U.S. patent application number 12/504510 was filed with the patent office on 2009-11-19 for pixel driver circuit and pixel circuit having the pixel driver circuit.
This patent application is currently assigned to IGNIS INNOVATION INC.. Invention is credited to ANIL KUMAR, AROKIA NATHAN, KAPIL SAKARIYA, PEYMAN SERVATI.
Application Number | 20090284501 12/504510 |
Document ID | / |
Family ID | 46322584 |
Filed Date | 2009-11-19 |
United States Patent
Application |
20090284501 |
Kind Code |
A1 |
NATHAN; AROKIA ; et
al. |
November 19, 2009 |
PIXEL DRIVER CIRCUIT AND PIXEL CIRCUIT HAVING THE PIXEL DRIVER
CIRCUIT
Abstract
A pixel driver circuit for driving a light-emitting element and
a pixel circuit having the pixel driver circuit are provided. The
pixel driver circuit includes a data line, address lines, switch
thin film transistors, feedback thin film transistors and drive
thin film transistors. The pixel circuit may include an organic
light emitting diode, which is driven by the pixel driver
circuit.
Inventors: |
NATHAN; AROKIA; (WATERLOO,
CA) ; SERVATI; PEYMAN; (WATERLOO, CA) ;
SAKARIYA; KAPIL; (SANTA CLARA, CA) ; KUMAR; ANIL;
(KITCHENER, CA) |
Correspondence
Address: |
Pearne & Gordon LLP
1801 East 9th Street, Suite 1200
Cleveland
OH
44114-3108
US
|
Assignee: |
IGNIS INNOVATION INC.
KITCHENER
ON
|
Family ID: |
46322584 |
Appl. No.: |
12/504510 |
Filed: |
July 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11220094 |
Sep 6, 2005 |
7569849 |
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12504510 |
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10468319 |
Jan 23, 2004 |
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PCT/CA02/00173 |
Feb 18, 2002 |
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11220094 |
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60268900 |
Feb 16, 2001 |
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Current U.S.
Class: |
345/204 ; 257/59;
257/E29.003 |
Current CPC
Class: |
H01L 29/78648 20130101;
H03K 17/6871 20130101; G09G 2300/0804 20130101; G09G 2320/0204
20130101; H01L 27/12 20130101; H01L 27/3276 20130101; G09G 2320/045
20130101; G09G 2320/0223 20130101; H03K 17/162 20130101; H01L
27/283 20130101; G09G 2300/0842 20130101; H01L 27/3244 20130101;
G09G 2300/0819 20130101; G09G 3/00 20130101; G09G 3/3241 20130101;
H05B 45/60 20200101; G09G 3/3233 20130101 |
Class at
Publication: |
345/204 ; 257/59;
257/E29.003 |
International
Class: |
G09G 5/00 20060101
G09G005/00; H01L 29/04 20060101 H01L029/04 |
Claims
1-35. (canceled)
36. A pixel driver circuit comprising: an address line; a data
line; a switch thin film transistor, a first node of the switch
transistor being connected to the data line and a gate of the
switch transistor being connected to the address line; a feedback
thin film transistor, a gate of the feedback transistor being
connected to the address line and a second node of the feedback
transistor being connected to a potential; a reference thin film
transistor, a first node of the reference transistor being
connected to a second node of the switch transistor, a gate of the
reference transistor being connected to the second node of the
switch transistor, and a second node of the reference transistor
being connected to a first node of the feedback transistor; and a
drive thin film transistor, a gate of the drive transistor being
connected to the gate of the reference transistor.
37. The pixel driver circuit according to claim 36, wherein at
least one of the thin film transistors is an amorphous silicon
based thin film transistor.
38. The pixel driver circuit according to claim 36, wherein at
least one of the thin film transistor is a polycrystalline silicon
based thin film transistor.
39. The pixel driver circuit according to claim 36, wherein at
least one of the thin film transistors is a n-type thin film
transistor.
40. The pixel driver circuit according to claim 36, wherein at
least one of the thin film transistors is a p-type thin film
transistor.
41. The pixel driver circuit according to claim 36, wherein the
thin film transistors each comprise a second gate.
42. The pixel driver circuit according to claim 36, wherein the
first node of the reference transistor is a drain node, and wherein
the second node of the reference transistor is a source node.
