
illustrate one aspect;
[0269] FIG. 14 shows a circuit diagram of a driver circuit to
illustrate some aspects;
[0270] FIG. 15 is a schematic representation of a display with
digital elements and the pixel array according to some of the
proposed aspects;
[0271] FIG. 16 shows a circuit to illustrate the clock for dark
pixels;
[0272] FIG. 17 is a representation of a global bias for the pixel
stream according to some aspects;
[0273] FIG. 18 shows a signal-time diagram with some signals
according to the embodiment of FIG. 15;
[0274] FIG. 19 shows another embodiment of a driver circuit with
reduced space consumption;
[0275] FIG. 20 shows embodiments of another driver circuit that
also has a reduced space consumption;
[0276] FIG. 21A shows a schematic diagram of a driver circuit for
two .mu.-LEDs to explain some aspects of dimmable control according
to some aspects;
[0277] FIG. 21B shows an embodiment of the dimmable control with a
.mu.-LED module;
[0278] FIG. 22 is a diagram of the LED current flowing through the
LED as a function of different capacitor voltages;
[0279] FIG. 23 shows a schematic representation of the brightness
of a lighting unit with LED when driven with a comparatively high
first voltage signal;
[0280] FIG. 24 is another schematic representation of the
brightness of a lighting unit with LED when driven with a
comparatively low first voltage signal;
[0281] FIG. 25 is a diagram showing the average light output of a
lighting unit with LED as a function of the voltage selected for
the capacitor voltage according to some aspects of the concept
presented here;
[0282] FIG. 26 shows a block diagram of the main components of a
PWM supply circuit for .mu.-LEDs;
[0283] FIG. 27 is an embodiment of a PWM supply circuit for
.mu.-LED according to the proposed principle;
[0284] FIG. 28 shows the embodiment of FIG. 27 in an operating
state with additional information on the signal flow;
[0285] FIG. 29 shows two principle illustrations of two simple
switch devices;
[0286] FIG. 30 illustrates a signal-time diagram of the proposed
embodiment with the signal points shown in FIG. 27;
[0287] FIG. 31 shows an illustrative version of an analogeue
ramp-based control circuit suitable for controlling the on/off
ratio for light-emitting devices in a .mu.-LED display;
[0288] FIG. 32 illustrates a signal-time diagram with different
signals of the concept according to FIG. 31;
[0289] FIG. 33 shows a circuit diagram of a pixel cell with
redundant .mu.-LEDs and fuses to separate a .mu.-LED;
[0290] FIG. 34 shows a further embodiment of a circuit with
redundant .mu.-LEDs, in which a defect of a .mu.-LED can be
compensated;
[0291] FIG. 35 illustrates a third embodiment of a circuit with
redundant .mu.-LEDs according to some aspects of the presented
concept;
[0292] FIG. 36 shows a fourth embodiment of a circuit with
redundant .mu.-LEDs in which a defective .mu.-LED can be
replaced;
[0293] FIG. 37 shows a fifth embodiment of a circuit with redundant
.mu.-LEDs;
[0294] FIG. 38 is a sixth embodiment of a circuit with redundant
.mu.-LEDs, in which a defect of a .mu.-LED is compensated;
[0295] FIG. 39 shows an outline of a procedure for testing and
configuring a pixel cell that is driven by one of the circuits
presented above;
[0296] FIG. 40 illustrates a circuit for driving and testing
.mu.-LEDs according to aspects of the proposed concept of a slot
antenna based on the principle disclosed in this application;
[0297] FIG. 41 is an embodiment of a controller with a different
.mu.-LED concept according to some aspects;
[0298] FIG. 42 shows a further embodiment of a control with a
.mu.-LED concept presented here;
[0299] FIG. 43 shows an embodiment of a display device consisting
of a monolithic pixel array with a monolithic IC in cross-sectional
view according to some aspects of the proposed concept;
[0300] FIG. 44 shows the previous embodiment of the proposed
display device in cross-sectional view with a sketched possible
light path;
[0301] FIG. 45 illustrates a second embodiment of the proposed
display device with monolithic pixel array and IC in
cross-sectional view;
[0302] FIG. 46 is a third embodiment of the proposed display device
in cross-sectional view according to further aspects of the
proposed principle;
[0303] FIG. 47 shows a fourth embodiment of the proposed display
device in cross-sectional view with additional measures for light
guidance;
[0304] FIGS. 48A and 48B show two alternative embodiment to improve
the localization of charge carriers in one of the proposed display
devices with further aspects from this disclosure;
[0305] FIG. 49A illustrates a circuit diagram for a control circuit
of one or more LEDs, taking into account the requirements for
geometry and size;
[0306] FIG. 49B shows an alternative embodiment of a schematic
diagram of a driver circuit for several .mu.-LEDs, taking into
account the requirements for geometry and size;
[0307] FIG. 49C shows a version of a comparator circuit, as it can
be used in a comparator instead of an OR gate as used in FIG.
49A;
[0308] FIG. 49D shows a time diagram for the various counter words
1D to 3D and the memory registers as they are used to generate the
output signal;
[0309] FIG. 50A shows a sectional view of a .mu.-LED display
arrangement;
[0310] FIG. 50B shows various examples of how the different
sections are connected after the execution of FIGS. 49A and
50A;
[0311] FIG. 51 shows an example of an inverted transistor of offset
type using amorphous silicon for use in the analogeue part of a
.mu.-LED driver;
[0312] FIG. 52 illustrates some examples of polysilicon transistors
suitable for a .mu.-LED driver circuit;
[0313] FIG. 53 shows a circuit diagram of a .mu.-LED or LED
display;
[0314] FIG. 54 shows a circuit diagram of a .mu.-LED display
segmented into different sub-matrices;
[0315] FIG. 55 illustrates a conventional approach for a driver
circuit for an LED in one pixel of a display;
[0316] FIG. 56 illustrates a version of a conventional gap driver
suitable for use in a display;
[0317] FIG. 57 shows a version of a conventional line driver
suitable for use in a display.
DETAILED DESCRIPTION
[0318] Augmented reality is usually generated by a dedicated
display whose image is superimposed on reality. Such device can be
positioned directly in the user's line of sight, i.e. directly in
front of it. Alternatively, optical beam guidance elements can be
used to guide the light from a display to the user's eye.
[0319] In both cases, the display may be implemented and be part of
the glasses or other visually enhancing devices worn by the user.
Google's.TM. Glasses is an example of such a visually augmenting
device that allows the user to overlay certain information about
real world objects. For the Google.TM. glasses, the information was
displayed on a small screen placed in front of one of the lenses.
In this respect, the appearance of such an additional device is a
key characteristic of eyeglasses, combining technical functionality
with a design aspect when wearing glasses. In the meantime, users
require glasses without such bulky or easily damaged devices to
provide advanced reality functionality. One idea, therefore, is
that the glasses themselves become a display or at least a screen
on or into which the information is projected.
[0320] In such cases, the field of vision for the user is limited
to the dimension of the glasses. Accordingly, the area onto which
extended reality functionality can be projected is approximately
the size of a pair of spectacles. Here, the same, but also
different information can be projected on, into or onto the two
lenses of a pair of spectacles.
[0321] In addition, the image that the user experiences when
wearing glasses with augmented reality functionality should have a
resolution that creates a seamless impression to the user, so that
the user does not perceive the augmented reality as a pixelated
object or as a low-resolution element. Straight bevelled edges,
arrows or similar elements show a staircase shape that is
disturbing for the user at low resolutions.
[0322] In order to achieve the desired impression, two display
parameters are considered important, which have an influence on the
visual impression for a given or known human sight. One is the
pixel size itself, i.e. the geometric shape and dimension of a
single pixel or the area of 3 subpixels representing the pixel. The
second parameter is the pixel pitch, i.e. the distance between two
adjacent pixels or, if necessary, subpixels. Sometimes the pixel
pitch is also called pixel gap. A larger pixel pitch can be
detected by a user and is perceived as a gap between the pixels and
in some cases causes the so-called fly screen effect. The gap
should therefore not exceed a certain limit.
[0323] The maximum angular resolution of the human eye is typically
between 0.02 and 0.03 angular degrees, which roughly corresponds to
1.2 to 1.8 arc minutes per line pair. This results in a pixel gap
of 0.6-0.9 arc minutes. Some current mobile phone displays have
about 400 pixels/inch, resulting in a viewing angle of
approximately 2.9.degree. at a distance of 25 cm from a user's eye
or approximately 70 pixels/.degree. viewing angle and cm. The
distance between two pixels in such displays is therefore in the
range of the maximum angular resolution. Furthermore, the pixel
size itself is about 56 .mu.m.
[0324] FIG. 1A illustrates the pixel pitch, i.e. the distance
between two adjacent pixels as a function of the field of view in
angular degrees. In this respect, the field of view is the
extension of the observable world seen at a given moment. This is
because human vision is defined as the number of degrees of the
angle of view during stable fixation of the eye.
[0325] In particular, humans have a forward horizontal arc of their
field of vision for both eyes of slightly more than 210.degree.,
while the vertical arc of their field of vision for humans is
around 135.degree.. However, the range of visual abilities is not
uniform across the field of vision and can vary from person to
person.
[0326] The binocular vision of humans covers approximately
114.degree. horizontally (peripheral vision), and about 90.degree.
vertically. The remaining degrees on both sides have no binocular
area but can be considered part of the field of vision.
[0327] Furthermore, color vision and the ability to perceive shapes
and movement can further limit the horizontal and vertical field of
vision. The rods and cones responsible for color vision are not
evenly distributed.
[0328] This point of view is shown in more detail in FIGS. 1B to
1D. In the area of central vision, i.e. directly in front of the
eye, as required for Augmented Reality applications and partly also
in the automotive sector, the sensitivity of the eye is very high
both in terms of spatial resolution and in terms of color
perception.
[0329] FIG. 1B shows the spatial density of rods and cones per
mm.sup.2 as a function of the fovea angle. FIG. 1C describes the
color sensitivity of cones and rods as a function of wavelength. In
the central area of the fovea, the increased density of cones (L, S
and M) means that better color vision predominates. At a distance
of about 25.degree. around the fovea, the sensitivity begins to
decrease and the density of the visual cells decreases. Towards the
edge, the sensitivity of color vision decreases, but at the same
time contrast vision by means of the rods remains over a larger
angular range. Overall, the eye develops a radially symmetrical
visual pattern rather than a Cartesian visual pattern. A high
resolution for all primary colors is therefore required, especially
in the center. At the edge it may be sufficient to work with an
emitter adapted to the spectral sensitivity of the rods (max.
sensitivity at 498 nm, see FIG. 1D and the sensitivity of the
eye).
[0330] FIG. 1C shows the different perceptual capacity of the human
eye by means of a graph of the angular resolution A relative to the
angular deviation .alpha. from the optical axis of the eye. It can
be seen that the highest angular resolution A is in an interval of
the angular deviation .alpha. of +/-2.5.degree., in which the fovea
centralis 7 with a diameter of 1.5 mm is located on the retina 19.
In addition, the position of the blind spot 22 on the retina 19 is
sketched, which is located in the area of the optic nerve papilla
23, which has a position with an angular deviation a of about
15.degree..
[0331] The eye compensates this non-constant density and also the
so-called blind spot by small movements of the eye. Such changes in
the direction of vision or focus can be counteracted by suitable
optics and tracking of the eye.
[0332] Furthermore, even with glasses, the field of vision is
further restricted and, for example, can be approximately in the
range of 80.degree. for each lens.
[0333] The pixel pitch in FIG. 1A on the Y-axis is given in .mu.m
and defines the distance between two adjacent pixels. The various
curves C1 to C7 define the diagonal dimension of a corresponding
display from 5 mm to approximately 35 mm. For example, curve C1
corresponds to a display with the diagonal size of 5 mm, i.e. a
side length of approximately 2.25 mm. For a field of view of
approximately 80.degree., the pixel pitch of a display with a
diagonal size of 5 mm is in the range of 1 .mu.m. For larger
displays like curve C7 and 35 mm diagonal size, the same field of
view can be implemented with a pixel pitch of approximately 5
.mu.m.
[0334] Nevertheless, the curves in FIG. 1A illustrate that for
larger fields of view, which are preferred for extended reality
applications, very high pixel densities with small pixel pitch are
required if the well-known fly screen effect is to be avoided. One
can now calculate the size of the pixel for a given number of
pixels, a given field of view and a given diagonal size of a
.mu.-display.
[0335] Equation 1 shows the relationship between dimension D of a
pixel, pixel pitch pp, number N of pixels and the edge length d of
the display. The distance r between two adjacent pixels calculated
from their respective centers is given by
r=d/2+pp+d/2.
D=d/N-pp
N=d/(D+pp) (1)
[0336] Assuming that the display (e.g. glasses) is at a distance of
2.54 cm (1 inch) from the eye, the distance r between two adjacent
pixels for an angular resolution of 1 arcminute as roughly
estimated above is given by
r=)tan(1/60.degree.*30 mm
r=8.7 .mu.m
[0337] The size of a pixel is therefore smaller than 10 .mu.m,
especially if some space is required between two different pixels.
With a distance, r between two pixels and a display with the size
of 15 mm.times.10 mm, 1720.times.1150 pixels can be arranged on the
surface.
[0338] FIG. 2B shows an arrangement, which has a carrier 21 on
which a large number of pixels, 20 and 20a to 20c are arranged.
Pixels arranged side by side have the pixel pitch pp, while pixels
20a to 20c are placed on carrier 21 with a larger pixel pitch pp.
The distance between two pixels is given by the sum of the pixel
pitch and half the size for each adjacent pixel. Each of the pixels
20 is configured so that its illumination characteristic or its
emission vector 22 is substantially perpendicular to the emission
surface of the corresponding LED.
[0339] The angle between the perpendicular axes to the emission
surface of the LED and the beam vector is defined as the
collimation angle. In the example of emission vector 22, the
collimation angle of LEDs 20 is approximately zero. LED 20 emits
light that is collinear and does not widen significantly.
[0340] In contrast, the collimation angle of the emission vector 23
of the LED pixels 20a to 20c is quite large and in the range of
approximately 45.degree.. As a result, part of the light emitted by
LED 20a overlaps with the emission of an adjacent LED 20b.
[0341] The emission of the LEDs 20a to 20c is partially
overlapping, so that its superposition of the corresponding light
emission occurs. In case the LEDs emit light of different colors,
the result will be a color mixture or a combined color. A similar
effect occurs between areas of high contrast, i.e. when LED 20a is
dark while LED 20b emits a certain light. Because of the overlap,
the contrast is reduced and information about each individual
position corresponding to a pixel position is reduced.
[0342] In displays where the distance to the user's eye is only
small, as in the applications mentioned above, a larger collimation
angle is rather annoying due to the effects mentioned above and
other disadvantages. A user is able to see a wide collimation angle
and may perceive displayed objects in slightly different colors
blurred or with reduced contrast.
[0343] FIG. 2A illustrates in this respect the requirement for the
collimation angle in degrees against the field of view in degrees,
independent of specific display sizes. For smaller display sizes
such as the one in curve C1 (approx. 5 mm diagonal), the
collimation angle increases significantly depending on the field of
view.
[0344] As the size of the display increases, the collimation angle
requirements change drastically, so that even for large display
geometries such as those illustrated in curve C7, the collimation
angle reaches about 10.degree. for a field of view of 100.degree..
In other words, the collimation angle requirements for larger
displays and larger fields of view are increasing. In such
displays, light emitted by a pixel must be highly collimated to
avoid or reduce the effects mentioned above. Consequently, strong
collimation is required when displays with a large field of view
are to be made available to a user, even if the display geometry is
relatively large.
[0345] As a result of the above diagrams and equations, one can
deduce that the requirements regarding pixel pitch and collimation
angle become increasingly challenging as the display geometry and
field of view grow. As already indicated by equation 1, the
dimension of the display increases strongly with a larger number of
pixels. Conversely, a large number of pixels is required for large
fields of view if sufficient resolution is to be achieved and fly
screens or other disturbing effects are to be avoided.
[0346] FIG. 3A shows a diagram of the number of pixels required to
achieve an angular resolution of 1.3 arc minutes. For a field of
view of approximately 80.degree., the number of pixels exceeds 5
million. It is easy to estimate that the size of the pixels for a
QHD resolution is well below 10 .mu.m, even if the display is 15
mm.times.10 mm. In summary, advanced reality displays with
resolutions in the HD range, i.e. 1080p, require a total of 2.0736
million pixels. This allows a field of view of approximately
50.degree. to be covered. Such a quantity of pixels arranged on a
display size of 10.times.10 mm with a distance between the pixels
of 1 .mu.m results in a pixel size of about 4 .mu.m.
[0347] In contrast, the table in FIGS. 3B-1 and 3B-2 shows several
application areas in which .mu.-LED arrays can be used. The table
shows applications (use case) of .mu.-LED arrays in vehicles (Auto)
or for multimedia (MM), such as automotive displays and exemplary
values regarding the minimum and maximum display size (min. and
max. size X Y [cm]), the pixel density (PPI) and the pixel pitch
(PP [.mu.m]) as well as the resolution (Res.-Type) and the distance
of the viewer (Viewing Distance [cm]) to the lighting device or
display. In this context, the abbreviations "very low res", "low
res", "mid res" and "high res" have the following meaning:
TABLE-US-00001 very low res pixel pitch approx. 0.8-3 mm low res
Pixel pitch approx. 0.5-0.8 mm mid res Pixel pitch approx. 0.1-0.5
mm high res Pixel pitch less than 0.1 mm
[0348] The upper part of the table, entitled "Direct Emitter
Displays", shows inventive applications of .mu.-LED arrays in
displays and lighting devices in vehicles and for the multimedia
sector. The lower part of the table, titled "Transparent Direct
Emitter Displays", names various applications of .mu.-LED arrays in
transparent displays and transparent lighting devices. Some of the
applications of .mu.-displays listed in the table are explained in
more detail below in the form of embodiments.
[0349] The above considerations make it clear that challenges are
considerable in terms of resolution, collimation and field of view
suitable for extended reality applications. Accordingly, very high
demands are placed on the technical implementation of such
displays.
[0350] Conventional techniques are configured for the production of
displays that have LEDs with edge lengths in the range of 100 .mu.m
or even more. However, they cannot be automatically scaled to the
sizes of 70 .mu.m and below required here. Pixel sizes of a few
.mu.m as well as distances of a few .mu.m or even less come closer
to the order of magnitude of the wavelength of the generated light
and make novel technologies in processing necessary.
[0351] In addition, new challenges in light collimation and light
direction are emerging. Optical lenses, for example, which can be
easily structured for larger LEDs and can also be calculated using
classical optics, cannot be reduced to such a small size without
the Maxwell equations. Apart from this, the production of such
small lenses is hardly possible without large errors or deviations.
In some variants, quantum effects can influence the behaviour of
pixels of the above-mentioned size and have to be considered.
Tolerances in manufacturing or transfer techniques from pixels to
sub mounts or matrix structures are becoming increasingly
demanding. Likewise, the pixels must be contacted and individually
controllable. Conventional circuits have a space requirement, which
in some cases exceeds the pixel area, resulting in an arrangement
and space problem.
[0352] Accordingly, new concepts for the control and accessibility
of pixels of this size can be quite different from conventional
technologies. Finally, a focus is on the power consumption of such
displays and controllers. Especially for mobile applications, a low
power consumption is desirable.
[0353] In summary, for many concepts that work for larger pixel
sizes, extensive changes must be made before a reduction can be
successful. While concepts that can be easily up scaled to LEDs at
2000 .mu.m for the production of LEDs in the 200 .mu.m range,
downscaling to 20 .mu.m is much more difficult. Many documents and
literature that disclose such concepts have not taken into account
the various effects and increased demands on the very small
dimensions and are therefore not directly suitable or limited to
pixel sizes well above 70 .mu.m.
[0354] In the following, various aspects of the structure and
design of .mu.-LED semiconductors, aspects of processing, light
extraction and light guidance, display and control are
presented.
[0355] These are suitable and designed to realize displays with
pixel sizes in the range of 70 .mu.m and below. Some concepts are
specifically designed for the production, light extraction and
control of .mu.-LEDs with an edge length of less than 20 .mu.m and
especially less than 10 .mu.m. It goes without saying, and is even
desired, that the concepts presented here can and should be
combined with each other for the different aspects. This concerns
for example a concept for the production of a .mu.-LED with a
concept for light extraction. In concrete terms, a .mu.-LED
implemented by means of methods to avoid defects at edges or
methods for current conduction or current constriction can be
provided with light extraction structures based on photonic crystal
structures. Likewise, a special drive can also be realized for
displays whose pixel size is variable. Light guidance with
piezoelectric mirrors can be realized for .mu.-LEDs displays based
on the slot antenna aspect or on conventional monolithic pixel
matrices.
[0356] In some of the following embodiments and described aspects,
additional examples of a combination of the different embodiments
or individual aspects thereof are suggested. These are intended to
illustrate that the various aspects, embodiments or parts thereof
can be combined with each other by the skilled person. Some
applications require specially adapted concepts; in other
applications, the requirements for the technology are somewhat
lower. Automotive applications and displays, for example, may have
a longer pixel edge length due to the generally somewhat greater
distance to a user. Especially there, besides applications of
extended reality, classical pixel applications or virtual reality
applications exist. This is in the context of this disclosure for
the realization of .mu.-LED displays, whose pixel edge length is in
the range of 70 .mu.m and below, also explicitly desired.