43. The pixel driver circuit according to claim 36, wherein the
second node of the feedback transistor is connected to a ground
potential.
44. The pixel driver circuit according to claim 36, wherein the
second node of the feedback transistor is connected to a voltage
supply.
45. The pixel driver circuit according to claim 36, comprising a
capacitor connected to the gate of the drive transistor and a
ground potential.
46. The pixel driver circuit according to claim 36, comprising a
capacitor connected to the gate of the drive transistor and a
voltage supply.
47. A pixel circuit comprising: a pixel driver circuit according to
claim 36; and an organic light emitting diode, the one of a first
node and a second node of the drive transistor being connected to
the organic light emitting diode.
48. The pixel circuit according to claim 47, wherein the one of the
first node and the second node of the drive transistor is connected
to the organic light emitting diode, and wherein the other node is
connected to a ground potential.
49. The pixel circuit according to claim 47, wherein the one of the
first node and the second node of the drive transistor is connected
to the organic light emitting diode, and wherein the other node is
connected to a voltage supply.
50. The pixel circuit according to claim 47, wherein the one of the
first node and the second node of the drive transistor is a drain,
and wherein the other is a source.
51. The pixel circuit according to claim 47, comprising a capacitor
connected between the gate of the drive transistor and a ground
potential.
52. The pixel circuit according to claim 47, comprising a capacitor
connected between the gate of the drive transistor and a voltage
supply.
53. The pixel circuit according to claim 47, wherein the pixel
circuit is arranged for a monochrome display.
54. The pixel circuit according to claim 47, wherein the pixel
circuit is arranged for a full color display.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuation-in-part application of U.S. patent
application Ser. No. 10/468,319 filed on Jan. 23, 2004, which is
the U.S. National Phase of PCT/CA02/00173 having an International
Filing Date of Feb. 18, 2002, which claims the benefit of U.S.
provisional patent application Ser. No. 60/268,900 filed on Feb.
16, 2001, the contents of all of the foregoing applications are
incorporated herein by reference in their entirety.
FIELD OF INVENTION
[0002] The present invention relates to a display technology, and
more particularly to a pixel driver circuit for driving a
light-emitting element and a pixel circuit having the pixel driver
circuit.
BACKGROUND OF THE INVENTION
[0003] Organic light emitting diode (OLED) displays have gained
significant interest recently in display applications in view of
their faster response times, larger viewing angles, higher
contrast, lighter weight, lower power, amenability to flexible
substrates, as compared to liquid crystal displays (LCDs). Despite
the OLED's demonstrated superiority over the LCD, there still
remain several challenging issues related to encapsulation and
lifetime, yield, color efficiency, and drive electronics, all of
which are receiving considerable attention.
[0004] Although passive matrix addressed OLED displays are already
in the marketplace, they do not support the resolution needed in
the next generation displays, since high information content (HIC)
formats are only possible with the active matrix addressing
scheme.
[0005] Active matrix addressing involves a layer of backplane
electronics, based on thin film transistors (TFTs) fabricated using
amorphous silicon (a-Si:H), polycrystalline silicon (poly-Si), or
polymer technologies, to provide the bias voltage and drive current
needed in each OLED based pixel. Here, the voltage on each pixel is
lower and the current throughout the entire frame period is a low
constant value, thus avoiding the excessive peak driving and
leakage currents associated with passive matrix addressing. This in
turn increases the lifetime of the OLED.
[0006] In active matrix OLED (AMOLED) displays, it is important to
ensure that the aperture ratio or fill factor (defined as the ratio
of light emitting display area to the total pixel area) should be
high enough to ensure display quality.
[0007] Conventional AMOLED displays are based on light emission
through an aperture on the glass substrate where the backplane
electronics is integrated. Increasing the on-pixel density of TFT
integration for stable drive current reduces the size of the
aperture. The same happens when pixel sizes are scaled down. One
solution to having an aperture ratio that is invariant on scaling
or on-pixel integration density is to vertically stack the OLED
layer on the backplane electronics, along with a transparent top
electrode as shown in FIG. 2. In FIG. 2, reference numerals S and D
denote a source and a drain, respectively. This implies a
continuous back electrode over the OLED pixel.
[0008] However, this continuous back electrode can give rise to
parasitic capacitance, whose effects become significant when the
electrode runs over the switching and other TFTs. The presence of
the back electrode can induce a parasitic channel in TFTs giving
rise to high leakage current. The leakage current is the current
that flows between source and drain of the TFT when the gate of the
TFT is in its OFF state.
SUMMARY OF THE INVENTION
[0009] It is an object of the invention to provide a system that
obviates or mitigates at least one of the disadvantages of existing
systems.
[0010] The present invention relates to a pixel driver circuit for
driving a light-emitting element (e.g. OLED), and a pixel circuit
having the pixel driver circuit.
[0011] In accordance with an aspect of the present invention, there
is provided a pixel driver circuit, which includes: an address
line; a data line; a switch thin film transistor, a first node of
the switch transistor being connected to the data line and a gate
of the switch transistor being connected to the address line; a
feedback thin film transistor, a first node of the feedback
transistor being connected to the data line and a gate of the
feedback transistor being connected to the address line; a
reference thin film transistor, a drain of the reference transistor
being connected to a second node of the feedback transistor, a gate
of the reference transistor being connected to a second node of the
switch transistor and a source of the reference transistor being
connected to a ground potential; and a drive thin film transistor,
a gate of the drive transistor being connected to the gate of the
reference transistor.
[0012] In accordance with a further aspect of the present
invention, there is provided a pixel circuit, which includes: the
pixel driver circuit described above; and an organic light emitting
diode, the source of the drive transistor being connected to the
ground potential and the drain being connected to the organic light
emitting diode.
[0013] In accordance with a further aspect of the present
invention, there is provided a pixel driver circuit, which
includes: an address line; a data line; a switch thin film
transistor, a first node of the switch transistor being connected
to the data line and a gate of the switch transistor being
connected to the address line; a feedback thin film transistor, a
gate of the feedback transistor being connected to the address line
and a second node of the feedback transistor being connected to a
ground potential; a reference thin film transistor, a drain of the
reference transistor being connected to a second node of the switch
transistor, a gate of the reference transistor being connected to
the second node of the switch transistor and a source of the
reference transistor being connected to a first node of the
feedback transistor; and a drive thin film transistor, a gate of
the drive transistor being connected to the gate of the reference
transistor.
[0014] In accordance with a further aspect of the present
invention, there is provided a pixel circuit, which includes: the
pixel driver circuit described above; and an organic light emitting
diode, the source of the drive transistor being connected to the
ground potential and the drain being connected to the organic light
emitting diode.
[0015] In accordance with a further aspect of the present
invention, there is provided a pixel circuit, which includes: the
pixel driver circuit described above; and an organic light emitting
diode, the source of the drive transistor being connected to the
organic light emitting diode and the drain being connected to a
voltage supply.
[0016] In accordance with a further aspect of the present
invention, there is provided a pixel driver circuit, which
includes: an address line; a data line; a switch thin film
transistor, a first node of the switch transistor being connected
to the data line and a gate of the switch transistor being
connected to the address line; a feedback thin film transistor, a
first node of the feedback transistor being connected to the data
line and a gate of the feedback transistor being connected to the
address line; a reference thin film transistor, a drain of the
reference transistor being connected to a second node of the
feedback transistor, the gate of the reference transistor being
connected to a second node of the switch transistor and a source of
the reference transistor being connected to a ground potential; a
diode-use thin film transistor, a drain and a gate of the diode-use
transistor being connected to a potential, and a source of the
diode-use transistor being connected to the second node of the
feedback transistor; and a drive thin film transistor, a gate of
the drive transistor being connected to the gate of the reference
transistor.
[0017] In accordance with a further aspect of the present
invention, there is provided a pixel circuit, which includes: the
pixel driver circuit described above; and an organic light emitting
diode, the source of the drive transistor being connected to the
ground potential and the drain being connected to the organic light
emitting diode.
[0018] In accordance with a further aspect of the present
invention, there is provided a pixel circuit, which includes: the
pixel driver circuit described above; and an organic light emitting
diode, the source of the drive transistor being connected to the
organic light emitting diode, and the drain being connected to a
voltage supply.