[0357] A general illustration of the main components of a pixel in
a .mu.-display is shown schematically in FIG. 4A. It shows an
element 60 as a light generating and light emitting device. Various
aspects of this are described in more detail below in the section
on light generation and processing. Element 60 also includes basic
circuits, interconnects, and such to control the illumination,
intensity, and, when applicable, color of the pixel. Aspects of
this are described in more detail in the section on light control.
Apart from light generation, the emitted light must be collimated.
For this purpose, many pixels in microdisplays have such
collimation functionality in element 60. The parallel light in
element 63 is then fed for light guidance into some optics 64, for
further shaping and the like. Light collimation and optics suitable
for implementing pixels for microdisplays are described in the
section on light extraction and light guidance.
[0358] The pixel device of FIG. 4A illustrates the different
components and aspects as separate elements. An expert will
recognize that many components can be integrated into a single
device. In practice, the height of a .mu.-display is also limited,
resulting in a desired flat arrangement.
[0359] For displays a control of each pixel is done individually
and separated from a second pixel to provide the appropriate
flexibility to visualize any kind of information. In simple terms,
it requires it requires controlling separately approximately 2
million pixels in a matrix of 1920.times.1080 pixels as in
conventional TVs or monitors. Apart from the challenges of
addressing such a number of pixels individually, in augmented
reality and automotive applications, the display is quite small and
the pixel size, as mentioned above, is only a few .mu.m.
[0360] In conventional drivers for larger pixel sizes and displays,
the analogeue drivers, like the digital circuits, can be easily
placed under the corresponding pixels. In such conventional
displays with a pixel, size of for example 200 .mu.m.sup.2 the
available space "under" the pixel is of the same order of
magnitude. The driver circuit could easily be implemented in the
available space and the size of the pixel itself would not be the
limiting factor. However, with the reduced size of the pixels, the
available space is no longer sufficient for conventional circuitry.
A similar problem arises when using digital circuit technology in
the material systems used so far. Silicon technology offers
possibilities to further reduce the size of circuits, but this
material system cannot easily be combined with existing materials
for generating blue or green light.
[0361] Therefore, new concepts are needed, which can roughly be
divided into two areas. The first area refers to new designs of
transistors, capacitors or other elements. The designs themselves
may exist for completely different applications or fields of
technology but not in combination with the material systems used
for the .mu.-LED or in combination with .mu.-LEDs as such. The
second area relates to circuit design and the principles of driving
.mu.-LED pixels. Simply put, digital transmission paths to address
the pixels in rows and columns take up space, as does the
corresponding row and column decoding. The same applies to the
realization of current sources or buffers to supply the individual
.mu.-LEDs with the necessary current. The design in monolithic as
well as single .mu.-LEDs can allow different concepts to achieve a
good visual impression with new approaches in addressing the
.mu.-LEDs in a display.
[0362] FIG. 5A shows an embodiment of a current driver for
.mu.-LEDs with backgate and dual-gate transistor, respectively,
which is formed in NMOS technology. This design can be realized in
a particularly compact form with only little space consumption.
[0363] Such a back-gate transistor is often used as a current
driver transistor or as a current source. It is constructed in TFT
(thin-film technology), among other things, and has a second
control connection, also known as a back gate, in addition to its
standard control connection or gate. With the help of this
additional back gate, the conductive channel of the transistor can
be changed as explained below. Instead of an additional transistor
for pulse width modulation (PWM), the back gate of an existing
dual-gate transistor can now be modulated with a PWM signal.
[0364] FIG. 5A shows a cross-section of a backgated NMOS
field-effect transistor. On the left side is a source region S, on
the right side is a drain region D, with a current conducting
channel between the two regions. The resistance of the channel,
i.e. its ability to conduct current, is changed by a single gate in
a normal field effect transistor. In a dual-gate transistor, the
channel is changed by a first bottom gate B and a second top gate
T. The gates are located on different sides of the channel. In the
embodiment shown, the top gate (upper gate) provides the additional
rear side contact or back gate contact.
[0365] FIG. 5B shows two top views of the dual-gate transistor as
shown in FIG. 5A. As shown in the left-hand illustration, a power
line can be controlled by a left source area S and a right drain
area D via top gate T and/or bottom gate B. The right-hand
illustration in FIG. 5B shows a section of the arrangement shown in
FIG. 5A.
[0366] FIG. 5C shows an illustration of the dependence of a
threshold voltage on a top-gate voltage V.sub.TG and thus the
interaction of a back contact with the threshold voltage V.sub.TH.
The threshold voltage VTH is in particular the gate-source voltage
V.sub.GS, with which the field effect transistor becomes
conductive. FIG. 5C shows the x-axis, the voltage.sub.VTG applied
to a top gate T. As a function of this, the y-axis shows the
threshold voltage VTH for changing the conductivity of the channel
of the controlled NMOS field effect transistor. For example, a top
gate voltage of 0 V provided a threshold voltage of 0.5 V for
current conduction. By means of the additional top gate of the
insulated Gate ZO NMOS transistor, the threshold voltage V.sub.TH
of the transistor can be shifted almost linearly over a wide
range.
[0367] FIG. 6 shows a first embodiment of a device for electronic
control of a .mu.-LED, in particular a pixel or subpixel for a
display. The .mu.-LED can be manufactured using the various
technologies shown. These include monolithic production, but also
the arrangement in bar form, with current constriction or with the
antenna structure disclosed in here. Decoupling structures can be
provided to direct the light.
[0368] The .mu.-LED is connected in series with a dual-gate
transistor between a first potential GND and a second potential
Vdd. The arrangement comprises a threshold line PWM, which is
connected to the first control gate or the back-gate BG of the
dual-gate transistor T2. This has an additional control electrode.
This backgate BG with a rear contact is shown in FIG. 5A and FIG.
5B. As shown in FIG. 5C, the threshold voltage can be shifted
significantly via the back contact, i.e. the output current can be
modulated by means of the additional gate BG while the voltage
U.sub.GS between gate G and source S remains constant. In
principle, Gate G and Backgate BG can also be used in reverse. This
means that the current setting can be carried out by means of the
first control terminal BG and the pulse width modulation by means
of the second gate G. By means of the wide dynamic range provided
by the circuit, the threshold voltage can be shifted into ranges
that lead to a safe switch-off of the second transistor T2.
[0369] This enables pulse width modulation (PWM) operation.
[0370] Another advantage is the speed of the proposed circuit using
the dual-gate transistor T2. A fast switching can be carried out.
Since, in contrast to modulation via the "Data" line, no memory
capacity is used, modulation can be performed much faster with the
same driver performance.
[0371] Furthermore, the device comprises a data signal line data
and a selection signal line sel. Finally, the device also contains
a selection hold circuit with a charge storage Cs and a control
transistor T1. The charge accumulator is arranged between a second
control gate G of the dual-gate transistor T2 and a connection of
the .mu.-LED. The control terminal of the control transistor T1 is
connected to selection signal line Sel. During operation, a date
"data" is impressed on the data signal line via the selection
signal line on gate G of the dual-gate transistor T2. The voltage
U.sub.GS is stored in capacitor Cs and is still present even after
switching off selection transistor T1. The voltage is given by the
data signal, whereby addressing is done by means of the selection
signal Sel.
[0372] Gate G thus creates a fixed channel and thus a constant
current through the current path. In this way a constant current
source is provided by transistor T2, which is additionally pulse
width modulated by a PWM signal at the back gate of transistor T2.
The .mu.-LED thus switches by the PWM signal between a current
given by the date in the charge storage and the state "off".
[0373] Since the .mu.-LED in some embodiments comprises a slight
dependency of the color by the impressed current, the color can be
impressed to a small extent by the data signal and the intensity by
the PWM signal. If the color dependence is low, the intensity can
be adjusted via the date even with a fixed PWM.
[0374] FIG. 6 shows a pulse width modulation of an adjustable
constant current source with an NMOS TFT (Thin Film) transistor T2
without GND-based programming. However, this version is not
temperature stabilized. The temperature instability results from
the fact that the voltage across the charge storage Cs varies
slightly due to the temperature dependence of the voltage drop
across the LED.
[0375] FIG. 7 shows a second embodiment of a device for electronic
control of a .mu.-LED pixel cell, provided in NMOS technology.
Similar to the previous design, the current path includes a
.mu.-LED and a dual-gate transistor T2 connected in series between
the first potential terminal GND and the second terminal Vdd. The
charge memory Cs of the selection signal holding circuit comprises
one terminal connected to the gate G of transistor T2 and its other
terminal connected between source S and first potential GND. As a
result, the voltage across the charge accumulator Cs remains
constant and is no longer dependent on the forward voltage of the
light emitting diodes and thus no longer so dependent on
temperature. The selection signal holding circuit is programmed via
GND.
[0376] On the other side the .mu.-LED is connected between the
drain connection D and the supply potential Vdd. Thus, the .mu.-LED
is located on the side of the second potential connection Vdd,
which provides the electrically higher potential. The arrangement
is similar to FIG. 6, but the .mu.-LED is not located on the low
side, i.e. not with the cathode connected to GND (ground), but on
the high side or upper side of transistor T2. Thus, the cathode of
the micro light emitting diode is connected to the drain of
transistor T2 and its anode to the second potential connection Vdd.
Correspondingly, the .mu.-LED shows, for example, a common anode
topology instead of a previous "common cathode".
[0377] FIG. 8 shows a third embodiment of a device, an embodiment
shown in FIG. 6, but now implemented using PMOS thin-film
transistors instead of NMOS thin-film transistors (TFT). Only PMOS
transistors are used. In this embodiment, the charge memory is
connected between the source of the dual-gate transistor T2 and the
first potential Vdd.
[0378] The embodiments shown in FIGS. 6 to 8 allow classic control
in a pixel matrix. The "front gate" (normal) gate G of transistor
T2 is described with a voltage value Data, the holding capacitor Cs
stores this voltage value and controls the second transistor T2
accordingly. This is used, for example, to set a color mixture in
an RGB pixel. A pulse width modulation (PWM) voltage is now applied
to the second transistor T2 via the backgate BG. This voltage
modulates the micro light emitting diode current in time via pulse
width modulation (PWM) and is used, for example, to change the
general brightness of a pixel with a previously programmed color.
The color is programmed in advance via the first transistor T1 and
the capacitor Cs. The same pulse width modulation signal can also
be applied to all transistors of a display line, for example, to
the respective backgate. Thus, a whole line is "dimmed".
[0379] It is also possible that all back gates of a complete
display, i.e. all columns and all rows, are driven by a common
pulse width modulation signal PWM, so that the complete display is
"dimmed" without changing its picture content. This can be used,
for example, for a day-night mode for a display in a car or for
glasses on Augment Reality applications. In this way, the
brightness can be adjusted dynamically and continuously to an
external brightness. In the automotive sector, parts of a display
may also be individually controllable in this way, allowing dark
areas to be brightened and lighter areas to be darkened.
[0380] FIG. 9 shows a third embodiment of a device, namely a
further design of a control device. In addition to the
representation and device shown in FIG. 6, a third transistor T3 is
connected in parallel to the .mu.-LED, the control terminal of the
third transistor T3 being connected to the selection signal line
Sel. The transistor T2 as constant current source is here designed
with only one gate. By means of such an arrangement, programming
can be performed independently of the anode potential of the
.mu.-LED. The device shown here results from a combination of
NMOS-based IGZO processes and the requirement of a common cathode
from process technology with regard to an assembly of .mu.-LEDs. On
this basis an implementation of a 2T1C (two transistors and one
capacitance) current source is possible.
[0381] If a high potential Vdd is applied to the selection signal
line Sel, the first transistor T1 is connected to the data signal
line Vdata, in addition the third transistor T3 becomes conductive,
bridging the LED and connecting capacitor C to reference potential
(GND). In this way, the capacitor is programmed with the voltage
Vdata, referenced to the reference potential GND of the lower,
first potential connection and not to the anode potential of the
.mu.-LED. If the potential of the selection signal line Sel is at
the reference potential (GND), the first transistor T1 and the
third transistor T3 are blocked, so that the capacitor C maintains
its previously programmed voltage, which corresponds to the
gate-source voltage U.sub.GS of the second transistor T2. If the
anode potential shifts, the separation of Vdata also shifts the
gate potential to the second transistor T2, so that the gate-source
voltage U.sub.GS of transistor T2 remains constant. In this way,
the second transistor T2 can operate as a current source.
[0382] FIG. 10 shows a fourth example of a device, in the form of a
subpixel cell. FIG. 10 shows an arrangement as shown in FIG. 9 with
the difference that the second transistor T2 here is designed as a
dual-gate transistor whose additional gate terminal BG is connected
to a threshold line PWM for applying pulse width modulation. The
front gate G is connected to the charge storage C, the back gate BG
is fed with the pulse width modulated signal.
[0383] The transistors T1 to T3 in combination with the holding
capacitor C1 form a 3T1C cell in NMOS configuration. The 2T1C cell
consisting of transistor T1 and transistor T2 can also be designed
as a PMOS configuration. In this case, for example, the third
transistor T3 is not required. Transistor T2 is configured as a
so-called "dual-gate transistor".
[0384] FIG. 11 shows an illustration of an example of a device with
additional temperature stabilization. The transistors T1 and T2 in
combination with the holding capacitor C1 provide a 2T1C cell in
NMOS configuration. The LED is placed on the low side of transistor
T2, since a "common cathode" is provided for process-related
reasons. The T2 is designed as a "dual-gate transistor" and thus
comprises two control electrodes. Similar to some previous
examples, the gate (corresponding to the bottom gate in FIG. 5A) of
the dual-gate transistor T2 is also part of the topology of the
2T1C cell in this embodiment and provides the color and general
brightness of the .mu.-LED via the ground-related programming of
the charge storage C1 and the signal on line Data1. Via the
backgate BG (front gate of FIG. 5) a PWM signal can be applied to
transistor T2, which acts as a current source. The gate-source
voltage of transistor T2 is thus dependent on the forward voltage
of the LED. Since the voltage drop across the LED depends on both
the cross-current and the temperature, the output current is
considerably different from the actual expected value of the
programming. This can be described by the following equation 2:
I.sub.LED=K(Udata-U.sub.LED(T,I)-Uth).sup.2 (2)
[0385] Here U.sub.data is the voltage across the charge storage C1.
When the .mu.-LED heats up by itself, its forward voltage
decreases, which leads to an increase of the current through
transistor T2. Due to the absence of negative feedback, a change in
the operating parameters of the .mu.-LED therefore has a
significant effect on the current and thus on the brightness or
color of the .mu.-LED.
[0386] Therefore a negative feedback is proposed, which exploits
the functionality of transistor T2 as a dual-gate transistor and
allows compensation of such effects. The negative feedback
comprises a holding capacitor C2, which is connected between the
reference potential AVSS and a control terminal of a transistor T3.
The first terminal of this capacitor forms the control for the
backgate BG of the dual-gate transistor T2 and the other terminal
is connected to the source S of the dual-gate transistor T2. The
negative feedback comprises a further transistor T4, whose control
and drain terminals are connected to the supply potential AVDD. Its
source terminal is connected to the backgate BG and the drain of
transistor T3. Finally, a fifth transistor T5 is provided for
optional programming of a compensation, which stores a compensation
value on line Data 2 in the holding capacitor C2 on the basis of a
selection signal Set2.
[0387] The gate-source voltage of transistor T3 corresponds to the
voltage of holding capacitor C2 minus the forward voltage of the
LED. If this forward voltage Vf_LED increases, the gate-source
voltage U.sub.GS of the third transistor T3 decreases, since the
stored charge on the capacitor C2 remains the same. Thus, the
current through the third transistor T3 decreases. Since this
current also flows through transistor T4, the coupling of its gate
to the supply potential results in a smaller voltage drop U.sub.DS
via the fourth transistor T4. This results in a higher voltage at
the node to the back gate of transistor T2. This in turn results in
a lower threshold voltage at transistor T2. By means of an
appropriate design of the transistors T3 and T4 according to the
following equation 3
.beta. = - W 4 L 3 W 3 L 4 .times. .times. whereat .times. .times.
U th * .times. .times. I T .times. .times. 2 = U t .times. h
.times. .times. U t .times. h * .times. .times. I Nom * + .beta. U
BG - s * - S ( 3 ) ##EQU00001##
an almost complete compensation of the described feedback effect of
the forward voltage of the LEDs can be achieved. Typical values for
.beta.=-0.52 this results in W.sub.3=3.69W.sub.4 with
L.sub.3=L.sub.4=L.sub.min.
[0388] The fifth transistor T5 and the capacitance C2 can be used
to fine-tune the pixel cell Data2 including the feedback. As shown
in FIG. 11, a significant improvement of the current stability is
achieved without complex pre-calculation. The compensation of the
current instability is achieved with few components and without
complex precalculation of the "Data" signal. This allows
temperature fluctuations during operation to be compensated.
Furthermore, a reduction of the quiescent current caused by the
third transistor T3 can be achieved by the additional control input
Data2 via Sel2.
[0389] FIG. 12 shows a fifth embodiment of a .mu.-LED control
device. As in the previous examples, the .mu.-LED can be part of a
display or a module. In addition to the design as shown in FIG. 6,
further changes have been made to the temperature compensation and
influence of the forward voltage through the .mu.-LED.
[0390] The embodiment comprises a third electronic switch T3 with a
first power line contact connected to the second terminal of the
.mu.-LED, and a second power line contact of the third electronic
switch T3 connected to the first control terminal BG of the second
electronic switch T2. The device also includes a fourth electronic
switch T4. A control terminal of the third electronic switch T3 is
connected to a second power line contact of the fourth electronic
switch T4, which are connected in common to the supply potential
AVDD. A control terminal of the fourth electronic switch T4 is also
connected to the supply potential AVDD. Finally, the fourth
electronic switch T4 has its first power line contact connected to
the second power line contact of the third electronic switch
T3.
[0391] A fifth electronic switch T5 is provided to control the
second electronic switch T2 via the first control connection BG.
This is connected in parallel to the .mu.-LED. It is also connected
by its second power line contact to the first power line contact of
the third electronic switch T3. The control terminal of the fifth
electronic switch T5 is electrically connected to a terminal for
supplying a pulse width modulation signal PWM.
[0392] The behaviour and function of the device shown in FIG. 12 is
similar to the device shown in FIG. 11, but unlike FIG. 11, the
gate of the third transistor T3 is electrically connected to a
fixed electrical potential Vdd. As an option, an additional fifth
transistor T5 can be provided for safe switching off the LED
without a cross current from the third transistor T3. A fifth
transistor T5 is not necessary if a cross current from the third
transistor T3 into the .mu.-LED is not a problem. According to the
device presented here, the pulse width modulation PWM is controlled
without a holding capacitor. In this way, a possible pulse width
modulation resolution can be increased with the same cycle time.
Likewise a recharging of a storage capacitor is not necessary,
which increases the switching speed.
[0393] A further aspect concerns in the following a control for a
brightness adjustment or a dimming of pixels, or of the assigned
.mu.-LEDs. Such dimming is not only frequently used in the
automotive sector, for example to switch between day and night
vision, but also in AR applications. Basically, such dimming can be
useful and advantageous when contrasts have to be adjusted or when
external light makes it necessary to control the brightness of a
display in order to avoid dazzling a user or to show information
reliably.
[0394] Conventionally, this problem can be addressed with PWM
control and current dimming, but external parameters of the LED
often change, which requires complex compensation circuits.
Alternatively, so-called 2T1C circuits can be used, to which the
control signal for driver control is fed and stored in a
capacitor.
[0395] The brightness is then adjusted by the voltage applied to
the capacitor. The invention now makes use of an aspect, which
often occurs rather as a parasitic undesired effect, namely the
gate-source capacitance of the driver transistor. This forms a
capacitive voltage divider with the capacitance of the capacitor,
so that the voltage at the gate of the transistor drops. If the
gate-source capacitance is selected appropriately, the brightness
can be adjusted over a wider range.
[0396] In one aspect, a control circuit for adjusting a brightness
of at least one .mu.-LED comprises a current driver element with a
control terminal. This is connected in series with the .mu.-LED and
has its first terminal connected to a first potential. A charge
accumulator is arranged between the control terminal and the first
potential and forms a capacitive voltage divider with a defined
capacity between the control terminal and the first terminal.
[0397] According to the invention, a control element is now
provided which provides a control signal to the control terminal
during an initial period of time, on the basis of which a current
flowing through the at least one .mu.-LED can be adjusted during
the initial, first period of time. During a second time period
following the first time period, the current flowing through the
.mu.-LED is now determined by a reduced control signal resulting
from the control signal during the first time period and the
capacitive voltage divider.
[0398] Thus, when the control signal is selected by the control
element, the brightness of the .mu.-LED can be adjusted so that it
depends either substantially on the current during the first time
period or the current through the LED during the subsequent second
time period.
[0399] In other words, the control signal determines the total
current through the .mu.-LED during the first and second time
periods and, if the control signal is appropriately selected,
depends substantially on the current flowing through the .mu.-LED
during the first time period or on the current flowing through the
.mu.-LED during the second time period.
[0400] Thus the control element is set up to provide a first or a
second control signal during the first time period in order to
operate the .mu.-LED at at least two different brightness levels
during the entire time period. For this purpose, for example, the
second control signal is larger than the first control signal, so
that the reduced control signal derived from the second control
signal is sufficient to drive the current driver and thus provide a
current sufficient to operate the .mu.-LED.
[0401] As mentioned, the current driver element may include a field
effect transistor whose gate forms the control terminal and has a
gate-source capacitance specified by design. Accordingly, during
the second time period, the reduced control signal applied to the
control terminal of the transistor or current driver results from
the control signal during the first time period and the ratio of a
charge storage capacity and the sum of the charge storage capacity
and the defined capacity.
[0402] Such a circuit is operated at a certain frequency, so that
first and second time periods follow each other periodically. This
frequency can be 60 Hz, often also 100 Hz or 120 Hz, or can be in
the range of 60 Hz to 150 Hz. In one aspect, the control element is
configured to make a ratio of the second time span to the first
time span adjustable, whereby the ratio can be in the range from
300:1 to 100:1, in particular in the range from 100:1. For this
purpose, the control element comprises a control transistor at
whose control terminal the first and second time span and thus the
duty cycle can be set by means of a signal.
[0403] A brightness level can now be selected by means of various
control signals during the first time period of a period. For this
purpose, it is provided in one aspect to operate the .mu.-LED at a
first, darker brightness level if a voltage of the first control
signal is within a first voltage interval, and to operate the
.mu.-LED at at least a second, brighter brightness level if a
voltage of the second voltage signal is within a second voltage
interval which is at least partly above the first voltage
interval.
[0404] In this context, the brightness is determined by the current
flowing through the .mu.-LED during the whole time period. With a
control signal that lies within the first voltage interval, the
total current is essentially determined by the current during the
first time period, since due to the capacitive voltage divider and
the associated drop in a voltage of the reduced control signal
during the second time period, the current through the LED during
this time period is very small and not sufficient or relevant for
operation. The current driver is not or only very slightly driven
during this time period, the LED is hardly or not at all lit.
[0405] In contrast, the total current over a period is
substantially determined by the current during the second period if
the control signal during the first period is within the second
voltage interval. In this case, despite the capacitive voltage
divider and the associated drop in a voltage of the reduced control
signal during the second time interval, the current driver is still
sufficiently driven so that a sufficiently high current flows
through the .mu.-LED to operate it. Typical possible values for the
first voltage interval range from 1.3 V to 4.5 V. The second
voltage interval ranges from 4.0 V to 10.0 V.
[0406] A further aspect concerns a method for adjusting a
brightness of at least one .mu.-LED connected to a current driver
element with a control terminal, the first terminal of which is
connected to a first potential and in which a capacitor is
connected between the control terminal and the first potential so
that it forms a capacitive voltage divider with a defined
capacitance between the control terminal and the first terminal. In
the method, a control signal is applied to the control terminal
during a first time period, whereby a current flowing through the
at least one .mu.-LED is adjusted during the first time period.
During the second period following the first period, the control
signal is turned off, whereby the current flowing through the
.mu.-LED is set by a reduced control signal resulting from the
control signal during the first period and the capacitive voltage
divider. "Switching off the control signal" here means
disconnecting the control signal from the control terminal so that
only a reduced signal acts on the control terminal thereafter,
resulting from the control signal during the first time period and
the capacitive voltage divider.
[0407] This reduced control signal is thus smaller than the control
signal by the ratio of the capacitive voltage divider.
Specifically, in one aspect, the reduced signal applied to the
control terminal during the second time period results from the
control signal during the first time period from the ratio of a
capacity of the capacitor and the sum of the capacity of the
capacitor and the defined capacity.
[0408] At this point a further aspect should be mentioned, namely
that a ratio of the second time period to the first time period is
in the range of 300:1 to 100:1, in particular in the range of
100:1. In another aspect, it is proposed to operate the .mu.-LED at
a first, darker brightness level if a voltage of the first control
signal is within a first voltage interval, and to operate the
.mu.-LED at at least a second, brighter brightness level if a
voltage of second voltage signal is within a second voltage
interval that is at least partially above the first voltage
interval.
[0409] In this context, the proposed method determines the
brightness by the current flowing through the .mu.-LED during the
entire time period. For a control signal that is within the first
voltage interval, the total current is essentially determined by
the current during the first time period, since due to the
capacitive voltage divider and the associated drop in voltage
during the second time period, the current through the LED during
this time period is very small. The current driver is not or only
very slightly driven during this time period.
[0410] On the other hand, the total current is essentially
determined by the current during the second time period if the
control signal during the first time period is within the second
voltage interval. In this case, despite the capacitive voltage
divider and the associated drop in a voltage of the control signal
during the second time interval, the current driver is still
sufficiently driven so that a sufficiently high current flows
through the .mu.-LED to operate it. Typical possible values for the
first voltage interval range from 1.3 V to 4.5 V. The second
voltage interval ranges from 4.0 V to 10.0 V.
[0411] The first or second control signal required for control can
be obtained from a digital control word by digital/analogeue
conversion. The digital control word comprises a number of n bits
for this purpose. The least significant m bits (M<n, e.g. m=n-2
bits) correspond to the first control signal, i.e. the most
significant bits are 0. In other words, n bits correspond to the
second control signal. In another aspect, the most significant bits
are used for coarse brightness adjustment, the least significant
bits for more precise range adjustment.
[0412] FIG. 21A shows a control circuit for a lighting unit 1,
which comprises two .mu.-LEDs 4 as illuminates. From the basic
design, the control circuit can be implemented in a 2T1C
architecture as shown here. However, other architectures are also
conceivable.
[0413] Even if two .mu.-LEDs 4 are provided according to the shown
design form in order to ensure redundancy with respect to light
generation, it is generally irrelevant for the realization of the
invention whether one .mu.-LED 4 or a plurality of .mu.-LEDs 4 are
used as illuminates. For example, the light unit 1 or the .mu.-LEDs
4 can be a light unit or LEDs of one color of one pixel.
[0414] In the embodiment shown in FIG. 21A, the two .mu.-LEDs 4
connected in parallel are each supplied with the electrical energy
required to excite a light emission via a current driving
transistor 6. In addition to one transistor 6 for each .mu.-LED, a
common current source can also be provided for both .mu.-LEDs 4.
Current driving transistor 6 is connected in series with .mu.-LED 4
between supply potential terminal 2 and reference potential
terminal 2a. Supply potential connection 2 provides the electrical
energy or voltage required for the operation of lighting unit
1.
[0415] A capacitor, which stores the brightness value, is connected
between the gate of the current-driving transistors 6 and the
reference potential connection 2a. Together with the control
transistor 7 it forms a 2T1C cell. A pulse signal is applied to its
gate, which applies a control signal 8 from the other terminal of
transistor 7 to the control terminal of current driving transistor
6.
[0416] For operation according to the proposed concept in a circuit
according to FIG. 21A, a pulse signal is now applied to the gate of
transistor 7. For example, the duty cycle On/Off can be 200:1, i.e.
at a repetition frequency of 60 Hz the ON pulse duration is approx.
50 .mu.s while the Off pulse duration is approx. 16.6 ms.
[0417] Within a period, the control transistor is now closed via
the pulse signal for a first period (ON pulse duration), and the
control transistor is opened again in a second period (OFF pulse
duration). During the first period, the control signal 8 is thus
applied to the control terminal of the current driver transistor 6
and via the capacitor 3. The control signal controls the current
driver transistor 6 and a current caused by the control signal 8
flows through the .mu.-LED. At the same time, a charge is applied
to the capacitor until the voltage of the control signal is
established across the capacitor (referred to the potential at
terminal 2a).
[0418] After the first time period, control transistor 7 is opened
again. The voltage of control signal 8 is now stored in the
capacitor and should continue to drive the current driver
transistor. In practice, however, this is not the case, since in
the second time period, a capacitive voltage divider is formed,
which consists of the capacitance of the storage capacitor 3 and
the capacitance formed by the gate and source of transistor 7. This
regularly causes the effective voltage 9 on capacitor 3 to be
lowered by a discrete value. The reduced effective voltage 9
results from the voltage of the control signal multiplied by
C1/C1+Cp, where C1 is the capacitor capacitance and Cp is the
gate-source capacitance. Thus, compared to the first time period, a
slightly smaller control signal 9 (or slightly lower voltage) is
applied to the driver transistor 6, so that a current of lower
intensity flows through the .mu.-LEDs 4. The brightness of LEDs 4
thus decreases slightly during the second period of a period.
However, this is not noticed by an observer, since only the average
light output available in relation to the period is decisive for
the perception of brightness.
[0419] Thus, for an entire period, control signal 8 is applied to
the control terminal during the first period and the reduced
control signal 9 during the second period. At a frequency of 60 Hz,
this would be 0.05 ms to 0.06 ms for the first time period and
approximately 16.6 ms for the second time period. In terms of the
average light output of the .mu.-LED, this means that light emitted
by the .mu.-LED during the second time period has a comparatively
high proportion of the average light output of the .mu.-LED during
one period.
[0420] This is equivalent to the average current through the
.mu.-LED. The current flowing through the .mu.-LED during the
second period has a relatively high share of the average current
during the whole period.
[0421] It follows from this that if a low voltage is selected for
control signal 8, the total current flowing through the LEDs 4
during one period, and thus the average light output, is determined
decisively by the strength of the current flowing through the LEDs
4, while control signal 8 is applied during the first period. If a
low voltage value is selected for control signal 8, lighting unit 1
can therefore be operated at a low brightness level and dimmed as
required within this low brightness range.
[0422] If, on the other hand, a high voltage is selected for the
first voltage signal 8, for example 8V, the total current flowing
through the LED during one period is largely determined by the
current during the second period of the period in which the reduced
control signal 9 is applied to the current driver transistor 6. If
a high control signal 8 is selected, i.e. a higher voltage, the
lighting unit 1 is operated at a high brightness level and can be
dimmed as required at this brightness level.
[0423] During the second period of the period in which the reduced
control signal 9 is applied to the lighting unit, a current greater
than 1 .mu.A still flows through the LED in this operating state,
so that particularly effective operation of LEDs 4 is possible.
[0424] FIG. 21B is a supplement to this embodiment where the
proposed circuit is implemented in a backplane substrate. Contact
areas are provided on the backplane substrate to which a .mu.-LED
module is attached. This comprises two .mu.-LED base modules as
disclosed in this application, for example in FIG. 184. The two
contacts 26 are each connected to a current driver transistor 6.
The two outer contacts 25 of the .mu.-LED Module are connected to
the ground or reference potential connection. The current driver
transistor is adequately dimensioned. In some aspects this may be
the dual-gate transistor disclosed here, as described in FIGS. 5 to
12.
[0425] Furthermore, a photonic crystal 32 is incorporated in the
.mu.-LED module. This extends to just above the active layer 20 and
changes the emission properties there, for example in the area
above the active layer, where it can have an emission-promoting
effect.
[0426] FIG. 22 shows a graph showing the strength of the current
flowing through the LEDs 4 as a function of the voltage of control
signal 8 and the reduced control signal 9. It can be clearly seen
that when a control signal 8 with a voltage value of about 1V to 3V
is applied during the first time period, the current flowing
through the .mu.-LEDs 4 is largely determined by the first voltage
signal 8 applied during the first period. Meanwhile, in the second
time period, the control signal 9, which is reduced by the
capacitive voltage divider, and thus the current flowing through
the .mu.-LEDs 4 is almost zero.
[0427] Only from a voltage of the control signal of about 3.0 V
during the first time period does the voltage of the reduced
control signal 9 increase and thus also the strength of the current
flowing through the .mu.-LEDs 4 during the second phase.
[0428] It must be taken into account in each case that due to the
different length of the two phases of a period, namely a short
first phase in which the control signal 8 is applied to the
lighting unit 1, and a long second phase in which the reduced
control signal 9 is applied to the current driver transistor 6, the
influence of the second time period on the average light output of
the .mu.-LEDs 4 is significantly greater. As a result, the total
current through the .mu.-LED increases significantly during a
period when the voltages of control signal 8 exceed 3.0 V. It
follows from this that in the case of a control signal with a
comparatively high voltage greater than 3.0 V or 3.5 V, the
proportion of the total current flowing through the .mu.-LEDs 4
during one period is determined to a large extent by the proportion
of the current during the second time period.
[0429] In addition, FIG. 23 shows a schematic representation of the
time course of the control signals 8, 9 and the resulting light
spot 10 when a control signal 8 is applied with a comparatively
high voltage. The control signal 8, which is transmitted to the
lighting unit, has a voltage of 10 V in the embodiment shown.
Otherwise, the voltage of the reduced control signal 9, which is
applied to the lighting unit during the second phase, is reduced
but still has a voltage that is significantly higher than 0 V. Due
to such a voltage curve of the control signals 8, 9, a bright light
spot 10 is formed, the lighting unit is thus operated at a high
brightness level.
[0430] FIG. 24 illustrates an operating condition in which a
control signal 8 is applied to the lighting unit at a comparatively
low voltage, in this case 2.0 V. The reduced control signal 9 in
this case has a voltage of at least almost 0 V. The brightness of
the light spot 10, which is determined by the average light output
of the lighting unit 10 during a period, is significantly lower
than in the operating state shown in FIG. 23. The lighting unit and
the LEDs used for it are thus operated at a comparatively low
brightness level at which they can be dimmed as required.
[0431] Finally, FIG. 25 shows in a graphical representation how the
electrical energy conducted through the LEDs during a period,
sometimes referred to as the amount of current, behaves in relation
to the voltage signals applied to a lighting unit during the first
and second periods of a period. The x-axis is the voltage during
the first period, the y-axis the current during a period.
[0432] It can be seen that when a control signal with a
comparatively low voltage is applied, especially a voltage of up to
about 3V, the total current flowing through the LEDs is caused by
this control signal. Only when control signals with voltages higher
than 3V are applied does the voltage of the reduced control signal
also increase. Above all, in this operating state, a current flows
through the .mu.-LEDs of the lighting unit which, due to the length
of the second time period, has a considerable influence on the
amount of the total current flowing through the LEDs during the
period and thus on the average light output or brightness of a
lighting unit with at least one .mu.-LED.
[0433] Furthermore, FIG. 25 shows that a lighting unit controlled
in this way can be operated at two different brightness levels
depending on the voltage selected for the control signal. At the
two brightness levels it is in turn possible to continuously vary
the brightness of the lighting unit within a dimming range limited
by a lower and an upper voltage value for the control signal. The
course of the two characteristic curves shown in FIG. 25 can be
adapted to suit requirements with the aid of a suitable circuit
design, in particular by specifically defining the capacitance of
the capacitor and the gate-source capacitance of the transistor
used as the switching element. It is also conceivable to determine
the voltage levels, the control signal and the reduced control
signal by suitable selection and dimensioning of the electronic
components used.
[0434] As the embodiments explained show, the control circuitry
designed in accordance with the invention enables the operation of
a lighting unit, which has at least one .mu.LED, on at least two
brightness levels in a comparatively simple manner. The main
consideration here is that, depending on the level of the voltage
of the control signal, either the current flowing through the LED
during the first time period or the second time period of a period
is decisive for the total current flowing through the LED as well
as for the average light output and the brightness of the .mu.-LED
perceptible by an observer.
[0435] Another aspect deals with the question of how a retroactive
effect on the control of a current source can be reduced when PWM
control is used. In pulse width modulation, the current source is
switched on and off in rapid succession for contrast and brightness
adjustment. The frequency is several 100 kHz up to the MHz range.
With control loops within the current source, the switching
operations lead to spikes or other behaviour, which can bring the
control loop out of its control range.
[0436] FIG. 26 shows a schematic block diagram for a regulated
current source for .mu.-LEDs, which remains stable even during
switching operations. This current source can be used in
.mu.-displays or other display devices and is suitable for
automotive and augmented reality applications.
[0437] The supply circuit includes a reference branch 10, which
provides a reference signal and in particular a reference current
or, if necessary, a reference voltage. In the following, all
further supply currents and, if necessary, also voltages are
derived from the reference signal. Further reference signals can
also be generated from this signal. The reference signal, i.e. the
reference current is characterized by a high temperature stability
but also a stability against process fluctuations during
production. If necessary, it can include one or more correction
circuits, which together provide an accurate and stable reference
signal, for example a reference current.
[0438] In the present case, reference branch 10 is connected to a
reference input 22 of an error correction detector 20 as well as to
a controllable supply source 30. In addition to the reference
input, the error correction detector 20 also comprises an error
signal input 23 and a correction signal output 21. The detector 20
is designed to compare an error signal at input 23 with a reference
signal at input 22 or a signal derived therefrom and to generate a
correction signal at its output 21.
[0439] The controllable supply source 30 has a controllable current
source, which is not shown separately in this block diagram. In
addition, the supply source includes a second backup source 40,
which provides a feedback signal to the error detector in one
operating state of the circuit. A switch device 70 is provided for
this purpose, which, depending on the operating state, i.e. an
operating signal at input 74, either switches the current source to
the load or disconnects it from the load and switches on the
substitute source 40. In this way, either a signal from the current
source to the consumer or the signal from the replacement source is
detected at detector 50.
[0440] A current-voltage converter or a voltage drop detector can
be used for detection. A voltage or a voltage drop or a current can
be detected with detector 50. The detected signal is then fed back
to the error correction detector 20 and compared with the reference
signal or a signal derived from it. The resulting error correction
signal is used to adapt the controllable current source. If load 60
is now supplied by current source 30, error correction detector 20
adjusts the current through the load to a value defined by the
reference signal. With a .mu.-LED, the current flowing through the
diode can thus be precisely adjusted. If the voltage drop across
the load or the current through the load changes due to temperature
effects, the error correction detector readjusts the current
accordingly. This part of the circuit and its operation corresponds
to a control loop.
[0441] If the load were now disconnected from the current, for
example if the LED is switched off in the case of PWM modulation,
the control loop would first attempt to readjust, but then run out
of the control range. For this reason, the invention provides for a
substitute signal to be supplied to the error correction detector
20. This signal is essentially the same or at least very similar to
the nominal signal when the load is switched on. Thus, the error
correction detector 20 is operated in its optimum range regardless
of the operating state of the load and the control loop is not
moved out of its control range. This results in very fast control
and prevents detector 20 from falling outside its control
range.
[0442] The proposed supply circuit thus includes a correction
circuit as part of a control loop for high-precision control of a
current or voltage source as well as a substitute source. The
correction circuit is now fed either a signal derived from the
current or voltage source or the signal of the substitute source.
The supply of the latter enables the current source to be switched
off without the control loop running out of its control range.
[0443] FIG. 27 shows a specific embodiment for driving a power
source for a supply of a Light Emitting Diode 60, which is part of
a pixel matrix not shown here, for example a display, video wall or
other application requiring a high-precision power supply. In the
case of light-emitting diodes, a current through the diode also
changes with changing temperatures, which can lead to a change in
brightness as well as a change in color temperature. This effect is
compensated by regulating the current source. Displays, pixel
matrices for picture or video applications are often operated with
pulse width modulation, in which the light emitting diodes are
switched on and off at high frequencies. The ratio between the two
states gives the brightness of the respective light emitting
diode.
[0444] The power supply circuit shown in the following is
essentially designed in MOS circuit technology. Some field effect
transistors are of the n-type, others of the p-type as shown. In
this case the supply circuit is connected between supply potential
VDD and consumer. By exchanging the channel types of the field
effect transistors and an arrangement between consumer and
reference or ground potential VG an alternative embodiment is
created. It is also possible to replace individual transistors with
bipolar transistors, or to form assemblies such as current mirrors
with them. Bandgap references can be used to generate precise
voltages, which then provide a current via a converter.
[0445] Supply circuit comprises a combined reference branch 10
consisting of two parts 10a and 10b, which provide a reference
current. They form part of a current mirror. The reference branch
10a for a first reference current comprises two transistors
connected in series, an n-field effect transistor 12a and a p-field
effect transistor 11a. The former is connected to a supply
terminal, the latter to the reference potential. The gate of the
transistor 12a is connected to the drain terminal and thus
impresses a constant current. Transistor 11a reflects the current
through the reference branch into the four series-connected
transistors 24, which form the fixed current source for a
differential amplifier. The differential amplifier forms a
component of the error correction detector 20 and contains, in
addition to the current source from the transistors 24, an
inverting and a non-inverting input transistor in each branch,
which is connected to the supply potential VDD via a further
current mirror 26 consisting of two p transistors. The
non-inverting input transistor 27 forms the reference signal input
22, the inverting transistor 28 leads to the error signal input 21.
The two transistors comprise the same dimensions as the transistors
of mirror 26 in this embodiment. However, different amplification
factors may be provided for in versions due to geometric dimensions
such as channel width or length. This may be necessary if, as
described below, there is also an inherent factor between the error
signal and the reference signal. Such an inherent factor results
from the design of the current source and the signals (error signal
and reference signal) tapped for the detector 20 as described
below
[0446] The controllable current source 30 comprises a current
mirror with an output branch and a reference branch, which
simultaneously forms the replacement source 40. The reference
source 10b is connected to a reference branch input 32. This input
32 is also connected to the non-inverting transistor 27 and to the
reference signal input of the error correction detector 20. The
reference branch of the current mirror is thus impressed with an
exact current, whereby a defined voltage drop is fed through the
central tap to input 22 of the error detector. The reference branch
10b comprises two series-connected transistors for adjusting the
current flow through the reference branch of the current mirror of
the current source 30 and for defining the reference voltage or
reference signal at input 22. The gate of transistor 101 is
connected to the gate of transistor 11a (but not drawn here) and is
thus part of the current mirror of reference source 10. The
controllable current source 30 comprises a supply input to which
the supply potential VDD is applied and a p-type current mirror
transistor 34. A capacitor is connected between gate and terminal
32, so that the voltage in the reference branch is coupled to the
gate. This voltage also forms the reference signal for the error
detector.
[0447] The reason for using a capacitor with positive feedback
instead of the usual conduction for current mirrors is, among other
things, due to an additional frequency compensation for the
additional control signal terminal 31, which connects the gate of
transistor 35 with the error correction output 21 of detector 20.
The error correction signal is thus also fed to the gate.