[0019] In accordance with a further aspect of the present
invention, there is provided a pixel driver circuit for driving a
colour pixel of a colour display, which includes: a first address
line; a data line; a first switch thin film transistor, a first
node of the first switch transistor being connected to the data
line and a gate of the switch transistor being connected to the
first address line; a feedback thin film transistor, a first node
and a gate of the feedback transistor being connected to a second
node of the first switch transistor and a second node of the
feedback transistor being connected to a ground potential; a second
switch thin film transistor, a source of the second switch
transistor being connected to a second node of the first switch
transistor, a gate of the second switch transistor being connected
to a second address line; a first drive thin film transistor, a
gate of the first drive transistor being connected to a drain of
the second switch transistor; a third switch thin film transistor,
a source of the third switch transistor being connected to the
second node of the first switch transistor, a gate of the third
switch transistor being connected to a third address line; a second
drive thin film transistor, a gate, of the second drive transistor
being connected to the drain of the third switch transistor; a
fourth switch thin film transistor, a source of the fourth switch
transistor being connected to the second node of the first switch
transistor, a gate of the fourth switch transistor being connected
to a fourth address line; and a third drive thin film transistor, a
gate of the third drive transistor being connected to the drain of
the fourth switch transistor.
[0020] In accordance with a further aspect of the present
invention, there is provided a pixel circuit, which includes: the
pixel driver circuit described above; a first organic light
emitting diode, a source of the first drive transistor being
connected to the ground potential and a drain of the first drive
transistor being connected to the first organic light emitting
diode; a second organic light emitting diode, a source of the
second drive transistor being connected to the ground potential and
a drain of the second drive transistor being connected to the
second organic light emitting diode; and a third organic light
emitting diode, a source of the third drive transistor being
connected to the ground potential and a drain of the third drive
transistor being connected to the third organic light emitting
diode.
[0021] In accordance with a further aspect of the present
invention, there is provided a pixel circuit which includes: a
pixel driver circuit described above, a first organic light
emitting diode associated with the first drive transistor; a second
organic light emitting diode associated with the second drive
transistor; and a third organic light emitting diode associated
with the third drive transistor, the source of the first drive
transistor being connected to the first organic light emitting
diode, and a drain of the first drive transistor being connected to
a voltage supply.
[0022] This summary of the invention does not necessarily describe
all features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] These and other features of the invention will become more
apparent from the following description in which reference is made
to the appended drawings wherein:
[0024] FIG. 1 shows variation of required pixel areas with mobility
for 2-T and 5-T pixel drivers;
[0025] FIG. 2 shows a conventional pixel architecture for surface
emissive a-Si:H AMOLED displays;
[0026] FIG. 3 shows a cross section view of a dual-gate TFT
structure;
[0027] FIG. 4 shows forward and reverse transfer characteristics of
dual-gate TFT for various top gate biases;
[0028] FIG. 5 shows a panel architecture of a AMOLED display;
[0029] FIG. 6A shows a pixel circuit including a conventional 2-T
pixel driver circuit;
[0030] FIG. 6B shows input-output timing diagrams for the 2-T pixel
circuit of FIG. 6A;
[0031] FIG. 7A shows a pixel circuit including a 5-T pixel current
driver circuit for an OLED display in accordance with an embodiment
of the present invention;
[0032] FIG. 7B shows input-output timing diagrams of the 5-T pixel
circuit of FIG. 7A;
[0033] FIG. 8 shows transient performance of the 5-T pixel current
driver circuit of FIG. 7A for three consecutive write cycles;
[0034] FIG. 9 shows input-output transfer characteristics for the
2-T pixel driver circuit of FIG. 6A for different supply
voltages;
[0035] FIG. 10 shows input-output transfer characteristics for the
5-T pixel current driver circuit of FIG. 7A for different supply
voltages;
[0036] FIG. 11 shows variation in OLED current as a function of the
normalized shift in threshold voltage;
[0037] FIG. 12 shows a pixel circuit including a conventional 2-T
polysilicon based pixel driver circuit having p-channel drive
TFTs;
[0038] FIG. 13 shows a pixel circuit including a 4-T pixel current
driver circuit for an OLED display in accordance with a further
embodiment of the present invention;
[0039] FIG. 14 shows a pixel circuit including a 4-T pixel current
driver circuit for an OLED display in accordance with a further
embodiment of the present invention;