[0448] The gate of the transistor is also connected to the gate of
an output transistor 36 via a switching device 70. This is located
between supply potential VDD and output. The current of the
reference branch is thus mirrored into the output branch 37 of the
current source. By dimensioning the two transistors 34 and 36
accordingly, the ratio of the output current to the current through
the branch with transistor 34 can be adjusted accordingly. If, for
example, the channel width of output transistor 36 is 10 times that
of transistor 34, then the current is also increased by the same
factor in simple approximation. In the illustration in FIG. 27, the
output transistor 36 is a single transistor. However, it can also
be designed as several transistors arranged in parallel.
[0449] The switching device 70 in the current source 30 is
configured to connect, depending on a signal, the gate of the
output transistor 36 either to a fixed potential, here the supply
potential, or to the gate of the current mirror transistor 34. In
the former case, the output transistor 36 is de-energized, since
the potential VDD blocks the gate of the p-type transistor. Since
in this case the transistor does not conduct current, it is also
referred to as transistor 36 is open. In the second case, the
output transistor 36 is closed and the current through the current
mirror transistor 34 is mirrored into the output with the
above-mentioned factor and led to LED 60.
[0450] The output of the current source 30 is connected to the load
60 or the LED as well as to a second switching device 70, which
applies either the voltage at the output of the current source to
the error signal input of the error detector 20, or a substitute
signal. This is provided by the substitute source 40, which is
formed by a p-type output transistor 41 and a transistor 43
connected in series. The series connection of the two transistors
41 and 43 is arranged between supply potential VDD and ground
potential VG. A central node 42 forms the output for the substitute
signal. The gate of transistor 43 is connected to its drain
terminal and thus to node 42. The gate of p-type output transistor
41 is connected to the gate of transistor 34. Thus, a current
mirror is also formed from the transistors 34 and 41. However, a
different factor is selected here by appropriately dimensioning the
output transistor 41 so that the current through this branch is
significantly lower than that through the output branch.
[0451] The two switching devices 70 operate essentially
synchronously and are designed so that the output of the current
source 30 is connected to the error signal input 23 of the detector
20 when the gate of transistor 36 is connected to the gate of
transistor 34. If, on the other hand, the output transistor of the
current mirror is de-energized, the substitute signal of the
substitute source is present at the error signal input, i.e. tap 42
is connected to input 23.
[0452] In the version shown here, the spare source is always
activated, i.e. the output transistor always forms a current mirror
with transistor 34 and a current flows through the branch of the
spare source. In an alternative version, a switch can also be
provided here which works in the opposite direction to the
switching device 70, i.e. it switches the replacement source
currentless, for example, if a voltage is applied to the load or a
current is provided by the current source 30.
[0453] In an operation of the supply circuit, the switching device
70 is now switched in such a way that node 71 is connected to node
72 and simultaneously the gates of the transistors 34 and 36 are
connected to each other. The current source then provides an output
current for the load. This leads via LED 60 to a voltage drop of a
few volts, for example 2 to 3 volts. The voltage drop is detected
as an error signal by the differential amplifier of detector 20 and
compared with the reference signal. If the current through the LED
now changes, for example due to a temperature change, the error
signal also changes and the detector generates a correction signal
for the current mirror at the correction signal output 21 and feeds
this to the control signal connection 31.
[0454] The correction signal is now also applied to the gate of
output transistor 36, so that the current is adjusted accordingly.
The error detector 20 controls the output current mirror so that
the saturation voltage of the inverting and non-inverting
transistors 27 and 28 is equal. A load-independent current source
is formed by means of the error correction detector 20 and the
current mirror connected to the output.
[0455] Since light emitting diodes are often operated with pulse
width modulation, the current through the diode changes in defined
intervals, i.e. the diode is switched on or off at high frequency.
The pulse width results in the brightness of the diode 60, which is
achieved by the switching device 70 in the current mirror. However,
if the current is switched off, the error detector 20 counteracts
this for the first time. This can cause it to run regularly out of
its optimum dynamic range. The same happens when the current is
switched on. Here the differential amplifier needs some time to
reach its normal control range. In addition, oscillations or
overshooting can occur, which reduces the life of the diode, but
can also be visible to a user. The second switching device 70
prevents this by keeping the error detector in its control range by
means of the replacement source.
[0456] FIG. 28 shows a diagram with the main signal flows. With a
switched-off diode, the gate of the p-type field effect transistor
36 of the output branch is directly connected to the supply
potential VDD. The lower switching device 70 connects the tap 42 of
the substitute source 40 to the error signal input 23 of the
detector 20. The substitute source reflects the current with a
lower ratio and the second transistor connected in series is used
for the necessary voltage generation. This is selected so that it
is close to the expected voltage drop of the consumer during normal
operation. This keeps the fault detector within its control range
and the control loop remains in its steady state.
[0457] FIG. 29 shows two principle illustrations of two simple
switch devices. Besides these, other switches can be used. They can
also be easily operated with the PWM signal, which can be used to
adjust the brightness of the LED. In other applications, other
suitable switches are used. The switching device 70 is similar to a
known inverter with the difference that the transistors shown here
are transmission gates. The output 71 is connected to the error
signal input. Input 74 forms the switching input to which the
switching signal, for example, the PWM signal, is fed. Two
transmission gates of different types connected in series are
arranged in series, with output 71 being connected between the two
transmission gates. Gate 73 of the p-type with its terminal 73
forms the connection to the backup source. Terminal 72 of the
second transmission gate forms the connection for the voltage
signal.
[0458] FIG. 30 shows a signal-time diagram for different signals in
the supply circuit in the different operating states. V.sub.FWM
describes the pulse width modulation signal for operating LED 60,
which is also applied to circuit devices 70. It is a logic signal
and changes between two states "High" and "Low". In the High state
from about 8 .mu.s to 18 .mu.s and then between 26 .mu.s and 44
.mu.s the LED is switched on, at the other times it is switched
off. The current through the LED follows these switching times as
can be seen from the lowest curve marked I.sub.LED.
[0459] In contrast, the voltage VLED changes only slightly between
the switched on state and the switched off state. The voltage
decreases continuously and would reach the starting voltage of
approx. 1.4V over time, a current no longer flows, i.e. the LED is
switched off. When the LED is switched on, i.e. at the time of 8
.mu.s, the voltage drop across the LED essentially corresponds to
the substitute voltage or the substitute signal V.sub.H. At the
time of switching on, a small voltage drop can be detected in the
substitute signal, which can be process-related and depends, for
example, on the parameters of the field effect transistors used.
Since different types (p- or n-mos) are used, their switching
behaviour is not always the same, so that residual currents could
still flow during the switchover time.
[0460] V.sub.in shows the signal at the inverting input, i.e. the
error signal input 23. Before the switching time 8 .mu.s, the
voltage V.sub.H is equal to the voltage at the error signal input
because of the position of the switching device 70, after switching
on it corresponds to the voltage V.sub.LED. This is illustrated by
the "=" sign in FIG. 30. V.sub.H is again selected so that it is as
similar as possible to the LED voltage V.sub.LED expected in normal
operation.
[0461] The error correction detector 20 now compares the voltages
V.sub.in at error signal input 23 and Vip at reference input 22 and
generates a correction signal Vo. At switching time 8 .mu.s there
is a small dip of the voltage Vip at the non-inverting input, which
increases a small peak in the correction signal. This may be a
simulation artefact, but can also be caused by a sudden change in
load in the branch of the power source. In any case, the correction
signal is so small and fast that it has no effect.
[0462] The second switching point at 18 .mu.s shows no or if only a
significantly lower behaviour. Nevertheless, the control at the
switch-on time does not significantly affect the output behaviour
of the error detector, but rather provides a precise correction
signal due to the fast feedback, so that the output current and
voltage are quickly adjusted to the desired value and then remain
constant. The simulation of FIG. 30 shows a control of less than
0.5 .mu.s in this context.
[0463] The proposed supply circuit provides a high-precision
current source that is particularly suitable for accurate and
color-true control of light emitting diode applications. The
already known PWM can be used for the contrast adjustment of the
individual light emitting diodes in a pixel matrix, display or
similar. The effects of switching operations during pulse width
modulation on the current source are reduced by the proposed
measures. As a result, even small variations in the operating
current, which are only a few percent above the nominal value of
the input voltage, can be realized without the switching operations
affecting the stability.
[0464] In an implementation, it is possible to build the
transistors of the current source close to each other, so that they
are thermally strongly coupled. For the replacement branch, it
makes sense to equip it with Si-pn diodes or other measures, such
as amplifiers, etc., in order to approximate the replacement signal
to the voltage dropping across the load during operation.
[0465] To control .mu.-LEDs or generally pixels in a display, the
switching ratio can be controlled digitally in addition to setting
the current through the .mu.-LED. A digital driver circuit with low
own power consumption is still able to --despite the low power
consumption, drive a large number of optoelectronic elements and
especially .mu.-LEDs.
[0466] FIG. 13 illustrates a schematic circuit diagram of an
implementation of a 6-T static random access memory cell, SRAM-6-T
memory cell 1, which includes two cross-coupled inverters 2 as a
1-bit memory. The SRAM 6-T memory cell 1 has a compact memory size
in the range of 1.08 .mu.m2 to 1.7 .mu.m2 per bit in 65 nm CMOS
technology and a low power in the range of 0.26 .mu.W to 0.37 .mu.W
per bit.
[0467] FIG. 14 illustrates a schematic circuit diagram of a driver
circuit 10 configured to drive an optoelectronic element, which is
a .mu.-LED 11. The driver circuit 10 is completely digital and is
manufactured using CMOS technology. In this context, FIG. 14 shows
only the circuit diagram. The .mu.-LED 11 is manufactured in a
material system suitable for generating light of the desired
wavelength, the circuit may be manufactured in a different material
system. For the functionality shown, both elements are electrically
contacted. Possibilities for this are disclosed in this
application.
[0468] The driver circuit 10 includes two cross-coupled NOR gates
12, 13 which form a first memory cell or latch used to control the
current through .mu.-LED 11. Driver circuit 10 includes additional
first memory cells not shown in FIG. 14. The additional first
memory cells have the same structure as the first memory cell shown
in FIG. 14 and are used to control the current through additional
.mu.-LEDs.
[0469] Each of the NOR gates 12, 13 has two inputs and one output.
The output of each NOR gate 12, 13 is coupled to one of the inputs
of the other NOR gate 12, 13. The other input of NOR gate 12
receives a set signal S_i and the other input of NOR gate 13
receives a reset signal R_i. The NOR gate 13 generates a signal Q
at its output, which controls the gate of a transistor 14.
[0470] The shown interconnection of the two NOR gates 12 and 13
with their inputs R_i, S_i and the output Q corresponds to an RS
flip-flop. Accordingly, the NOR gates connected in this way can be
replaced in the circuits shown.
[0471] Depending on its gate voltage, transistor 14 switches a
current through .mu.-LED 11 on or off. The current is generated by
a transistor 15. The .mu.-LED 11 and the channels of transistors
14, are connected in series between a supply voltage VDD and ground
GND. The driver circuit 10 also includes two pull-up PMOS
transistors 16, 17 which are coupled to the transistors 18, 19
respectively. The transistors 16, 17 receive a signal non-S_i or a
signal non-R_i at the gate terminals.
[0472] The .mu.-LED 11 is arranged together with other .mu.-LEDs in
a pixel array. Each of the .mu.-LEDs is connected to a driver
circuit as shown in FIG. 14. To enable the selection of a line i,
the transistors 18, 19 are each coupled to the NOR gates 12, 13.
The transistors 18, 19 are controlled by a line selection signal
Line_i at the gate terminals. Pull-down resistors 20, 21 are also
provided to hold back states of the cross-coupled NOR gates 12, 13.
When the set non-signal S_i (active low set) is received by NOR
gate 12, the output of NOR gate 13 is triggered to a high state.
The cross-coupled NOR gates 12, 13 hold the high state until they
are reset to a low state by the non-R_i (active low set) reset
signal received from NOR gate 13.
[0473] FIG. 15 shows a schematic circuit diagram of an
optoelectronic device 30, the optoelectronic device 30 including a
pixel circuit array 31 comprising an array of .mu.-LED driver
circuits 10 as shown in FIG. 14. As an example, the array includes
2K rows and 2K columns. Each driver circuit 10 is connected to a
respective .mu.-LED. In addition, the .mu.-LED array is made of a
different III/IV material chip and each .mu.-LED in the array is
connected to each pixel driver circuit at the drain of transistor
14 in FIG. 14.
[0474] A line decoder and driver 32 selects the lines Line_1 to
Line_2K one after the other. The PWM signals controlling the
current through the .mu.-LEDs are generated by N loadable 8-bit
counters 33, where N is 2K for this example. The N counters 33
generate the set signals S_i and the reset signals R_i (or
alternatively the signals non-S_i and non-R_i) for N columns of
pixels simultaneously per selected row. When pixel pulse width
values, i.e., 8-bit pixel gray data, are loaded into counter 33,
the set signals S_i are activated to turn on the pixel stream, and
the counters 33 start with a pixel clock frequency of, for example,
between 40 MHz to 100 MHz. When counter 33 reaches the pixel data
values, the reset signals R_i are activated to turn off the pixel
stream.
[0475] There is also a 9-bit (MSB) counter 34, which generates the
global or common dimming for the pixel array. The 9-bit pixel
dimming data loaded into counter 34 thus determines the brightness
of the background of the pixel array. If the dimming pulse width is
zero, a line scan is performed so that the pixels in the lines
light up. Otherwise, global pixel illumination is performed first,
followed by line-by-line scanning. The set signals S_i and reset
signals R_i generated by counter 33 and the global or common
dimming signals generated by counter 34 are fed to N buffers and
multiplexers 35, which pass the signals to the columns of the pixel
circuit array 31.
[0476] The global dimming data can also be combined with the
greyscale data in the video/image signal processor IC or through
the .mu.-LED driver IC, so that no separate global dimming pulse is
required and then only the greyscale data is updated line by line.
The counters 33, 34 are controlled by a signal Load Counter.
Furthermore, the counters 33 receive a clock signal clk. The
counter 34 receives a clock signal clk-MSB.
[0477] To get rid of dark pixels, the driver circuit can include a
second memory cell or latch for each .mu.-LED. FIG. 16 illustrates
a schematic diagram of a driver circuit 40 design based on driver
circuit 10 as shown in FIG. 14. Driver circuit 40 includes a first
memory cell 41 and a second memory cell 42. Both the first memory
cell 41 and the second memory cell 42 have a set input S, a reset
input R and an output Q. Furthermore, the reset input R of the
first memory cell 41 is connected to the set input S of the second
memory cell 42. The outputs Q of the first and second memory cells
41, 42 are connected to inputs of an AND gate 43. The output of AND
gate 43 is connected to the gate of transistor 14.
[0478] As can be seen in the function time diagram shown in FIG.
16, a global reset is performed at the beginning of each frame so
that all pixels are dark. Then a global set signal S_d is applied
to the set inputs S of the second memory cells 42 to make all
pixels "normal pixels". Then the second memory cells 42 of the
pixel circuit array are loaded or reset row by row to implement
selective dark pixels. An implementation of the optoelectronic
device includes a spatial averaging pixel bias current. The
optoelectronic device includes a global N-bit digital-to-analogue
converter, DAC, covering a pixel current range of, for example, 22
nA to 1 .mu.A. As illustrated in FIG. 17, identical peripheral bias
currents are summed to produce a spatial average bias.
[0479] Turning the pixel stream on and off is controlled by the
state of the second memory cell or the latch for dark pixels and
the PWM signal for normal active pixels. FIG. 18 illustrates a
function-time diagram of the optoelectronic device. Line 1 of the
function timing diagram shows the duration of a frame. During the
frame, the display shows a content such as a video sequence.
[0480] At the beginning of the frame a global reset is performed so
that all pixels of the display are dark (see line 2). Then dark
pixels are loaded line by line so that these pixels are permanently
dark during this frame (see lines 3 to 4). Then a global dimming is
applied to ensure that the background has the same brightness (see
line 5). Then grayscale data is loaded to generate the PWM signals
starting at line_1 and ending at line_2K (see lines 6 to 7).
Finally line 8 shows when the pixels are switched on. After the
frame is finished, the next frame starts.
[0481] FIG. 19 illustrates a schematic circuit diagram of another
version of a driver circuit 50 configured to drive the .mu.-LED 11.
Driver circuit 50 is completely digital and requires even less
space than driver circuit 10 shown in FIG. 14.
[0482] In the driver circuit 50, the first memory cell includes an
NMOS transistor 51 and a PMOS transistor 52 connected in series
between the supply voltage VDD and ground GND, which means that the
channels of the two transistors 51, 52 are connected in series.
Additionally, an input of an inverter 53 is connected between the
transistors 51 and 52. The output of inverter 53 is connected to
the gates of transistors 51, 52.
[0483] Furthermore, an NMOS transistor 54 and a PMOS transistor 55
are connected in series between the supply voltage VDD and ground
GND. The transistors 54, 55 receive a set signal S1 or a reset
signal non-R1 at their gate terminals. To remove dark pixels, the
driver circuit 50 includes a second memory cell or latch that has
the same structure as the first memory cell and is also illustrated
in FIG. 19. The second memory cell includes an NMOS transistor 56
and a PMOS transistor 57 connected in series, an inverter 58 and an
NMOS transistor 59 and a PMOS transistor 60 connected in
series.
[0484] The transistors 59, 60 receive a set signal S2 or a reset
signal non-R2 at their gate terminals. The output of inverter 53 of
the first memory cell generates a signal Q1 and the output of
inverter 58 of the second memory cell generates a signal Q2. The
signals Q1 and Q2 are fed into the inputs of a NAND gate 61. An
inverter 62 is located downstream of the NAND gate 61, and the
output of inverter 62 is coupled to the gate of transistor 14,
which switches the current through .mu.-LED 11 on and off depending
on its gate voltage.
[0485] The function timing diagram of FIG. 19 shown above makes it
clear that a global reset is performed first by applying the reset
signal non-R1 to the first memory cell. Then the reset signal S1 is
applied to trigger the first memory cell at output Q1 to the high
state. The first memory cell holds the high state until it is reset
to the low state by the reset signal non-R1. A lower function
timing diagram of FIG. 19 shows the function of the second memory
cell during the loading of dark pixels. First a global set signal
is applied by signals S2. Then dark pixels are loaded line by line
by the non-R2 reset signal.
[0486] FIG. 20 illustrates a schematic circuit diagram of another
embodiment of a driver circuit 70, which is a variation of the
driver circuit 50 shown in FIG. 19. The driver circuit 70 contains
the same first and second memory cells as the driver circuit 50,
but the driver circuit 70 does not contain a NAND gate for
combining the output signals of the first and second memory cells.
Instead, driver circuit 70 includes an additional NMOS transistor
71 connected in series with transistor 54. In particular,
transistor 71 is located between transistor 54 and ground GND. The
gate of transistor 71 is controlled by the output signal Q2 of the
second memory cell.
[0487] FIG. 31 illustrates a version of an analogeue ramp for
current control in the form of a 2500 control circuit that includes
a pixel driver with a small footprint. It is built in a
semiconductor material and uses various techniques described here.
Such a concept is based on a analogeue ramp for lighting control
and is particularly space-saving and shows a hysteresis during
operation, which reduces noise and makes double buffering possible.
Double buffering allows longer operating cycles, which reduces the
total power consumption. This aspect can be advantageous,
especially when combined with other power saving functions.
[0488] The control circuit features a pixel driver as a combination
of a 2530 pulse generator with a column data buffer as input stage.
A common ramp generator 2502, which can also be used for several
pixels 2506, e.g. a row or column, is part of the control circuit
in this version. The control circuit is coupled with its output
2521 to a control input of an adjustable current source of a
.mu.-LED pixel. The current source can be selectively enabled and
disabled based on a pulse signal DW applied to the control input of
the adjustable current source. In response to the pulse signal DW
the .mu.-LED is switched on or off. In an alternative embodiment,
the power source can be replaced by a switch or similar element to
ensure that the .mu.-LED is selectively switched on or off. The
pulse length of signal DW corresponds to the brightness of the
.mu.-LED element of the pixel.
[0489] The control circuit 2500 comprises a line selection input
2503 for the line selection signal RS and a column data input 2504
for the data signal AV. These inputs are similar to the
conventional approach and in fact, they can be used in a similar
way. The control circuit also comprises a trigger input 2501 for a
trigger or "ramp start" signal RaS and a ramp signal input 2505 for
a ramp signal.
[0490] Similar to the conventional cell as shown in FIG. 55, the
column data input is connected via a switch 2510 to a capacitor
2509 to store data information corresponding to the brightness of
the .mu.-LED inside the capacitor 2509.
[0491] Switch 2510 is implemented as described here as a field
effect transistor in Si technology or also in Ga or In technology.
The gate or control input of switch 2510 is connected to the line
selection input to receive the line selection signal RS. However,
while the conventional approach uses the charge stored in the
capacitor to control the current directly through the light
emitting device, capacitor 2509 is used together with switch 2510
as an input buffer. The output 2511 of the input buffer and in
particular the capacitor and switch are connected to the pulse
generator 2530 to generate a pulse.
[0492] Pulse generator 2530 comprises a comparator 2508, which for
example contains a differential amplifier and an output buffer
stage 2507 implemented as an RS flip-flop, whose behaviour can be
expressed with NOR and NAND gates. The differential amplifier is
implemented in the same technology as switch 2510. For this
purpose, it may include transistors as described in this
application. The inverting input 2511 of the comparator is
connected to capacitor 2509, the non-inverting input 2512 is
connected to the ramp input signal 2505. Comparator 2508 can be
selectively switched off to reduce power consumption as explained
in detail later.