[0040] FIG. 15 shows a pixel circuit including a 4-T pixel current
driver circuit for an OLED display in accordance with a further
embodiment of the present invention;
[0041] FIG. 16 shows a pixel circuit including a 4-T pixel current
driver circuit for an OLED display in accordance with a further
embodiment of the present invention;
[0042] FIG. 17 shows a pixel circuit including a pixel current
driver circuit for a full color, OLED display in accordance with a
further embodiment of the present invention;
[0043] FIG. 18 shows a schematic diagram of the top gate and the
bottom gate of a dual gate transistor where the top gate is
electrically connected to the bottom gate;
[0044] FIG. 19 shows a pixel circuit including a 5-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0045] FIG. 20 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0046] FIG. 21 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0047] FIG. 22 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0048] FIG. 23 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0049] FIG. 24 shows a pixel circuit including a pixel current
driver circuit for a full color display in accordance with a
further embodiment of the present invention;
[0050] FIG. 25 shows a pixel circuit including a 5-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0051] FIG. 26 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0052] FIG. 27 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0053] FIG. 28 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention;
[0054] FIG. 29 shows a pixel circuit including a 4-T pixel current
driver circuit in accordance with a further embodiment of the
present invention; and
[0055] FIG. 30 shows a pixel circuit including a pixel current
driver circuit for a full color display in accordance with a
further embodiment of the present invention.
DETAILED DESCRIPTION
[0056] The following description is of a preferred embodiment.
[0057] The embodiments of the present invention are described using
an OLED display. However, the embodiments of the present invention
are applicable to any other displays, such as phosphorus displays,
inorganic electroluminescent (EL), and LED displays. A pixel driver
circuit in accordance with the embodiments of the present invention
includes a plurality of TFTs, which form a current mirror based
pixel current driver for automatically compensating for the shift
of threshold V.sub.th of a drive TFT. The TFTs are formed in a
current-programmed .DELTA. V.sub.T-compensated manner.
[0058] The pixel driver circuit is suitable for an OLED display.
The OLED layer may be vertically stacked on the plurality of TFTs.
The pixel driver circuit may be provided for monochrome displays or
for full colour displays. The OLED may be a regular (P-I-N) stack
OLED or an inverted (N-1-P) stack OLED, and may be located at
either the drain or source of the drive TFT(s)
[0059] The TFT may be an n-type TFT or a p-type TFT. The TFT may
be, but not limited to, an amorphous silicon (a-Si:H) based TFT, a
polysilicon-based TFT, a crystalline silicon based TFT, or an
organic semiconductor based TFT.
[0060] Although amorphous Si does not enjoy equivalent electronic
properties compared to poly-Si, it adequately meets many of the
drive requirements for small area displays such as those used in
pagers, cell phones, and other mobile devices. Poly-Si TFTs have
one key advantage in that they are able to provide better pixel
drive capability because of their higher mobility. Their higher
mobility can be of the order of .mu..sub.FE.about.100 cm.sup.2/Vs.
".mu..sub.FE" represents field effect mobility, which is typically
used to evaluate how well a semiconductor can conduct. "Vs" is a
unit where V stands for volt, and s stands for second. This makes
poly-Si highly desirable for large area (e.g. laptop size) Video
Graphics Array (VGA) and Super VGA (SVGA) displays. The lower
mobility associated with a-Si:H TFTs (.mu..sub.FE.about.1
cm.sup.2/Vs) is not a limiting factor since the drive transistor in
the pixel can be scaled up in area to provide the needed drive
current. The OLED drive current density is typically 10 mA/cm.sup.2
at 10V operation to provide a brightness of 100 cd/m.sup.2, which
is the required luminance for most displays. For example, with an
a-Si:H TFT mobility of 0.5 cm.sup.2/Vs and channel length of 25
.mu.m, this drive current requirement translates into required
pixel area of 300 .mu.m.sup.2, which adequately meets the
requirements of pixel resolution and speed for some 3-inch
monochrome display applications.
[0061] FIG. 1 illustrates simulation results for the variation of
the required pixel size with device mobility calculated for two
types of drivers, which will be elaborated later, a conventional
voltage-programmed 2-T pixel driver circuit (FIG. 6A) and a
current-programmed, .DELTA.V.sub.T-compensated 5-T pixel driver
circuit in accordance with an embodiment of the present invention
(FIG. 7A)
[0062] In FIG. 1, the graph having a mark ".box-solid." represents
the pixel size required by the 2T pixel driver circuit given a
reference mobility of the TFT, and the graph having a mark
".diamond-solid." represents the pixel size required by the 5T
pixel driver circuit given a reference mobility of the TFT. In FIG.