[0493] Comparator 2508 provides a status signal or comparison
result CS at its output. The output of the comparator is directly
connected to the reset input R of the RS flip-flop 2507. The set
input S is connected to the trigger input 2501.
[0494] The operation of the control circuit is explained in more
detail with reference to the various signals illustrated over time
in FIG. 32. It is assumed that the line selection signal RS is
applied and a constant charge is applied to capacitor 2509. A
constant signal IS is applied to the non-inverting input of the
comparator (corresponding to reference 2512). Signal IS corresponds
to the brightness of the .mu.-LED associated with the control
circuit.
[0495] At time T1, the trigger signal RaS changes from a low level
LOW to a high level HIGH and subsequently the set input S of the RS
flip-flop 2507 also goes to HIGH. At time T3, the trigger signal
RaS will change back to the LOW level. The ramp signal Rsig is
applied at the same time T1. Ramp signal Rsig increases linearly
over the time the trigger is HIGH. This means that ramp signal Rsig
starts from a first value corresponding to LOW and rises to a
second level, i.e. the HIGH level. Ramp signal Rsig is also applied
to the non-inverting input of the comparator. During the time
period from T1 to T2, the comparator compares the signal IS
buffered in capacitor 2509 with ramp signal Rsig. As long as the
signal at the non-inverting input is lower than the inverting
input, the output signal applied to the reset input R of the RS
flip-flop remains LOW. At time T2, the reset input R receives the
rising edge of the result signal CS when the output of the
comparator changes from LOW to HIGH. At this time, the ramp signal
becomes higher than the buffered signal IS.
[0496] As a result of this transition, output Q of the RS flip-flop
resets the control signal DW for the current source to LOW value
from time T2. It can thus be seen that the time T2 at which the
output signal DW switches off the current source again depends on
the charge stored in capacitor 2509, provided that a uniformly
rising ramp Rsig is assumed. The ramp signal RSig and the signal IS
thus define a pulse whose length essentially corresponds to the
time period from T0 to T2.
[0497] At time T3, the trigger signal changes from HIGH to "LOW".
At the same time, the ramp signal is switched off, causing the
comparator to output a "LOW" signal. Therefore, both signals at the
R and S input will change to LOW. Due to a small hysteresis in the
comparator, the transition for the trigger signal at input S will
be a little faster, causing the flip-flop to keep the output signal
DW LOW, regardless of the transition of signal CS at input R. At
time T5, trigger signal RaS is repeated at input S. Likewise, the
ramp signal Rsig starts again at its start value.
[0498] The period between time T3 to T5 is the blanking time used
to reprogram the corresponding columns in each row. For this
purpose, the row selection signal is triggered at time T7, which
connects the column data line to the capacitor via switch 2510.
Capacitor 2509 is then charged or discharged to a new value. In
this example, capacitor 2509 is discharged to a much smaller value
that corresponds to a different (lower) brightness. The recharging
is initiated at time T7 and ends at time T4, when the line
selection signal RS goes LOW again, opening the switch. Another row
can be addressed and reprogrammed during the cycle for the present
row at time T5.
[0499] Because of the lower level for signal IS, the comparator
2508 now changes its output much earlier at time T6 in the new
cycle. Consequently, output Q falls to "LOW" at time T6, which is
much shorter than for the previous period of the trigger signal
RaS. Output Q with its control signal DW controls the current
through the .mu.-LED coupled to it. The longer the output signal DW
remains at HIGH, the longer a current flow through the .mu.-LED,
resulting in a high brightness for the corresponding color.
Comparator 2508 and may be the RS-Flip-Flop can be switched off
during reprogramming and blanking time to reduce power consumption.
For this purpose, at least the comparator comprises a 2520 power
control unit connected to the trigger input. As long as the trigger
signal is Rsig HIGH, the comparator 2508 is powered to perform its
operation. During the sampling period, it is switched off in
response to the trigger signal.
[0500] Since in some examples the sampling time can be
significantly longer than the current time for the trigger signal,
the whole pulse generator can be switched off.
[0501] In an alternative embodiment, reference is again made to
time T2 in FIG. 32. The comparator switches its output signal CS
from LOW to HIGH as soon as the ramp signal reaches the threshold
of the buffered signal IS. Trigger signal S is still HIGH, which
causes the RS flip-flop to switch the output signal LOW. As you can
see, output Q remains LOW regardless of the level at the reset
input R. Therefore, the comparator could be switched off after a
reset because of the transition of the signal at input R. In some
variants, the power control unit 2520 can be coupled to output Q to
control the power supply to the comparator based on the state of
output Q.
[0502] Segmentation and additional ramps can be used if different
lines are addressed. This would allow implementing spatial-temporal
multiplexing, which reduces the generation of current peaks and
leads to less varying power consumption. While in the present
example signals have been applied to specific inputs on the
comparator, the skilled person can see that the design of this
principle can be changed. For example, inverting and non-inverting
inputs can be exchanged, resulting in inverse behaviour. The RS
flip-flop requires two transistors and resistors, which implements
a small asymmetry during the design in the RS flip-flop (e.g. by
adjusting the value of one resistor), adjusts the switching
behaviour and will prevent undefined states.
[0503] With some .mu.-displays, individual pixel errors may occur,
which damage the .mu.-LEDs. Such errors cannot be avoided. However,
a repair with the size of a .mu.-display is only possible with a
very large effort. Therefore, it is suggested to design not only
subpixels redundantly, i.e. to provide more than one subpixel of
the same color, but to provide redundant .mu.-LED branches with
selection fuse. These redundant pixels can also be connected to the
same power source. In a test, the functionality of each .mu.-LED is
now checked. If the test results in two functional .mu.-LEDs, one
of them can be specifically deactivated to compensate for color
changes or loss of brightness of the other .mu.-LED due to the
different current flow. If, on the other hand, a fault is detected,
the redundant .mu.-LED continues to be used.
[0504] FIG. 33 shows an embodiment of a proposed device that
provides such redundancy with simultaneous selection protection.
The illustration shows two pixel cells each with a first and second
branch, each having a .mu.-LED D1a and D1b, respectively. The
.mu.-LED D1a and D1b are connected to a common reference potential
connection GND. Their other terminals are each connected to an
electronic fuse Fa and Fb. These are, for example, a fuse, which
melts when the current through the fuse becomes large enough. The
second branch, i.e. the branch with the fuse Fb and the .mu.-LED
D1b also shows an imprinting component EPT. This is designed as a
MSOFET transistor and its drain terminal is connected between the
fuse and the .mu.-LED. Its source contact is connected to the
common reference potential, the gate can be supplied with the
selection signal Vburn via the imprinting signal line EPT. In
principle, lines or alternatively columns can be addressed,
controlled or selected via the imprinting signal line EP, depending
on the wiring.
[0505] The pixel cell also includes a 2T1C circuit with a current
driving transistor T1. This transistor is connected to the supply
potential on the one hand and to the first and second branch and
its fuses Fa and Fb on the other hand. A charge storage C is
electrically connected to the gate of the first transistor T1 and
to the source terminal of the first transistor T1. Furthermore, the
"t1C cell also comprises a transistor T2 which is connected between
the data terminal Vdata and the gate of the transistor T1. The
selection signal can be fed to its gate.
[0506] For each color of a pixel two .mu.-LEDs D1a and D1b, each
electrically connected in series to an electrical fuse Fa and Fb,
can be provided. In this way, redundancy is created for each pixel
in all sub-pixels.
[0507] In the case where .mu.-LEDs are electrically connected along
a row and along a column to a common imprinting signal line EP,
each pixel cell of a column, for example, can be electrically
connected and addressable to the supply potential terminal VDD by
means of a common supply line to a switching transistor arranged on
a common carrier outside the active display. Fuses of a column can
thus be triggered or made to melt.
[0508] In the following, the mode of operation of this circuit is
explained in more detail.
[0509] In the first case, one of the two .mu.-LEDs is defective in
such a way that it is "OPEN", i.e. there is no current flow through
the defective .mu.-LED. Then the test gives a corresponding result
and the respective other .mu.-LED is automatically used. On the
other hand, a "SHORT", i.e. a short circuit, can also be present.
If this short-circuit occurs, the resistance through the
short-circuited diode is very low, so that the current through the
respective fuse is significantly higher. This also cuts the fuse in
a SHORT.
[0510] A third case concerns the situation that both .mu.-LEDs
function as expected. In this case, the current of the power source
is split between both branches, which can lead to a color
error.
[0511] The dominant wavelength depends on the selected current.
Therefore, in such a case, the signal Vburn (high potential, e.g.
VDD) is applied so that the imprinting component EPT becomes
conductive. If transistor T1 is simultaneously fully switched
through by a corresponding signal on the data and selection line, a
high potential is thus applied to the fuse. The resulting high
current flow destroys fuse Fb, so that diode D1b is safely
disconnected.
[0512] When designed in PMOS technology, the potentials and signals
exchange their polarity accordingly.
[0513] The fuse can be designed as a metal strip with different
widths. For example, one length can be 33 [.mu.m], a width at one
longitudinal end 20 [.mu.m], at the other longitudinal end 9
[.mu.m] and in a 12 [.mu.m] long central area 2 [.mu.m]. The
longitudinal ends can be square or rectangular and have passages.
The square longitudinal end can be in the direction of the
transistor T1 and the rectangular longitudinal end can be in the
direction of a light emitting diode. A material can be IGZO, for
example.
[0514] Instead of the above-mentioned metal strips, a thin-film
transistor can also be used, especially in diode interconnection,
in which the gate and source are electrically connected
permanently. Each .mu.-LED can be equipped with its own thin-film
transistor. This can act as both a controllable current source and
an electrical fuse. By means of a signal, the thin-film transistor
can be pulled to zero potential, for example, so that it burns
through as a result of the increased current flow and the .mu.-LED
is switched off. In principle, all known types of electrical fuse
can be used. Activation or release does not have to destroy the
fuse, but in any case, it must safely disconnect the assigned
.mu.-LED from the power supply.
[0515] In this way, an end-of-line test can be carried out without
additional process steps such as laser cutting or similar. A
combination with embossing diodes as embossing components is also
possible.
[0516] FIG. 33 shows on the right side a neighboring cell of a
first pixel cell. For each line a selection signal line Vsel, an
imprint signal line EP and a data signal line Vdata can be
connected. With Vsel and Vdata the selection signal line generates
a signal for selecting the relevant line to activate the associated
fuses. The imprinting signal line EP provides a fusing voltage
V_burn for generating a fusing current I_burn.
[0517] FIG. 34 shows a second embodiment of a proposed device in
which the arrangement between the current source and .mu.-LEDs is
reversed. While FIG. 33 shows a configuration with a common
cathode, FIG. 34 shows a common anode configuration with the
.mu.-LEDs.
[0518] The anode connections of the .mu.-LEDs D1a and D1b are
connected to the supply potential connection VDD. A first current
line contact of a first transistor T1 is connected to the reference
potential terminal GND. The drain terminal of the first transistor
T1 is connected to the common terminal of the electrical fuses Fa
and Fb. The selector holding circuit comprises a charge storage C
connected to the control contact of the first transistor T1 and to
a source terminal of the first transistor T1.
[0519] The function of this arrangement is similar, but the
transistor EPT is connected between the fuse Fb and .mu.-LED D1b
and the supply potential. A voltage V_burn can be applied to the
gate of the imprinting transistor EPT via an imprinting signal line
EP, thus causing the electrical fuse Fb, which is a fuse, to
melt.
[0520] FIG. 35 shows a third embodiment of a device with redundant
branches of .mu.-LEDs, which can be selected by means of selection
fuses. In contrast to the embodiment in FIG. 35, the series
connection of fuse and .mu.-LED is swapped in each branch. Thus,
the fuse is directly connected to the supply potential terminal,
the .mu.-LED of each branch is connected on the cathode side to a
common base point and to the current driving transistor T1.
Furthermore, the imprinting transistor EPT is connected with its
drain terminal between fuse Fb and .mu.-LED Dib. Its source
terminal also leads the current driving transistor T1 to the common
base point for the .mu.-LEDs. The 2T1C cell is constructed in the
same way as in the previous figure. To melt the fuse, the diode D1b
is bridged with the imprinting transistor EPT and the signal Vburn,
so that a high current melting the fuse flows through the fuse
Fb.
[0521] As the LEDs are not connected together to the potential
connections for VDD or GND, no common electrode of the .mu.-LEDs
can be realized, i.e. one electrode for several pixels. This
arrangement is suitable, for example, if no common electrode is
required for process technology.
[0522] FIG. 36 shows a slight modification of the embodiment
according to FIG. 33, where the transistors are PMOS (especially
transistor T1) and the charge storage is connected between the gate
and the fixed supply potential. The advantage of this embodiment is
that the voltage across the charge storage is independent, unlike
the R design in FIG. 33, in which the voltage across charge storage
C can vary slightly due to the forward voltage or changes in it due
to temperature fluctuations. The same advantage of independence
from temperature variations is also shown in the design of FIG.
34.
[0523] FIG. 37 shows another alternative version of the embodiment
shown in FIG. 36. The imprinting component here is an imprinting
diode EPD with one terminal connected to a second terminal of the
.mu.-LED D1b, to which the imprinting diode EPD is assigned, and
the other terminal connected to an imprinting signal line EP, by
means of which addressing can be performed. As shown in FIG. 37, a
first terminal of the imprinting diode EPD is connected between
fuse Fb and .mu.-LED D1b and a second terminal of the imprinting
diode EPD is connected to the imprinting signal line EP. The
melting voltage V_burn is also applied to the latter, with which
the electrical fuse melts.
[0524] During operation, a selection of an electrical fuse Fb to be
triggered is made by switching through the first transistor T1.
This is done by programming a voltage on the charge storage C via
the data line Data and the selection line Sel. The VDD connection
is connected to 0 volts or a negative voltage, in contrast to
normal operation. A voltage V_burn is then applied to the imprint
signal line EP, which is more positive than the voltage at VDD. In
this way, a high current IF or I_burn flows via the imprinting
diode EPD via the electrical fuse Fb and the first transistor T1,
which is switched on, whereby the fuse Fb is triggered in the
selected pixel cell. The fuse Fb melts and the corresponding LED
D1b is switched off. In addition, the potential at the first
potential connection GND should ideally also be greater than 0
Volt, for example equal to the melting voltage V_burn, so that no
large current flows via LED D1b or D1a and can damage them.
[0525] According to this embodiment, the current (IF, I_burn)
required to trip the electrical fuse Fb flows in the opposite
direction to that which would flow in "normal operation". After
this procedure in an EOL test, no additional process steps, such as
laser cutting or similar, are required.
[0526] FIG. 38 shows a modification of the embodiment according to
FIG. 37, in which the imprinting diode was only turned upside down.
It is now connected on the anode side between fuse Fb and .mu.-LED
D1b of the second branch. The arrangement according to FIG. 38 is
created using PMOS thin-film transistors as current driver
transistor T1 and a common cathode arrangement for the .mu.-LEDs.
All imprint signal lines EP of a line of a display are connected
together here. The electrical fuse Fb to be triggered is selected
by switching through the first transistor T1. For this purpose, the
charge storage C is set to o V or another voltage so that T1
becomes conductive. A voltage of 10 V or another positive voltage
is applied to the VDD connection. The voltage V_burn, which is
applied to the imprint signal line EP, is here more negative than
the voltage at the supply potential connection VDD and is 0 Volt,
for example. In this way, a high current I_burn flows through the
imprinting diode EPD, through the electrical fuse Fb and the
conducting first transistor T1, whereby the fuse Fb in the selected
pixel cell is triggered and thus melted.
[0527] Meanwhile, the potential at the first potential connection
GND should ideally be just as high as the potential at the second
potential connection VDD, so that the LEDs D1a and D1b are switched
in reverse direction and so that no high current flows over the LED
D1b or D1a and can damage them despite the first transistor T1
being conductive. According to this embodiment, the current (IF)
I_burn required to trigger fuse Fd flows in the same direction as
it would in "normal operation" of the arrangement.
[0528] FIG. 39 shows an embodiment of a method for the electronic
configuration of a plurality of .mu.-LEDs. In a first step S1 the
.mu.-LEDs of the first branch and the second branch are tested for
their functionality. This results in several possibilities, of
which the following is probably the most common. In this case, both
.mu.-LEDs function as expected. If this is the case, in a second
step S2 an imprint signal is applied to the electronic imprinting
component. A current is then provided by the current driver or
current source, which flows through the now conductive current
imprinting element. The current is selected so that the .mu.-LEDs
are not damaged, but the fuse of the respective branch is
destroyed. This deactivates the respective branch. In case of a
fault, however, only one of the two branches is still functional.
The other is either "OPEN", i.e. no current flows over the faulty
branch, or "SHORT", i.e. a short circuit is present. In the latter
case, the increased current and the low resistance in this branch
can destroy the fuse in the faulty branch, so that the fuse in the
faulty branch changes from SHORT and OPEN and does not affect the
function of the whole arrangement any more.
[0529] With the method described above, the imprinting signal line
can be designed as a global line, i.e. one connected to all pixels.
Addressing is done via the supply line via transistor circuits on a
panel outside an active display, as well as via the selection lines
and appropriate programming of the charge accumulators of the 2T1C
cells.
[0530] This results in a reduced wiring effort. Likewise, a
reduction of the necessary layers can be achieved, which can lead
to a reduction in costs. However, the switching transistors must be
designed in such a way that they can carry the current of a column.
Furthermore, there is an increased power dissipation in the panel
or in the common carrier during this process.
[0531] The described circuit design with two fuses can be used for
a variety of .mu.-LED embodiments.
[0532] FIG. 40 shows an embodiment of the circuit based on the
proposed concept combined with a slot antenna arrangement disclosed
in this application. The slot antenna has a stack of semiconductor
layers with a lower contact area 1005 and an upper contact area
1011. The upper contact area 1011 of each slot antenna is connected
to a common ground potential terminal GND within the substrate 1007
via a transparent cover electrode 1002. The substrate 1007 also
houses the other circuit elements for driving and testing the slot
antenna. The contact areas 1005 of both slot antennas are now
connected to a fuse F.sub.a or F.sub.b. Between the fuse F.sub.b
and the contact area 1005 of the right slot antenna there is also a
tap, which leads to the imprint diode EPD and the imprint signal
line EP.
[0533] The respective other terminals of the fuses F.sub.a and F
are connected to the output of the current driver transistor T1.
Together with the selection transistor T2 and the capacitor located
between the supply potential V.sub.DD and the control terminal of
the current driver transistor T1, the current driver transistor T1
forms a 2T1C cell to supply the two slot antennas. In one aspect,
the current driver transistor T1 is the dual-gate transistor
disclosed in this application.
[0534] As explained in the previous examples, a test step evaluates
whether the two slot antennas are functional. If this is the case,
the fuse Fb is destroyed by the isolating element EPD and thus the
right slot antenna is cut off from the power supply. If one of the
two slot antennas is defective, the power supply for the remaining
slot antenna is provided by the 2T1C cell.
[0535] FIG. 41 shows a similar embodiment, in which .mu.-LEDs are
provided in the form of horizontally aligned microrods. These are
connected with their respective contact 2 to a contact area 3 on
the substrate not shown here. The contact area 3 is in turn
connected to the common reference potential GND. The rear contact
connection of each microrod is connected to the respective fuse
F.sub.a or Fb. .mu.-LED D1b is the redundant diode and its rear
contact terminal is connected to the imprinting transistor EPT. To
melt the fuse Fb, the imprinting signal Vburn is applied to the
control connection, whereby a high current flows through the fuse
from the current driver transistor T1 to the imprinting signal line
EP. In this way, the horizontally oriented microrod Dib is
disconnected from the power supply. In the event of a
production-related failure of the microrod D1a due to either a
short-circuit or a disconnection, the 2T1C cell with its current
driver transistor T1 supplies the microrod Dib.
[0536] FIG. 42 shows a further embodiment, in which a series of
basic modules 5 are provided. Two first adjacent contacts each are
connected to the supply potential connection V.sub.DD via
respective fuses F.sub.a and F.sub.b. Between the fuse F.sub.b and
the respective contact, an imprint transistor is connected, which
is connected to the output of the current driver transistor T1 of a
respective current source. In this version, a pair of base modules
is connected to a common current source comprising a 2T1C cell
consisting of the current driver transistor T1, the selection
transistor T2 and a capacitor. In the event of a positive test of
the two base modules, a switching signal is applied via the control
terminal of the imprinting transistor EPT and thus a fusing current
is switched via transistor T1 to the respective fuse F.sub.b.
[0537] Small-scale display arrangements with a high resolution are
particularly desirable for AR systems, such as head-up displays or
glasses with a light field display that projects a raster image
directly onto the retina. For .mu.-displays with pixel-sized light
sources, so-called .mu.-displays in matrix form based on GaN or
InGaN are proposed, among others.
[0538] FIG. 43 shows a display device comprising an IC substrate
component and a monolithic pixelated optochip mounted thereon as a
first embodiment of a cross-sectional view. An IC substrate
component 1 is shown with monolithic integrated circuits 2.1, 2.2,
2.3 and with IC substrate contacts 3.1, 3.2, 3.3 controlled by
them. The IC substrate component 1 can comprise further components
for control, power supply and for signal exchange with peripheral
devices, whereby an interface 23 is sketched as an example. In this
context, reference is made to further different versions in this
application, which describe the digital and analogeue circuit
components in more detail. FIGS. 49A to 49C, 50A and 50B with their
corresponding descriptions are given as examples.
[0539] The IC substrate contacts 3.1, 3.2, 3.3. are metallic and
are each separated by an insulating layer. A monolithic pixelated
optochip 4 is arranged on the IC substrate component 1 and
electrically and mechanically connected to the IC substrate
contacts 3.1, 3.2, 3.3. To be more precise, contacts 22.1m 22.2 and
22.3 are inserted on the surface of the pixelated optochip 4 in
such a way that they are opposite the IC substrate contacts 3.1,
3.2, 3.3 when they are positioned exactly on the IC. As shown, the
contacts have the same size in each case, so that even a slight
offset as shown has no negative effects and a short circuit is
avoided. Various techniques for such a connection are disclosed in
this application.
[0540] The monolithic pixelated optochip 4 comprises a
semiconductor layer sequence 5 with a first semiconductor layer 6
with p-doping and a second semiconductor layer 7 with n-doping,
wherein the first semiconductor layer 6 and second semiconductor
layer 7 are applied over a large area and extend in the lateral
direction perpendicular to the stacking direction 8 substantially
over the entire monolithic pixelated optochip 4. Embodiments of the
semiconductor layers 6, 7 with several individual layers of
different doping levels or made of different semiconductor
materials are not shown in detail. Between the first semiconductor
layer 6 and the second semiconductor layer 7 there is an active
layer, not shown in detail, with quantum wells in the area of which
an active zone 24 emitting electromagnetic radiation forms when a
current flows through the semiconductor layer sequence 5 in stack
direction 8.
[0541] On the front side 17 above the semiconductor layer sequence
5, a transparent contact layer 16, for example of indium tin oxide
(ITO), is applied flat. In order to achieve a .mu.-LED 9 with a
small pixel size P, in the present embodiment of 2 .mu.m to 5 .mu.m
diagonal size, the first light source contact 10.1, 10.2, 10.3 on
the underside of the first semiconductor layer 6 facing the IC
substrate component 1 is considerably smaller than the pixel size
P. For the embodiment, a maximum diagonal MD of the first light
source contact 10.1, 10.2, 10.3 of 300 nm is selected so that the
feature is fulfilled according to which the projection area 13 of
the first light source contact 10.1, 10.2, 10.3 on the .mu.-LED
back side 12 corresponds at most to half the area of the .mu.-LED
back side 12. For the present embodiment, the projection surface 13
comprises a diagonal of 4 .mu.m and covers approximately 5% of the
area of the .mu.-LED rear surface 12. This results in a laterally
limited current path 25 within the .mu.-LED 9 between the first
light source contact 10.2 and the second light source contact 11
formed by a section of the transparent contact layer 16, which
leads to a laterally limited active zone 24. Additionally,
non-radiative recombination at the edges of the active zone 24 are
suppressed. To improve the lateral confinement of the current path
25, the doping of the first semiconductor layer 6 and the second
semiconductor layer 7 is preferably selected such that they have a
p or n conductivity of less than 10.sup.4 Sm.sup.-1, preferably
less than 3*10.sup.3 Sm.sup.-1, more preferably less than 10.sup.3
Sm.sup.-1. In addition, it is advantageous to select a small layer
thickness SD of the first semiconductor layer 6. It is preferred
that the layer thickness SD of the first semiconductor layer 6 in
stack direction 8 is at most ten times and preferably at most five
times the maximum diagonal MD of the first light source contact
10.1, 10.2, 10.3 in lateral direction.
[0542] According to the invention, the first light source contact
10.2 is surrounded in a lateral direction perpendicular to the
stacking direction 8 by a rear absorber 15.1, 15.2 with an optical
blocking effect, the rear absorber 15.1, 15.2 preferably consisting
of silicon, germanium or gallium arsenide and/or having a graphene
or soot particle intercalation. From the light path 26 shown in
FIG. 44 for the first embodiment, it can be seen that this measure
reduces crosstalk from a driven .mu.-LED 9 into adjacent
pixels.
[0543] For the second embodiment shown in FIG. 45, the same
reference characters are used for the components that are identical
to the first embodiment. Shown are three-dimensional structures on
the top of the second semiconductor layer 7, which improve the
light extraction to the front side 17. It can be seen that the
degree of total reflections is reduced and the output coupling cone
is enlarged. For a design alternative not shown in detail, 17
Fresnel lens structures are provided on the front side. In another
alternative, photonic crystal structures are arranged on the
surface. In some alternatives, structures are arranged above the
.mu.-LEDs and partly extend into the active layer. Such a
combination is also possible to create a constriction and
localization of the recombination zone.
[0544] FIG. 46 shows a third embodiment with a rear absorber 15.2,
15.2, which comprises sections 27.1, 27.2 projecting into the
semiconductor layer sequence 5, which additionally shield the
boundary area between adjacent .mu.-LEDs 9. Structured elements of
reflective materials such as aluminium, gold or silver or of
dielectric materials whose refractive index is lower than that of
the first semiconductor layers 6, 7 can be used for the subsections
27.1, 27.2. For further embodiment, subsections 27.1, 27.2
additionally improve the lateral limitation of the current
path.
[0545] The fourth embodiment shown in FIG. 47 further reduces the
optical crosstalk between adjacent .mu.-LEDs 9 by a frontal
absorber 21.1, 21.2, 21.3, 21.4, which laterally surrounds the
second light source contacts 11.1, 11.2, 11.3. If the frontal
absorber 21.1, 21.2, 21.3, 21.4 is electrically insulating, the
lateral restriction of the current path for the localization of the
active zone 24 can additionally be improved.
[0546] For the embodiments shown in the figures, an optochip
contact element 22.1, 22.2, 22.3 is arranged between the first
light source contact 10.1, 10.2, 10.3 and the respectively assigned
IC substrate contact 3.1, 3.2, 3.3. The cross-sectional area of the
optochip contact element 22.1, 22.2, 22.3 is larger than that of
the first light source contact 10.1, 10.2, 10.3, so that the
monolithic pixelated optochip 4 can be contacted in a simplified
manner on the IC substrate component 1.
[0547] FIG. 48A shows an alternative embodiment, which is basically
based on the previous example in FIG. 43. However, additional
measures have been taken to reduce the current and prevent optical
and electrical crosstalk. The embodiment is similar to that of FIG.
133 in this respect. In particular, a trench 20 was created between
the middle and right .mu.-LED after the application of layers 6 and
the active layer, which comprises an optically reflective but also
insulating material (at least on the trench wall). The latter to
avoid a short circuit between the pixels, the former to avoid
optical crosstalk. Between the left and the middle pixel a larger
trench is created, which essentially extends through layers 6 and
7. It forms not only an optical barrier, but also an electrical
barrier between the pixels or .mu.-LEDs. Further aspects of this
embodiment can be found in relation to FIGS. 131 to 137 and other
places in this application.
[0548] FIG. 48B shows a further embodiment based on the previous
examples. Identical elements carry the same reference points. In
this embodiment a doping 32 is introduced in layer 6 between the
individual .mu.-LEDs. The doping changes the band structure in this
area and leads to an increase of the band gap. Injected charge
carriers thereby experience a field and are kept away from this
area. Together with the light source contact 10.2, effective
localization is thus also achieved in the area of the recombination
zone shown in FIG. 48B.
[0549] Another aspect is the photonic structure 32 on the surface
of layer 16, where a transparent material 31a with a high
refractive index (e.g. Nb.sub.2O.sub.5) is directly applied as a
column or pillar over a recombination zone. Light generated in zone
24 is bundled by the column as waveguide and thus directed. Another
column of the same material 31b is located in this configuration
between two adjacent pixels. Between them, a transparent material
with a lower refractive index is filled in. This result in a
refractive index variation in lateral alignment similar to the
structures described above. The periodic variation of the
refractive index leads to an optical band gap. The size and shape
of this band gap depends, among other things, on the periodicity,
so this diagram is only an example, other periodicities are also
conceivable. Such a combination of the different techniques results
in a strong localization on the one hand and a good directional
radiation on the other hand. Crosstalk is prevented. The IC
structure and the comparatively large contacts also improve the
alignment and fastening of the two layer structures.
[0550] FIG. 49A illustrates a general overview of digital and
analogeue concepts of the three essential parts of a .mu.-LED
display array with its main functionality. Sections I and II
concern analogeue sections of the .mu.-display with a plurality of
pixels arranged in rows and columns. Each pixel 141 can either
consist of subpixels with different colors. Alternatively, displays
with pixels of similar size can be used to obtain the different
colors. The .mu.-LED display is implemented in this embodiment as a
monolithic display comprising a first substrate carrier on or in
which the .mu.-LED pixels are integrated. However, other designs,
in particular, the designs disclosed here, are also conceivable,
including the antenna slot structure and the realization of the
.mu.-LED in bar form or in modules.
[0551] In some cases, the first substrate carrier also includes the
circuit for the analogue section II. In an alternative, the
substrate of the .mu.-LED is thinner and comprises a large number
of contacts on its underside. The contacts on the underside are
then bonded or otherwise attached to a carrier that includes the
analogue section II. Alternatively, the analogue section II can be
grown on a thinned substrate that also carries the .mu.-LED pixels
on the other side. Such an approach can reduce misalignment between
the analogue section and the .mu.-LED pixels. On the other hand, a
material system is required that is suitable for integrating an
analogue circuit.
[0552] The analogue section II of the arrangement contains the
control for the current through the respective pixels. For this
purpose, each pixel 141 with its anode contact is brought into
contact with a common source potential 1411. The respective cathode
of the .mu.-LED pixels is connected to an adjustable driver, which
in this case is implemented as current source 142, which is
integrated in section II and in turn connected to terminal 1412. In
this design, a common anode contact is thus realized. Cover
electrodes as disclosed in this application may provide such a
function. However, the other case of a common cathode also exists.
In this case, the .mu.-LED is located between cathode potential
terminal 1412 and the current source. The advantage of such an
arrangement is that the supply voltage can be somewhat lower and
the .mu.-LED does not have to process a large input voltage.
[0553] Section II also includes a reference current source 1410,
for example a temperature-stabilized current mirror or the like, to
supply the same reference current to the respective current sources
142. While only one current source is shown in this example,
multiple reference current sources may be used to provide a
respective reference current for different pixels. For example,
each pixel line can be assigned to a reference current source. If
such reference current sources are switchable, the current sources
for each row can be switched on or off periodically, thus reducing
power consumption. In terms of embodiments, Section II is
manufactured in polysilicon, which thus comprises a different
material system than that used for the realization of the .mu.-LEDs
in Section I.
[0554] In addition to the reference current supplied to each of the
current sources 142, the current sources also include a switch
input to work selectively with each current source and then
separately with each pixel. Switching the current sources using PWM
techniques to adjust the brightness of each pixel, as explained,
further reduces overall power consumption. The PWM signal is
generated in digital section III of the array.
[0555] The digital section III comprises a clock input CLK and a
data input DAT. The data input DAT is coupled to 12-bit shift
registers 148, which are connected in series. The shift register
receives the incoming data stream and delivers a corresponding word
to a 12-bit memory 147 for storage. The 12-bit memory may comprise
flip-flops or a similar circuit to store the 12-bit words in
memory. The memories are coupled to the other input of each
comparator 144. In this way, a data stream can be used to store
temporarily a whole series of brightness values in the flip-flops
of memory 147.
[0556] The clock signal at input CLK defines the clock for a
counter 149 that supplies a 12-bit counter word D0 . . . 11.
Counter word D0 . . . 11 is applied to the respective comparators
144, which are connected to the current sources 142 of each
.mu.-LED pixel. In an alternative embodiment, other components can
also be used if necessary, for example, a combination of different
gates, which check whether counter word D0 . . . 11 is smaller than
the word of the memory connected to it.
[0557] When operating such an arrangement, comparator 144 compares
counter word D0 . . . 11 with the memory word, i.e. the contents of
the 12 bit memory. Depending on the result, for example whether the
comparison with the comparator indicates whether the counter word
D0 . . . 11 is larger or smaller than the memory word, the current
source is switched on or off. In other words, the comparison with
the comparator results in a pulse width based on the clock signal
in counter 149 to operate each pixel. For example, the first pixel
in the displayed chain should have a dark value, i.e. be switched
off, the second pixel should have a light value or be completely
switched on. The data stream then has the following relevant string
of zeros and ones in two words, strung together in the form of
"0000000000111111111111". After the words are stored in one of the
two memories 147 each, they are sent in inverted form to the
comparator 144. The comparison is made in the comparator. As long
as the counter word D is smaller than the memory word M, the driver
remains switched on (in the example with the inverting comparator,
"111111111111" and "000000000000" are thus compared with the
counter word).
[0558] The .mu.-LED display array contains different parts that
have different requirements and limitations, making it difficult to
implement in a single semiconductor material. Nevertheless, the
main challenge is the size predefined by the pixel size of the
.mu.-LED. Transistors or other active elements in the analogue or
digital part face this limitation, which excludes certain
implementations.
[0559] FIG. 49B shows another version of the three sections of a
.mu.-LED display arrangement with its main functionality. While the
first section is essentially the same as the corresponding section
I of FIG. 49A, section II is slightly different. Section II now
includes a DEMUX Demultiplexer, which switches between the
different pixels using a higher clocked sync signal Sync. The
frequency of this signal Sync has a higher frequency than the
refresh rate and depends on the number of signals O1 to O3
generated by the DEMUX demultiplexer. In one configuration, the
demultiplexer controls all pixels of a row or a column. In an
alternative configuration, a demultiplexer can be used for each
subpixel of a pixel. Combinations of these are also possible. This
allows the number of necessary contact areas between section II and
section III to be reduced.
[0560] Section III again comprises a multiplexer between the
outputs of the respective comparators Comp. D>M and the
demultiplexer of the second section II. The synchronization signal
Sync is the same as for the demultiplexer in section II and is
generated together. Another change compared to the execution of
FIG. 49A is that the counter word (D0 . . . 11) determining the PWM
modulation for the individual comparators is fed directly to the
comparators individually and not jointly. In contrast to the
embodiment of FIG. 49A, the implementation of a multiplexer and
demultiplexer has the advantage that the number of interconnects,
i.e. the number of connections between the purely digital section
III and section II can be reduced. In contrast, an additional
higher-frequency synchronization signal must be routed between
Sections III and II via one of these interfaces.
[0561] FIG. 49C shows a functional circuit diagram of a version of
a known comparator, as it can be used in parts in principle in the
embodiment of FIGS. 49A and 49B. The circuit represents a 2 BIT
comparator, but can be extended to several bits. In practical
implementation, the inverting inputs can also be omitted. Since
there is also a comparison with the counter word, it is sufficient
to implement the circuit part A>B or A<B.
[0562] FIG. 49D shows a time diagram for the various counter words
1D to 3D and the memory registers as they are used to generate the
output signal. The counter words D0 . . . 11 are time-shifted so
that each time word starts when the previous one has passed
through. With the comparator or an OR function the output signal O1
to O3 is generated, which is then fed to the multiplexer.
[0563] The .mu.-display arrangement comprises various parts with
different requirements and limitations, making it difficult to
implement in a single semiconductor material. A challenge also lies
in the available space, which is essentially determined by the
pixel size of the .mu.-LED. Transistors or other active elements in
the analogue and digital part are subject to this limitation, which
excludes certain implementations.
[0564] FIG. 50A shows an exemplary sectional view of a .mu.-display
to illustrate different aspects of contacting and wiring of the
individual sections. Similar to FIG. 49A or 49B, the .mu.-display
comprises a .mu.-LED section I, an analogue section II and a
digital section III. The .mu.-LED portion is based on GaN, InGaP or
another semiconductor material capable of emitting light of blue,
red or green color. The .mu.-LED section I comprises the common
cathode or anode (+) contact layer 1411 extending on the upper
surface and connecting each of the active regions of the .mu.-LED
pixel 141. Not shown is an additional out-coupling or light-shaping
structure on the surface of layer 1411, which may include photonic
structures, converters or the like.
[0565] The pixels are arranged in a substrate and are optically and
electrically separated from each other so that their emission does
not disturb neighbouring .mu.-LED pixels and the pixels can be
controlled separately. For example, .mu.-LED pixel 141 can be
implemented using the current limiting doping described above. In
this case, the current flow is limited to a smaller area by doping.
The doping changes the band gap so that the charge carriers are
effectively limited. Examples of such limitations or other
structural measures to improve quantum efficiency and/or radiation
characteristics are disclosed in the other sections. The pixels may
also contain LED nanorods arranged in a slotted antenna structure,
as also described above. Also bars or the other .mu.-LED structures
disclosed in this application are conceivable.
[0566] The underside comprises in some areas an insulating material
to prevent leakage current. The surface is shaped in such a way
that area II is aligned so that the elements are mainly below the
respective pixel element. Each .mu.-LED pixel includes a contact
area facing area II, which forms the connection to area II of the
.mu.-LED display.
[0567] The analogue section II of the .mu.-display of FIG. 50A can
be implemented from or based on the same semiconductor material
system. For example, active and passive components used for the
power sources can be implemented in GaN InGaP or InAlP systems,
provided that space requirements can be met. In such cases, the
forming of the components can be achieved using several
conventional deposition techniques. This has the advantage that
contacts of the .mu.-LED pixels in the interface of section I can
easily be aligned with the traces within section II. Stress and
strain due to different temperature coefficients can also be
minimized. Alternatively, section II is formed with a different
semiconductor material. For example, polycrystalline silicon or
amorphous silicon structures are suitable and are understood to
form small components. Both sections can be formed, aligned and
joined separately.
[0568] Due to the size requirements, the alignment must be very
precise, as the size of the contacts of the .mu.-LED can only be in
the range of a few nm.sup.2. As a further alternative, polysilicon
material can be deposited on the lower surface layer by various
growth processes to form subsequently the required circuit
components. To reduce the voltage, one or more sacrificial layers
can also be implemented. Furthermore, the polysilicon layer can be
formed first, and then the .mu.-LED pixels can be formed using the
desired material system. In the present example, different material
systems are used for area II and I, but the expansion and other
parameters are adapted so that a joint production is possible.
[0569] For this purpose, section II is manufactured with
polycrystalline silicon. Polycrystalline silicon or amorphous
silicon structures are well understood to form particularly small
dimensioned components. For this purpose, polysilicon material is
applied to a suitable carrier and the necessary components are
formed in it. To reduce the thermal expansion, several intermediate
or sacrificial layers are provided, which do not take over any
further function, but adapt the thermal parameters by the different
crystal structure. Such layers are also located between area II and
area I. There a change of the material system to the material
system intended for .mu.-LED pixel production takes place. Then the
.mu.-LED pixels are formed.
[0570] Alternatively, all sections can be formed separately,
aligned and then bonded together. Due to the size requirements, the
alignment must be very precise, because the size of contacts of the
.mu.-LED may only be in the range of a few .mu.m.sup.2.
[0571] Depending on complexity, area II as illustrated in FIG. 50A
by elements 151 and interconnection layers 152 contains one or more
transistors that are part of a power source or switch.
Interconnection layers 152, arranged in several layers of Section
II, connect the contacts on the surface of Section II to the
various components in Section II. For example, contact 165s of
transistor 152 is connected to the top contact via an
interconnection layer and to the corresponding .mu.-LED. Similarly,
gate contact 169, which controls transistor switching or resistor
behaviour, is coupled to contact interface 153 on the bottom
surface of the portion adjacent to Digital Section III.
[0572] Digital section III is based on silicon and comprises some
digital circuits 170. It is normally formed separately and then
electrically connected to the analogue section II by a bonding
process. Forming the digital and analogue sections separately
allows for optimized manufacturing techniques and testing of the
analogue and .mu.-LED sections prior to bonding to the digital
section. Similar to the analogue section, the digital section III
contains some interconnections for digital and analogue signals.
Power may also be provided via the digital section III.
[0573] The small space available may require different setups and
implementations. One aspect is the integration of transistors
within the analogue section to form the power source and control
circuitry. FIG. 51 and FIG. 52 illustrate various examples of the
implementation of field effect transistors with small space
requirements in the semiconductor material.
[0574] FIG. 51 illustrates an inverted stacked transistor formed
with amorphous silicon. The transistor has an insulating gate layer
155 formed of SiN over gate contact 156. The gate contact 156 is
shaped by a small bump so that the gate layer 155 follows the bump,
which has a central region 157 and two sloping sidewalls 158. A
layer of amorphous silicon 154 is formed over the gate layer, thus
also forming a central area and two sloped sides. The surface of
the amorphous layer 154 can be highly n-doped to form a highly
n-doped layer of amorphous silicon 151 with high conductivity.
Alternatively, the highly n-doped layer 151 is deposited on layer
154.
[0575] Finally, a metal layer is applied to the n-doped layer 151,
which also extends to the side edges of the silicon layer 154 and
SiN layer 155. A gap in the metal layer and the layer 151 divides
the structure and thus forms a source and a drain contact. In
particular, metal layer 152 forms a drain contact, while metal
layer 153 forms the source contact of the field effect transistor.
The conductive channel is then formed in the polysilicon layer in
the central region between source and drain. The highly n-doped
polysilicon layer 151 provides a good electrical connection to the
channel in layer 154. This structure allows the gate to be
contacted from a side other than source and drain, taking up very
little space.
[0576] FIG. 52 shows two examples of space-saving polysilicon
transistors. The transistors are formed on a glass carrier with a
grown SiO.sub.2 layer as base substrate. Each transistor comprises
two highly n-doped polysilicon regions 165s and 165d, separated by
an undoped polysilicon layer 170, which is located between the
regions 165s and 165d. Adjacent to the drain region is a lightly
doped drain region 166, which is located between the polysilicon
region 170 and the drain region 165d.