1, ".mu..sub.0" denotes a reference mobility whose value is in the
range 0.1 to 1 cm.sup.2/Vs.
[0063] For instance, the area of the pixel for the 2-T pixel driver
(FIG. 6A) has the area of the switching transistors, the area of
the drive transistor, and the area occupied by interconnects, bias
lines, etc. In FIG. 1, the drive current and frame rate are kept
constant at 10 .mu.A and 50 Hz, respectively, for a 230.times.230
array. It is clear that there is no significant savings in area
between the 2-T and 5-T pixel drivers but the savings are
considerable with increasing mobility. This stems mainly from the
reduction in the area of the drive transistor where there is a
trade-off between TFT and TFT aspect ratio, W/L (Width/Length).
[0064] In terms of threshold voltage (V.sub.T) uniformity and
stability, both poly-Si and a-Si:H share the same concerns,
although in comparison, the latter provides far better spatial
uniformity but not stability (.DELTA.V.sub.T). Thus the inter-pixel
variation in the drive current can be a concern in both cases,
although clever circuit design techniques can be employed to
compensate for .DELTA. V.sub.T hence improving drive current
uniformity. In terms of long-term reliability, it is not clear with
poly-Si technology. Although there are already products based on
a-Si:H technology for displays and imaging, the reliability issues
associated with OLEDs may yet be different.
[0065] The fabrication processes associated with a-Si:H technology
are standard and adapted from mainstream integrated circuit (IC)
technology, but with capital equipment costs that are much lower.
One of the main advantages of the a-Si:H technology is that it has
become a low cost and well-established technology, while poly-Si
has yet to reach the stage of manufacturability. The technology
also holds great promise for futuristic applications since
deposition of a-Si:H, a-SiN.sub.x:H, and TFT arrays can be achieved
at low temperatures (<120.degree. C.) thus making it amenable to
plastic substrates, which is a critical requirement for
mechanically flexible displays.
[0066] To minimize the conduction induced in all TFTs in the pixel
by the back electrode, an alternate TFT structure based on a
dual-gate structure is employed as shown in FIG. 7A. In the dual
gate TFT (e.g. FIG. 3), a top gate electrode is added to the TFT
structure to prevent the OLED electrodes from biasing the a-Si:H
channel area (FIG. 2). The voltage on the top gate can be chosen
such so as to minimize the charge induced in the (parasitic) top
channel of the TFT. The objective underlying the choice of the
voltage on the top gate is to minimize parasitic capacitance in the
driver circuits and leakage currents in the TFTs so as to enhance
circuit performance. In what follows, the operation of the
dual-gate TFT is described.
[0067] FIG. 3 illustrates the structure of a dual-gate TFT
fabricated for this purpose, wherein reference numerals S and D
denote a source and a drain, respectively. The fabrication steps
are the same as of that of a normal inverted staggered TFT
structure except that it requires a sixth mask for patterning the
top gate. The length of the TFT may be around 30 .mu.m to provide
enough spacing between the source and drain for the top gate. The
width may be made large (e.g. 1600 .mu.m) by interconnecting four
TFTs with W=400 .mu.m (with four of these TFTs) in parallel to
create a sizeable leakage current for measurement. A delay time is
inserted in the measurement of the current to ensure that the
measurement has passed the transient period created by defects in
the a-Si:H active layer, which give rise to a time-dependent
capacitance.
[0068] FIG. 4 shows results of static current measurements for four
cases: first when the top gate is tied to -10V, second when the top
gate is grounded, third when the top gate is floating, and lastly
when the top gate is shorted to the bottom gate. In FIG. 4,
V.sub.tg represents the bias voltage applied to the top gate of the
TFT, and V.sub.bg represents the voltage applied to the bottom gate
of the TFT.