[0577] Alternatively, a gold-doped region 167 is formed between
polysilicon 170 and drain region 165d. The source 165s, drain 165d
and undoped areas 170 are then completely covered by a SiO.sub.2
layer, which extends on the sidewalls of the areas 165s and 165d
respectively. Holes are etched over areas 165s and 165d to gain
access to the source and drain areas. The holes are filled with a
metal, for example Al, to create electrical contacts. The contact
also runs over the sidewall of the SiO.sub.2 layer, creating a
larger area for contacting. In the center above the polysilicon
layer 170 a gate is formed by applying an aluminum layer 169 on top
of the insulating SiO.sub.2 layer. Gate 169 is electrically
insulated from the metal contacts for source and drain.
[0578] The limited space available may also require new concepts
for the implementation of control circuits. In conventional
circuits for controlling LED displays, the pixels are arranged in
addressable rows and columns. Each pixel consists of one LED of a
certain color or alternatively of a triplet of three different
LEDs. In the latter case, a pixel can also be referred to as a
pixel containing three sub-pixels, each of which comprises an LED
of a particular color.
[0579] Referring again to the example of FIG. 49A or 49B, FIG. 50B
shows various designs for connecting .mu.-LED structures to digital
circuit sections. The two sections can be based on different
material systems or technologies. The upper first section comprises
the .mu.-LED elements or pixels or subpixels arranged in rows and
columns. Depending on the desired color, different material systems
and technologies are used, for example the materials InGaN and
InGaAlP. In a first example, the wafer or .mu.-LED structure is
connected to a wafer based on crystalline silicon using a W2 W
(wafer to wafer) process, which includes the digital circuit
section and any necessary analogue sections. In the example of FIG.
49B, section I is realized by the upper wafer, the lower wafer
comprises sections II and III. In the second example of FIG. 50B,
thin film layers of polycrystalline silicon are deposited on the
bottom of the first wafer with the first section at low
temperatures. This section either provides pure interconnects to
connect to the digital section III or additionally houses driver
circuits or other components to drive the .mu.-LEDs. In these two
examples, the wafers are interconnected together to produce the
desired display or matrix. The third example shows an alternative
embodiment, in which individual chips are provided with digital
circuits and are operatively connected to section II. The chips
include, for example, rows and column drivers for driving parts of
the display.
[0580] FIG. 54 shows an embodiment described in more detail below.
In this way, individual parts of the display can be controlled
separately. In addition, this type of separation during production
allows individual faulty circuits to be sorted out without having
to replace the entire wafer in case of a fault in one element of
the digital circuit in section III.
[0581] The limited space under the analogue sections makes new
concepts for the required implementation of digital control
concepts. In conventional circuits for controlling LED displays,
the pixels are arranged in addressable rows and columns. The same
principle can also be applied in the present case. Each pixel has
one LED of a certain color or alternatively a triplet of three
different LEDs. In the latter case, it can also be called a pixel
if it contains three subpixels, each of which comprises a .mu.-LED
of a certain color.
[0582] FIG. 53 shows a diagram with the elements required to
address a conventional LED display. For simplicity, only one color
type is shown, although each pixel contains three LEDs with
different colors. The pixels are arranged in addressable columns
and rows. The display comprises an 1800 pixel matrix with 1920
pixels per row and 1020 lines. The pixel matrix was constructed in
a monolithic way. The display has several line drivers 1802 and
several column drivers 1803 to address each pixel in the pixel
matrix individually. Both driver types can be integrated into the
matrix or provided as external components coupled to the matrix via
an interface. A combination is also possible.
[0583] Each of the line drivers 1812 has an individual driver
device that is coupled to a corresponding line 1805a, 1805b and
drives the current through it. Likewise, each column driver has a
driver element 1813, each driver element being connected to a data
line 1804a, 1804b. Pixel drivers 1801 are located at the
intersections of the rows and columns. The pixel driver 1801 is
connected to the rows and columns and drives the corresponding
pixel.
[0584] The display includes some control and address signals from
external components, two of which are specially marked here, namely
DATA and SYNC. The latter signal SYNC is used to synchronize the
row and column drivers with each other to avoid artefacts and
ensure clean programming. By addressing a corresponding row, the
pixels connected to the corresponding row are selected. The DATA
signal is then applied to the appropriate columns to program each
of the pixel drivers 1801 in the selected rows.
[0585] In the case of a display with a large number of pixels, the
clocking for conventional display programming can lead to high
frequencies for the programming signal. For example, in the display
of FIG. 53, the frequency for the programming frequency per bit and
row may be several MHz depending on the color depth of each
subpixel in the range. For example, with a brightness depth of 10
bits, corresponding to 1024 different illumination values, the
programming frequency for 1080 display lines and a frame rate of 60
Hz is about 66 MHz.
[0586] The table below shows the frequency of the programming
signal and the programming time per bit and row in .mu.s. With
increasing color or illumination depth, the PWM time units for
programming and therefore the programming frequency increases.
TABLE-US-00002 Programming Programming frequency Color bits PWM
units time in .mu.s (MHz) 8 255 0.06 17 10 1023 0.02 66 12 4096
0.00 265 14 16383 0.00 1062
[0587] The very short programming time, especially with high color
or illumination bits (i.e. 12 bits or 14 bits), leads to a high
load on the corresponding line and column drivers. In the extreme
case of a change from white to black or vice versa of a single
pixel, the column driver must reprogram (reload) the pixel in a few
ns. For comparison, ultramodern DDR4 rams run at an internal
frequency of about 800 MHz to 1.5 GHz, i.e. in the range of the
programming frequency of 14 bit illumination depth.
[0588] In order to reduce the programming frequency, the rising and
falling clock edge can be used for programming, as similarly in
memories. It is also possible to segment the display and divide the
display matrix into different segments. Depending on the production
technology, segmentation allows individual segments to be tested
separately, so that they can be replaced in the event of
errors.
[0589] FIG. 54 shows an example where a display of 1920.times.1080
pixels is segmented into a 2.times.2 matrix with sub-displays. Each
subdisplay 1800a to 1800d contains a pixel matrix of 960.times.540
pixels. Similar to the display in FIG. 53, each subdisplay
comprises its own column and row drivers 1802a, up to 1802d, and
1803a to 1803d. DATA and SYNC signals are also supplied to the
respective segments. The smaller number of lines reduces the
programming frequency accordingly. Further segmentation of the
columns as shown in FIG. 54 will also reduce the demand on the
column drivers and the load with each programming cycle is reduced.
The following table shows an example of programming time and
programming frequency for 108 display rows per segment (there are
10 such segments in total, again with a refresh rate of 60 Hz.
TABLE-US-00003 Programming Programming frequency Color bits PWM
units time in .mu.s (Mhz) 8 255 0.61 1.7 10 1023 0.15 6.6 12 4096
0.04 26.5 14 16383 0.01 106
[0590] As shown, the reduced number of lines due to segmentation
reduces the requirements for programming time and programming
frequency by roughly the factor of segmentation. Each of the
segments is implemented in a similar way. Each pixel matrix 1800,
1800a to 1800d contains lines and rows on which the pixel drivers
and light emitting devices are arranged.
[0591] FIG. 55 shows an example of a conventional pixel driver such
as a 2T1C structure in which the current through the LED is
controlled by a charge programmed during the blanking time of the
display. The driver is located at the interface of a line 1805 and
a data line 1804. Furthermore, a supply line 2002, which provides a
supply voltage V.sub.DD and a current I.sub.DAC, is coupled to the
light emitting device 2004 via a driver transistor 2003.
[0592] The driver transistor 2003 thus operates as a controllable
current source. The current through the driver transistor 2003 is
controlled by the 1T1C structure 2002. In particular, a field
effect transistor M2 has its gate connected to the line selection
line for programming and acts as a switch.
[0593] When activated by a "HIGH" signal on the line selection
line, transistor M1 closes and data line 1804 charges capacitor C1
to the desired level. During this programming, the power supply
line may be switched off that the light emitting device is
basically off. This will prevent various artefacts during
programming. After reprogramming, transistor M2 is open again and
the charge stored in the capacitor drives current transistor M1 so
that a current flows through the light emitting device. The current
corresponds to the stored charge and thus to the desired lighting
level.
[0594] FIG. 56 shows the schematic for a conventional column or
data driver. The driver comprises a digital section and an analogue
section to drive the corresponding data lines. Alternatively, the
output can control dedicated drivers for the data lines. Apart from
power supply connections in GND, VDD and VSS, further control
signals CLK and DIR are provided. Digital values R, G and B for the
different colors are stored in a buffer. They are forwarded and
processed by a level shifter and then fed to a digital-analogue
converter. The DAC can also correct some values by using a
separately generated correction signal Vg-cor. After conversion to
analogue signals, they are stored in an output buffer and then
applied to an output buffer. The analogue rgb signals are then
applied to the data lines. Although only 3 data output lines are
shown here, the column data driver provides signals for all data
lines in the display matrix.
[0595] FIG. 57 shows an example of a conventional line driver. The
driver comprises a shift register that receives the CLK and DIR
signals and is coupled to a large number of logical AND gates via a
level shifter. The gates also receive an ENABLE signal, to which
the corresponding outputs in the output buffer go HIGH. During
operation, the shift register shifts the bits with each CLK signal
to apply selectively a HIGH signal to one of the corresponding
gates. The ENABLE signal is required to activate globally line
selection during reprogramming.
[0596] FIG. 348 generally shows a possible embodiment of a
semiconductor layer stack. This comprises an n-doped layer 3, which
is epitaxially deposited on a substrate not shown. The n-doped
layer 3 is followed by the active region. This contains a
multiquantum well structure with the quantum well layers 3.1 and
3.2. The multiquantum well structure can have a plurality of such
successive layers, which are also formed with different material
systems. Adjacent to this is the p-doped layer 2, followed by a
current widening layer 1.
[0597] In the following, various devices and arrangements as well
as methods for manufacturing, processing and operating as items are
again listed as an example. The following items present different
aspects and implementations of the proposed principles and
concepts, which can be combined in various ways. Such combinations
are not limited to those listed below:
[0598] 836. Device for electronic control of a .mu.-LED pixel cell,
in particular created with NMOS technology, comprising [0599] a
data signal line, a threshold line and a selection signal line;
[0600] a .mu.-LED electrically connected in series to a dual-gate
transistor and together with it between a first and second
potential terminal; [0601] wherein the dual-gate transistor is
arranged with its current conduction contacts between a terminal of
the .mu.-LED and a potential terminal, and a first control gate of
the dual-gate transistor is connected to the threshold line; [0602]
a selection hold circuit comprising a capacitor coupled to a second
control gate of the dual-gate transistor and to a current
conduction contact of the dual-gate transistor, and a control
transistor having its control terminal connected to the selection
signal line.
[0603] 837. Device according to item 836, the dual-gate transistor
comprising a backgate transistor in which the backgate forms the
first control gate.
[0604] 838. Device according to item 836 or 837, where the first
control gate of the dual-gate transistor is configured to set a
threshold voltage.
[0605] 839. Device according to any of the preceding items, in
which the dual-gate transistor comprises a thin-film transistor
with two opposite control gates.
[0606] 840. Device according to any of the preceding items, which
is configured in such a way that a switching signal (PWM signal) is
applied to the threshold line during operation.
[0607] 841. Device according to any of the preceding items, in
which a first terminal of the .mu.-LED is connected to the first
potential terminal; and in which the dual-gate transistor is
arranged with its current conducting contacts between a second
terminal of the .mu.-LED and the second potential terminal; and the
capacitor is connected to the second control gate of the dual-gate
transistor and to the second terminal of the optoelectronic
component.
[0608] 842. Device according to any of the preceding items, in
which the first terminal of the .mu.-LED is connected to a second
current line contact of the dual-gate transistor and its second
terminal is connected to the second potential terminal;
[0609] the dual-gate transistor with its current line contacts is
located between a first terminal of the .mu.-LED and the first
potential terminal;
[0610] the capacitor is connected to the second control gate of the
dual-gate transistor as well as to the first potential
terminal.
[0611] 843. Device according to any of the preceding items, in
which [0612] the first terminal of the .mu.-LED is connected to the
first potential terminal; [0613] the dual-gate transistor with its
current line contacts is arranged between a second terminal of the
.mu.-LED and the second potential terminal; [0614] the capacitor is
connected to the second control gate of the dual-gate transistor
and to the second potential terminal.
[0615] 844. Device according to any of the preceding items, in
which the selection hold circuit comprises another control
transistor.
[0616] 845. Device according to item 844, in which the further
control transistor is connected in parallel with the .mu.-LED and
its control terminal is connected to the selection signal line.
[0617] 846. Device according to item 845, in the case of the
dual-gate transistor is configured as a transistor with one gate
providing the second control gate.
[0618] 847. Device according to any of the preceding items, in
which the charge storage is connected to the second control gate of
the dual-gate transistor and to the first potential terminal, and
further comprising:
[0619] a temperature compensation circuit with a negative feedback
based on the detection of a forward voltage by the .mu.-LED, the
temperature compensation circuit being configured on the output
side to output a signal on the threshold line.
[0620] 848. Device according to item 847, in which the temperature
compensation circuit comprises a control path arranged in parallel
with the dual-gate transistor and having two paths connected in
series.
[0621] 849. Device according to item 847, in which the threshold
line is connected from a node between the two controlled paths
provided by a third control transistor and a fourth control
transistor, to the first control gate of the dual-gate
transistor.
[0622] 850. Device according to item 849, in which the control
terminal of the fourth control transistor is connected to the
second potential terminal.
[0623] 851. Device according to any of items 847 to 850, in which
the temperature compensation circuit comprises a second charge
storage device connected to a control terminal of a control
transistor providing one of the two paths and to the first
potential terminal.
[0624] 852. Device according to item 851, in which a second data
signal line for programming a negative feedback factor is provided,
which is coupled to the second charge storage and the third control
transistor.
[0625] 853. Device according to item 852, in which the coupling is
established via a fifth control transistor controlled by a second
selection signal line.
[0626] 854. Device according to any of the items 847 to 850, in
which the temperature compensation circuit is connected to the
second potential terminal via its third control transistor
[0627] 855. Device according to any of the items, in which a fifth
control transistor is connected in parallel to the .mu.-LED, on
which a switching signal (PWM signal) is applied to its control
terminal during operation.
[0628] 856. Device according to any of the preceding items, in
which the transistors are field-effect transistors using NMOS
technology.
[0629] 857. Method of operating a device according to any of the
preceding items, wherein an analogue data drive signal for color
control of the .mu.-LED is applied to the .mu.-LED via the
selection hold circuit by means of the selection signal, and
brightness control of the .mu.-LED is effected by means of a
coupled pulse width modulation signal.
[0630] 858. Use of a device according to any of the items 836 to
856 for driving a .mu.-LED or m-LED array or optoelectronic device
according to any of the preceding items.
[0631] 859. Driver circuit for driving a plurality of
optoelectronic elements, comprising:
[0632] a plurality of first memory cells, each comprising a set
input,
[0633] a reset input and an output,
[0634] wherein each first memory cell at said output is triggered
to a first state by a set signal at said set input and maintains
said first state until reset to a second state at said reset input,
and
[0635] wherein the output of each first memory cell is configured
to control a respective one of said optoelectronic elements.
[0636] 860. Driver circuit according to item 859, wherein each
first memory cell provides a pulse width modulation signal, PWM
signal at the output, and the PWM signal controls a switch
configured to switch a current through the respective
optoelectronic element on and off.
[0637] 861. Driver circuit according to any of the preceding items,
wherein each first memory cell comprises two cross-coupled NOR
gates or two cross-coupled NAND gates.
[0638] 862. Driver circuit according to any of the preceding items,
each first memory cell comprising an NMOS transistor and a PMOS
transistor connected in series, and an inverter having an input
connected between the NMOS transistor and the PMOS transistor and
an output connected to the gates of the NMOS and PMOS
transistors.
[0639] 863. Driver circuit according to any of the preceding items,
further comprising a plurality of counters each configured to
activate a set signal when a data value is loaded into the
respective counter and to activate a reset signal when the
respective counter reaches the loaded data value.
[0640] 864. Driver circuit according to any of the preceding items,
further comprising a common counter configured to generate a common
dimming signal for the plurality of optoelectronic elements.
[0641] 865. Driver circuit according to any of the preceding items,
further comprising a plurality of second memory cells, each second
memory cell being coupled to a respective one of the first memory
cells and configured to override an output signal of the respective
first memory cell as necessary to leave the respective
optoelectronic element turned off.
[0642] 866. Optoelectronic device, comprising:
[0643] a plurality of optoelectronic elements, in particular
.mu.-LEDs or .mu.-LED arrangements according to any of the
preceding items, and
[0644] a driving circuit for driving the plurality of
optoelectronic elements according to one of the preceding
items.
[0645] 867. Optoelectronic device according to item 866, where the
optoelectronic elements are .mu.-LEDs.
[0646] 868. Method of operating an optoelectronic device according
to item 866, comprising the following steps performed in the
specified order during a frame: [0647] switching off all
optoelectronic elements; [0648] controlling the optoelectronic
elements that darken during framing by means of the second memory
cells; and [0649] controlling the current through the
optoelectronic elements by means of the first memory cells.
[0650] 869. Method according to item 868, in which a common dimming
of the optoelectronic elements is carried out before the current
through the optoelectronic elements is controlled by means of the
first memory cells.
[0651] 870. Control circuit for adjusting a brightness of at least
one .mu.-LED, comprising a current driving element having [0652] a
control terminal, the first terminal of which is connected to a
first potential; [0653] a capacitor connected between the control
terminal and the first potential and forming a capacitive voltage
divider with a defined capacitance between the control terminal and
the first terminal; [0654] a control element adapted to apply a
control signal to the control terminal during a first time period,
on the basis of which a current flowing through the at least one
.mu.-LED is adjustable during the first time period;
[0655] wherein during a second time period subsequent to the first
time period, a current flowing through the .mu.-LED is determined
by a reduced control signal formed by the control signal during the
first time period and the capacitive voltage divider; and
[0656] the control element is arranged to provide a first or a
second control signal during the first time period in order to
operate the .mu.-LED at at least two different brightness
levels.
[0657] 871. Control circuit according to item 870, in which the
current driving element comprises a field effect transistor whose
gate forms the control terminal and the defined capacitance is a
gate-source capacitance predetermined by design.
[0658] 872. Control circuit according to any of the preceding
items, in which the reduced control signal applied to the control
terminal during the second time period is obtained from the control
signal during the first time period and the ratio of a capacitance
of the capacitor and the sum of the capacitance of the capacitor
and the defined capacitance.
[0659] 873. Control circuit according to any of the preceding
items, characterised in that the control is set to operate the
first and second time periods at a repetition frequency of 60 Hz or
more.
[0660] 874. Control circuit according to any of the preceding
items, in which the control element comprises a control transistor
at whose control terminal the first and second time periods are
adjustable by means of a signal.
[0661] 875. Control circuit according to any of the preceding
items, in which a ratio of the second time period to the first time
period is in the range of 300:1 to 100:1, in particular in the
range of 100:1
[0662] 876. Control circuit according to any of the preceding
items, adapted to operate the .mu.-LED at a first, darker
brightness level when a voltage of the first control signal is
within a first voltage interval, and to operate the .mu.-LED at at
least a second, brighter brightness level when a voltage of the
second control signal is within a second voltage interval which is
at least partially above the first voltage interval.
[0663] 877. Control circuit according to item 876, characterized in
that the first voltage interval is in the range of 1.3 V to 4.5
V.
[0664] 878. Control circuit according to item 876 or 877,
characterized in that the second voltage interval is in the range
of 4.0 V to 10.0 V.
[0665] 879. Method for adjusting a brightness of at least one
.mu.-LED which is connected to a current driver element having a
control terminal, the first terminal of which is connected to a
first potential and in which a capacitor is connected between the
control terminal and the first potential so that it forms a
capacitive voltage divider with a defined capacitance between the
control terminal and the first terminal, comprising the steps
[0666] applying a control signal to the control terminal during a
first period of time, thereby adjusting a current flowing through
the at least one .mu.-LED during the first period of time; and
[0667] switching off the control signal during a second time period
following the first time period, whereby the current flowing
through the .mu.-LED is adjusted by a reduced control signal formed
by the control signal during the first time period and the
capacitive voltage divider.
[0668] 880. Method according to item 879, in which the reduced
control signal applied to the control terminal during the second
time period is obtained from the control signal during the first
time period by the ratio of a capacity of the capacitor and the sum
of the capacity of the capacitor and the defined capacity.
[0669] 881. Method based on one of the preceding articles, in which
a ratio of the second period to the first period is in the range
300:1 to 100:1, in particular in the range 100:1.
[0670] 882. Method according to any of the preceding items, in
which the .mu.-LED is operated at a first, darker brightness level
if a voltage of the first control signal is within a first voltage
interval, and the .mu.-LED is operated at at least a second,
brighter brightness level if a voltage of the second control signal
is within a second voltage interval which is at least partially
above the first voltage interval.
[0671] 883. Method according to any of the preceding items, in
which the control signal is derived from a digital control word
having a number n of bits, the n bits corresponding to the second
control signal and the least significant m bits corresponding to
the first control signal.
[0672] 884. Use of a control circuit according to any of the
preceding items for driving a .mu.-LED, .mu.-LED array or .mu.-LED
module according to any of the preceding items. O
[0673] 885. Supply circuit, comprising: [0674] an error correction
detector having a reference signal input, an error signal input and
a correction signal output; [0675] a controllable current source
with current output and a control signal terminal, the control
signal terminal being connected to the correction signal output to
form a control loop for the controllable current source, the
current source configured to provide a current at the current
output in dependence on a signal at the control signal terminal;
[0676] a substitute source with one output configured to provide a
substitute signal; [0677] a switching device which is configured to
supply, depending on a switching signal, either a signal derived
from the current at the current output or the substitute signal to
the error signal input with additional disconnection of the current
output of the current source
[0678] 886. Supply circuit according to item 885, in which the
substitute signal is substantially the same as the signal derived
from the current signal.
[0679] 887. Supply circuit according to any of the preceding items,
in which the variable current source comprises a current mirror
having a switchable output branch connected to the current
output.
[0680] 888. Supply circuit according to item 887, in which the
output branch comprises an output transistor whose control terminal
is connected via the switching device to a fixed potential for
opening the transistor in dependence on a switching signal.
[0681] 889. Supply circuit according to any of the preceding items,
in which the adjustable current source comprises an input branch to
which a reference current can be supplied and which has a node
which is connected to the reference signal input of the error
correction detector.
[0682] 890. Supply circuit according to any of the preceding items,
in which the controllable current source comprises a current
mirror, the control signal terminal being connected to the control
terminal of an output transistor of the current mirror.
[0683] 891. Supply circuit according to any of the preceding items,
in which the error correction detector comprises a differential
amplifier, the two branches of which are connected together to a
supply potential via a current mirror.
[0684] 892. Supply circuit according to item 891, in which the two
branches of the differential amplifier each comprise an input
transistor, which have different geometric parameters.
[0685] 893. Supply circuit according to any of the preceding items,
in which the replacement source comprises an element coupled to the
output for generating a voltage so that the replacement signal is
substantially equal to the signal derived from the current
signal.
[0686] 894. Supply circuit according to any of the preceding items,
in which the replacement source comprises a series connection of a
current-generating element and a voltage-generating element, the
output being disposed between the two elements.
[0687] 895. Supply circuit according to any of the preceding items,
in which the replacement source comprises a transistor whose
control terminal is connected to the control terminal of the
current mirror transistor of the current source.
[0688] 896. Supply circuit according to any of the preceding items,
in which the switching device comprises one or more transmission
gates.
[0689] 897. Supply circuit according to any of the preceding items,
comprising a reference current mirror configured to supply a
current defined on the input side to the error correction detector
and to the current source on the output side.
[0690] 898. Method for powering a .mu.-LED comprising: [0691]
detecting of a supply current by the .mu.-LED; [0692] comparing the
supply current with a reference signal and deriving a correction
signal from the comparison; [0693] changing the supply current in
response to the correction signal to control the supply current to
a setpoint; [0694] switching off a supply current through the
.mu.-LED and simultaneous supply of a substitute signal for the
comparison step.
[0695] 899. Method according to item 898, in which the substitute
signal is substantially equivalent to a supply current through the
.mu.-LED or a signal derived therefrom.
[0696] 900. Use of a supply circuit according to any of the
preceding items for supplying a .mu.-LED or .mu.-LED device, in
particular according to any of the preceding items, which is
operated by a signal which pulse-width modulates the power
supply.
[0697] 901. Arrangement with [0698] the supply circuit implemented
in a substrate according to any of the preceding items; and [0699]
a .mu.-display according to any of the preceding items or
comprising a matrix of pixels arranged in rows and columns and
having at least one .mu.-Led or .mu.-LED array according to any of
the preceding items.
[0700] 902. Display matrix control circuit comprising a plurality
of light emitting devices arranged in rows and columns, comprising:
[0701] a row selection input for a row selection signal and a
column data input for a data signal; [0702] a ramp signal input for
a ramp signal having a level between a first value and a second
value and a trigger input for a trigger signal; [0703] a column
data buffer configured to buffer the data signal in response to the
row select signal; [0704] a pulse generator coupled to the column
data buffer and said ramp signal input and configured to provide a
buffered output signal to control the on/off ratio of at least one
of said plurality of light emitting devices in response to the
trigger signal, the data signal and the ramp signal.
[0705] 903. Control circuit according to item 902, wherein the
pulse generator comprises [0706] comparator means for comparing the
buffered data signal with the ramp signal; and [0707] an output
buffer coupled to an output of the comparator device and the
trigger input.
[0708] 904. Control circuit according to object 903, wherein the
output buffer comprises a flip-flop, in particular an RS flip-flop
with its input coupled to the output of the comparator device and
the trigger input respectively.
[0709] 950. Control circuit according to any of items 902 to 904,
wherein the column data buffer comprises a capacitor for storing
the data signal and a switch disposed between the capacitor and the
column data input.
[0710] 906. Control circuit according to any one of items 902 to
905, the comparator device comprising a power control input coupled
to the trigger input to adjust its power consumption based on the
trigger signal.
[0711] 907. Control circuit according to any one of items 902 to
906, wherein the comparator device is coupled to the output buffer
to control its power consumption based on an output state of the
output buffer.
[0712] 908. Control circuit according to one of the items 902 to
907, wherein the comparator is coupled with its inverting input to
the data column buffer and with its non-inverting input to the ramp
signal input.
[0713] 909. Control circuit according to any of the items 902 to
907, further comprising: [0714] a ramp generator for supplying the
ramp signal to the ramp signal input, the ramp generator being
configured to generate a signal varying between an initial value
and a final value in response to the trigger signal.
[0715] 910. Method of controlling the illuminance of a light
emitting device in a matrix display having a plurality of light
emitting devices arranged in addressable rows and columns, the
method comprising [0716] providing a data signal for a selected row
and at least one light emitting device; [0717] supplying a trigger
signal; [0718] converting a level of the data signal to a pulse
with respect to a trigger signal; and [0719] controlling the on/off
ratio of the light emitting device with the pulse.
[0720] 911. Method according to item 910, wherein the step of
converting a level of the data signal comprises: [0721] generating
a ramp signal between a first value and a second value; [0722]
comparing the data signal with the ramp signal to generate a
comparison signal; [0723] generating of a pulse based on the
trigger signal and a change in the comparison signal.
[0724] 912 Method according to item 910, wherein the generation of
a pulse comprises setting a level of an output signal to a first
value in response to a trigger signal and resetting the level of
the output signal to a second value in response to the change in
the comparison signal.
[0725] 913. Method according to items 911 or 912, where the ramp
signal is generated in response to the trigger signal.
[0726] 914. Method according to any of the items 910 to 913,
wherein delivery of a data signal comprises pre-buffering the data
signal, in particular pre-buffering the data signal in a memory
device.
[0727] 915. Use of the control circuit according to any of the
preceding items in a .mu.-display or for driving a .mu.-LED,
.mu.-LED array or an array of .mu.-LEDS, in particular according to
any of the items 94 to 111 or 7 to 50 and 484 to 536.
[0728] 916. Device for electronically driving a plurality of
.mu.-LEDs, comprising [0729] a first and at least one second branch
each having a .mu.-LED connected therein and an electronic fuse
arranged in series with the .mu.-LED, the first and the at least
one second branch being connected to a potential terminal on one
side; [0730] a driver circuit having a data signal input, a
selection signal input and a driver output connected to the other
side of the first and at least one second branch; [0731] an
imprinting component associated with the at least one second
branch, which is designed to generate a current flow triggering the
electronic fuse arranged in series.
[0732] 917. Device according to item 916, in which [0733] the
.mu.-LED is configured according to any of the subsequent or
preceding items; and/or [0734] the .mu.-LED comprises a
light-shaping or light-guiding element on its surface according to
any of the subsequent or preceding items; and/or [0735] the
.mu.-LED of each branch comprises a common electrically conductive,
in particular transparent, contact layer
[0736] 918. Device according to any of the preceding items,
characterised in that
[0737] the imprinting component comprises an imprinting transistor,
which is electrically connected with its current line contacts in
parallel with the .mu.-LED to which the imprinting transistor is
assigned and whose control contact is connected to an imprinting
signal line.
[0738] 919. Device according to any of the preceding items,
characterised in that
[0739] the imprinting component comprises an imprinting diode
having one terminal connected to a second terminal of the .mu.-LED
with which the imprinting diode is associated and the other
terminal of which is connected to an imprinting signal line.
[0740] 920. Device according to any of the preceding items,
characterised in that
[0741] first terminals of the .mu.-LED are connected to a reference
potential terminal;
[0742] a first transistor with its current conduction contacts is
arranged between a common terminal of the fuses of the .mu.-LED and
a supply potential terminal;
[0743] a charge storage device is electrically connected to a
control contact of the first transistor and to a first current
conduction contact of the first transistor.
[0744] 921. Device according to any of the preceding items,
characterised in that second terminals of the .mu.-LED are
connected to a supply potential terminal;
[0745] a first current conducting contact of a first transistor is
connected to a reference potential terminal and a second current
conducting contact of the first transistor is connected to a common
terminal of the electrical fuses;
[0746] a capacitor is connected to a control contact of the first
transistor and to the first current conduction contact of the first
transistor.
[0747] 922. Device according to any of the preceding items,
characterised in that second terminals of the .mu.-LED are
connected to the fuse assigned to the .mu.-LED;
[0748] a first current conducting contact of a first transistor is
connected to a reference potential terminal and a second current
conducting contact of the first transistor is connected to first
terminals of the .mu.-LED;
[0749] a capacitor is connected to a control contact of the first
transistor and to the first current conduction contact of the first
transistor.
[0750] 923. Device according to any of the preceding items,
characterised in that first terminals of the .mu.-LED are connected
to a reference potential terminal;
[0751] a first transistor with its current line contacts is
arranged between a common terminal of the fuses of the .mu.-LED and
a supply potential terminal;
[0752] the charge storage device is electrically connected to a
control contact of the first transistor and to a second current
conduction contact of the first transistor.
[0753] 924. Device according to any of the preceding items,
characterised in that first terminals of the .mu.-LED are connected
to a first reference potential terminal;
[0754] a first transistor with its current conduction contacts is
arranged between a common terminal of the fuses of the .mu.-LEDs
and a supply potential terminal;
[0755] a capacitor is electrically connected to a control contact
of the first transistor and to a second current conduction contact
of the first transistor, a first terminal of the imprinting diode
being connected to a second terminal of the .mu.-LED and a second
terminal of the imprinting diode being connected to the imprinting
signal line.
[0756] 925. Device according to any of the preceding items,
characterised in that first terminals of the .mu.-LEDs are
connected to a reference potential terminal;
[0757] a first transistor with its current line contacts is
arranged between a common terminal of the fuses of the .mu.-LEDs
and a supply potential terminal;
[0758] a capacitor is electrically connected to a control contact
of the first transistor and to a second current conduction contact
of the first transistor, a second terminal of the imprinting diode
being connected to the second terminal of the .mu.-LED and a first
terminal of the imprinting diode being connected to the imprinting
signal line.
[0759] 926. Device current line contact, characterised in that the
driver circuit comprises the first transistor, a second transistor
and the charge storage, the selection signal line being applied to
a control contact of the second transistor and the data signal
input being applied to a current conduction contact of the second
transistor, and a first or a second current conduction contact of
the first transistor providing the driver output which is connected
to the .mu.-LEDs of the first branch and a second branch to provide
a power supply.
[0760] 927. .mu.-display or .mu.-display module comprising a
plurality of the devices according to any of the preceding items,
in which pixel cells of the .mu.-display are each electrically
connected along a row and/or along a column on a common imprinting
signal line, and each pixel cell of a column is electrically
connected to the supply potential terminal by means of a common
supply line to a switching transistor arranged on a common carrier
outside the .mu.-display.
[0761] 928. .mu.-display or .mu.-display module according to item
927, in which the .mu.-LEDs connected in the first and at least one
second branch at least comprise [0762] features according to any of
the preceding items, in particular items 94 to 111 or 7 to 50;
[0763] features according to any of the items 484 to 536; [0764] a
photonic structure according to any of items 607 to 679.
[0765] 929. Method for electronically configuring a plurality of
.mu.-LEDs according to any of the preceding items articles,
comprising the steps of: [0766] testing a function of the .mu.-LED
of the first and second branch; [0767] if there is no error in the
.mu.-LED in the first and second branch: [0768] applying of an
imprinting signal to the electronic imprinting component; [0769]
imprinting into the second branch a current flow which triggers the
fuse connected in series to the .mu.-LED of the second branch.
[0770] 930. Use of a device according to any of the preceding items
in a display arrangement according to any of the preceding or
subsequent items.
[0771] 931. Display arrangement comprising
[0772] an IC substrate component with monolithic integrated
circuits and with IC substrate contacts arranged as a matrix;
and
[0773] a monolithic pixelated optochip comprising a semiconductor
layer sequence with a first semiconductor layer having a first
doping and a second semiconductor layer having a second doping,
wherein the polarity of the charge carriers in the first
semiconductor layer differs from that of the second semiconductor
layer and the semiconductor layer sequence defines a stacking
direction; and
[0774] wherein .mu.-LEDs arranged as a matrix are present in the
monolithic pixelated optochip; and
[0775] wherein each .mu.-LED has a .mu.-LED rear side facing the IC
substrate component and a first light source contact which adjoins
the first semiconductor layer in a contacting manner and is
electrically conductively connected to a respective one of the IC
substrate contacts;
[0776] characterised in that
[0777] the projection area of the first light source contact on the
.mu.-LED rear surface is at most half the area of the .mu.-LED rear
surface; and
[0778] the first light source contact in a lateral direction
perpendicular to the stacking direction is surrounded by an
absorber on the rear side.
[0779] 932. Display arrangement according to item 931,
characterized in that the first semiconductor layer and the second
semiconductor layer comprise a p- or n-conductivity lower than 104
Sm.sup.-1, preferably lower than 3*103 Sm.sup.-1, more preferably
lower than 103 Sm.sup.-1.
[0780] 933. display arrangement according to any of the preceding
items, characterized in that the layer thickness of the first
semiconductor layer in the stacking direction is at most ten times
and preferably at most five times the maximum diagonal of the first
light source contact in the lateral direction.
[0781] 934. Display arrangement according to any of the preceding
items, characterized in that the pixel size of the .mu.-LED is
<10 .mu.m and preferably <5 .mu.m and particularly preferably
<2 .mu.m.
[0782] 935. Display arrangement according to one of the preceding
items, characterized in that the projection area of the first light
source contact on the .mu.-LED back is at most 25% and preferably
at most 10% of the area of the .mu.-LED back.
[0783] 936. Display arrangement according to any of the preceding
items, characterized in that the rear absorber extends in the
stacking direction in the semiconductor layer sequence.
[0784] 937. Display arrangement according to any of the preceding
items, characterized in that a second light source contact made of
a transparent material is arranged in the stacking direction above
the second semiconductor layer for each .mu.-LED, which is
electrically conductively connected to a transparent contact layer
on the front side of the monolithic pixelated optochip.
[0785] 938. Display arrangement according to item 937,
characterized in that the second light source contact is formed by
the transparent contact layer itself.
[0786] 939. display arrangement according to any of the preceding
items, characterized in that the second light source contact is
adjacent to the transparent contact layer and the second light
source contact of adjacent .mu.-LEDs are separated from each other
by an absorber on the front side in a lateral direction
perpendicular to the stacking direction.
[0787] 940. Display arrangement according to any of the preceding
items, characterized in that the front absorber extends against the
stacking direction up to and preferably into the second
semiconductor layer.
[0788] 941. Display arrangement according to any of the preceding
items, characterized in that, with respect to the stacking
direction, an optochip contact element whose cross-sectional area
is larger than that of the first light source contact is adjacent
below the first light source contact.
[0789] 942. Display arrangement according to any of the preceding
items, further comprising:
[0790] a light-shaping structure, in particular a microlens or a
photonic crystal, which is arranged on the monolithic pixelated
optochip and directs light emitted by the monolithic pixelated
Optochip.
[0791] 943. Display arrangement according to any of the preceding
items, further comprising a light-converting element on the surface
of the monolithic pixelated Optochip.
[0792] 944. Display arrangement according to any of the preceding
items, wherein, in the case of two adjacent .mu.-LEDs, one .mu.-LED
is configured as a redundant element to the other .mu.-LED, to
which a fuse element in the IC substrate component is assigned,
which fuse element is designed to replace the other .mu.-LED by the
redundant element if the other .mu.-LED fails or to disconnect the
redundant element from a power supply if the other .mu.-LED is
functional.
[0793] 945. Method of manufacturing a display device,
[0794] wherein an IC substrate component with monolithic integrated
circuits and with IC substrate contacts arranged as a matrix and a
monolithic pixelated optochip are electrically conductively
connected; and
[0795] a semiconductor layer sequence with a first semiconductor
layer having a first doping and a second semiconductor layer having
a second doping is grown in the monolithic pixelated optochip, the
polarity of the charge carriers in the first semiconductor layer
differing from that of the second semiconductor layer and the
half-conductor layer sequence defining a stacking direction;
and
[0796] wherein .mu.-LEDs arranged in the monolithic pixelated
optochip as a matrix are applied, each .mu.-LED comprising a
.mu.-LED rear side facing the IC substrate component and a first
light source contact which adjoins the first semiconductor layer in
a contacting manner and is electrically conductively connected to a
respective one of the IC substrate contacts;
[0797] characterised in that
[0798] the first light source contact is applied with a size such
that its projection area perpendicular to the stacking direction
occupies at most half the area of the .mu.-LED backside; and
[0799] the first light source contact in a lateral direction
perpendicular to the stacking direction is surrounded by an
absorber on the rear side.
[0800] 946. Display arrangement with a .mu.-display comprising a
plurality of pixels arranged in rows and columns, comprising:
[0801] a first substrate structure with .mu.-LEDs arranged therein
or applied thereto, the edge length of which is less than 50 .mu.m,
in particular less than 20 .mu.m, and which form the pixel
structure arranged in rows and columns, wherein the .mu.-LEDs are
individually controllable; and a plurality of contacts are arranged
on the surface of the first substrate structure opposite to a light
emission direction; [0802] a second substrate structure comprising
on a surface a plurality of contacts corresponding to the contacts
of the first substrate structure and having a plurality of digital
circuits for addressing the .mu.-LEDs;
[0803] wherein the first and second substrate structures are
connected together and the plurality of contacts are electrically
connected to the corresponding contacts, and wherein the first
substrate structure is formed with a first material system and the
second substrate structure is formed with a second material system
different therefrom.
[0804] 947. Display arrangement according to item 946, in which at
least some contacts of the plurality of contacts have an edge
length of less than 10 .mu.m or an area of less than 20
.mu.m.sup.2.
[0805] 948. Display arrangement according to any of the preceding
items, in which the .mu.-LEDs are formed with an edge length of
less than 10 .mu.m and/or have a distance to adjacent .mu.-LED of
less than 7 .mu.m.
[0806] 949. Display arrangement according to any of the preceding
items, comprising an adhesive or other form-fitting element
partially disposed between and holding together the first and
second substrate structures
[0807] 950. Display arrangement according to any of the preceding
items, wherein the .mu.-display comprises a plurality of pixels
arranged in rows and columns, at least some of the .mu.-LEDs or
.mu.-LED arrays or optoelectronic devices according to any of the
preceding items, or elements according to any of the preceding
items.
[0808] 951. Display arrangement according to any of the preceding
items, in which the second substrate structure comprises at least
some of the circuitry according to any of the preceding items.
[0809] 952. Display arrangement according to any of the preceding
items, further comprising at least one light guiding arrangement
having features according to any of the preceding items.
[0810] 953. Display arrangement according to any of the preceding
items, in which the first substrate structure is separated from the
second substrate structure by an intermediate structure through
which at least contact lines extend which connect the contacts of
the first substrate structure with contacts of the second substrate
structure.
[0811] 954. Display arrangement according to any of the preceding
items, in which the first system of materials comprises at least
one of the following compounds GaN, GaP, GaInP, InAlP, GaAlP or
GaAlInP, GaAs, AlGaAs, and the second material system comprises at
least one of the following material systems: monocrystalline,
polycrystalline, amorphous silicon, indium-gallium-zinc oxide, GaN
or GaAs.
[0812] 955. Display arrangement according to any of the preceding
items, in which in the first carrier structure comprises a
plurality of switchable current sources, each of which is connected
to a pixel for the supply thereof, and whose switch inputs are
coupled to the contacts for supplying switching signals from the
digital circuits.
[0813] 956. Display arrangement according to item 955, in which the
switchable current sources are arranged in a material system, which
is different from the material system used for the .mu.-LEDs or
from the first material system.
[0814] 957. Display arrangement according to any of the preceding
items, in which the plurality of digital circuits of the second
substrate structure are adapted to generate a PWM-like signal from
a clock signal and a data word for each pixel.
[0815] 958. Display arrangement according to item 957, in which the
plurality of digital circuits comprises a number of serially
connected shift registers, the respective length of which
corresponds to the data word for one pixel, each shift register
being connected to a buffer for intermediate storage.
[0816] 959. Display arrangement according to any of the preceding
items, wherein the plurality of digital circuits comprise a
multiplexer electrically coupled to a demultiplexer in the first
substrate structure for driving multiple optoelectronic
devices.
[0817] The description with the help of the exemplary embodiments
does not limit the various embodiments shown in the examples to
these. Rather, the disclosure depicts several aspects, which can be
combined with each other and also with each other. Aspects that
relate to processes, for example, can thus also be combined with
aspects where light extraction is the main focus. This is also made
clear by the various objects shown above.
[0818] The invention thus comprises any features and also any
combination of features, including in particular any combination of
features in the subject-matter and claims, even if that feature or
combination is not explicitly specified in the exemplary
embodiments.
* * * * *