[0069] With a floating top gate, the characteristics are almost
similar to that of a normal single gate TFT. The leakage current is
relatively high particularly when the top gate is biased with a
negative voltage. The lowest values of leakage current are obtained
when the top gate is pegged to either OV or to the voltage of the
bottom gate. In particular, with the latter the performance of the
TFT in the (forward) sub-threshold regime of operation is
significantly improved. This enhancement in sub-threshold
performance can be explained by the forced shift of the effective
conduction path away from the bottom interface to the bulk a-Si:H
region due to the positive bias on the top gate. This in turn
decreases the effect of the trap states at the bottom interface on
the sub-threshold slope of the TFT.
[0070] It is noted that although the addition of another metal
contact as the top gate reduces the leakage current of the TFT, it
may potentially degrade pixel circuit performance by possible
parasitic capacitances introduced by vertically stacking the OLED
pixel. Thus the choice of top gate connection becomes important.
For example, if the top gates in the pixel circuit are connected to
the bottom gates of the associated TFTs, this gives rise to
parasitic capacitances located between the gates and the cathode,
which can lead to undesirable display operation (due to the
charging up of the parasitic capacitance) when the gate driver
drives the TFT switch as illustrated in FIG. 5. On the other hand,
if the top gates are grounded, this results in the parasitic
capacitance being grounded to yield reliable and stable circuit
operation.
[0071] The OLED driver circuits considered here are the
voltage-programmed 2-T driver of FIG. 6A, and the
current-programmed .DELTA.V.sub.T-compensated 5-T version of FIG.
7A. The 5-T driver circuit is a significant variation of the
previous designs, leading to reduced pixel area, reduced leakage,
lower supply voltage, higher linearity (.about.30 dB), and larger
dynamic range (40 dB).
[0072] Before discussing the operation of the 5-T pixel driver
circuit, the operation of the conventional voltage-driven 2-T pixel
driver circuit will be described. FIG. 6A shows a 2-T pixel circuit
including the 2-T pixel driver circuit, an OLED and a capacitor
C.sub.s. The 2-T pixel driver includes two TFTs T.sub.1 and
T.sub.2. FIG. 6B shows input-output timing chart of the 2-T pixel
circuit of FIG. 6A. I.sub.OLED represents the current passing
through the OLED element and transistor T.sub.2.
[0073] Referring to FIGS. 6A and 6B, when the address line is
activated by V.sub.address, the voltage on the data line
(V.sub.data) starts charging capacitor CS and the gate capacitance
of the driver transistor T.sub.2. Depending on the voltage on the
data line, the capacitor charges up to turn the driver transistor
T.sub.2 on, which then starts conducting to drive the OLED with the
appropriate level of current. When the address line is turned off,
T.sub.1 is turned off. However, the voltage at the gate of T.sub.2
remains since the leakage current of T.sub.1 is trivial in
comparison. Hence, the current through the OLED remains unchanged
after the turn off process. The OLED current changes only the next
time around when a different voltage is written into the pixel.
[0074] FIG. 7A illustrates a 5-T pixel circuit having the 5-T pixel
current driver circuit for an OLED display, an OLED, and a
capacitor C.sub.s. The 5-T pixel current driver circuit has five
TFTs T.sub.1-T.sub.5. Unlike the 2-T pixel driver circuit of FIG.
6A, the data that is written into the 5-T pixel in this case is a
current (I.sub.data).
[0075] FIG. 7B shows input-output timing diagrams of the 5-T pixel
circuit of FIG. 7A. Referring to FIGS. 7A and 7B, the address line
voltage V.sub.address, and the data line current I.sub.data are
activated or deactivated simultaneously. When V.sub.address is
activated, it forces T.sub.1 and T.sub.2 to turn on. T.sub.1
immediately starts conducting but T.sub.2 does not since T.sub.3
and T.sub.4 are off. Therefore, the voltages at the drain and
source of T.sub.2 become equal. The current flow through T.sub.1
starts charging the gate capacitor of transistors T.sub.3 and
T.sub.5, like the 2-T driver. The current of these transistors
starts increasing and consequently T.sub.2 starts to conduct
current. Therefore, T.sub.1's share of I.sub.data reduces and
T.sub.2's share of I.sub.data increases. This process continues
until the gate capacitors of T.sub.3 and T.sub.5 charge (via
T.sub.1) to a voltage that forces the current of T.sub.3 to be
I.sub.data. At this time, the current of T.sub.5 is zero and the
entire I.sub.data goes through T.sub.2 and T.sub.3. At the same
time, T.sub.5 drives a current through the OLED, which is ideally
equal to I.sub.data*(W.sub.5/W.sub.3). (W.sub.5/W.sub.3) signifies
a current gain where W.sub.5 represents channel width of T.sub.5,
and W.sub.3 represents channel width of T.sub.3. Now if I.sub.data
and V.sub.address are deactivated, T.sub.2 will turn off, but due
to the presence of capacitances in T.sub.3 and T.sub.5, the current
of these two devices cannot be changed easily, since the
capacitances keep the bias voltages constant. This forces T.sub.4
to conduct the same current as that of T.sub.3, to enable the
driver T.sub.5 to drive the same current into the OLED even when
the write period is over. Writing a new value into the pixel then
changes the current driven into the OLED.
[0076] The result of transient simulation for the 5-T current
driver circuit of FIG. 7A is shown in FIG. 8. As can be seen, the
circuit has a write time of <70 .mu.s, which is acceptable for
most applications. The 5-T current driver circuit does not increase
the required pixel size significantly (FIG. 1), since the sizes of
T.sub.2, T.sub.3, and T.sub.4 are scaled down. This also provides
an internal gain (W.sub.5/W.sub.3=8), which reduces the required
input current to <2 .mu.A for 10 .mu.A OLED current.
[0077] The transfer characteristics for the 2-T and 5-T driver
circuits of FIGS. 6A and 7A are illustrated in FIGS. 9 and 10,
respectively, generated using reliable physically-based TFT models
for both forward and reverse regimes. A much improved linearity
(.about.30 dB) in the transfer characteristics
(I.sub.data/I.sub.OLED) is observed for the 5-T driver circuit due
to the geometrically-defined internal pixel gain as compared to
similar designs. In addition, there are two components (OLED and
T.sub.5) in the high current path, which in turn decreases the
required supply voltage and hence improves the dynamic range.
According to FIG. 10, a good dynamic range (.about.40 dB) is
observed for supply voltage of 20V and drive currents in the range
I.sub.OLED.ltoreq.10 .mu.A, which is realistic for high
brightness.
[0078] FIG. 11 illustrates variation in the OLED current with the
shift in threshold voltage for the 2-T and 5-T driver circuits of
FIGS. 6A and 7A.
[0079] In FIG. 11, the graph having a mark ".box-solid." represents
the OLED current when using the 2-T pixel driver circuit, and the
graph having a mark ".diamond-solid." represents the OLED current
when using the 5-T pixel driver circuit.
[0080] The 5-T current driver circuit compensates for the shift in
threshold voltage particularly when the shift is smaller than 10%
of the supply voltage. This is because the 5-T current driver
circuit is current-programmed. In contrast, the OLED current in the
2-T driver circuit changes significantly with a shift in threshold
voltage. The 5-T current driver circuit described here operates at
much lower supply voltages, has a much larger drive current, and
occupies less area.
[0081] The pixel architectures are compatible to surface (top)
emissive AMOLED displays that enable high on-pixel TFT integration
density for uniformity in OLED drive current and high aperture
ratio. The 5-T driver circuit of FIG. 7A provides on-pixel gain,
high linearity (-30 dB), and high dynamic range (40 dB) at low
supply voltages (15-20V) compared to the similar designs (27V). The
results described here illustrate the feasibility of using a-Si:H
for 3-inch mobile monochrome display applications on both glass and
plastic substrates. With the latter, although the mobility of the
TFT is lower, the size of the drive transistor can be scaled up yet
meeting the requirements on pixel area as depicted in FIG. 1.
[0082] As described above, the TFT may be, but not limited to, a
polysilicon-based TFT. Polysilicon has higher electron and hole
mobilities than amorphous silicon. The hole mobilities are large
enough to allow the fabrication of p-channel TFTs.
[0083] The advantage of having p-channel TFTs is that bottom
emissive OLEDs can be used along with a p-channel drive TFT to make
a good current source. One such circuit is shown in FIG. 12. FIG.
12 illustrates a pixel circuit having a conventional 2-T
polysilicon based pixel current driver circuit. The 2-T polysilicon
based pixel current driver circuit has a p-channel drive TFT. In
FIG. 12, T.sub.1 and T.sub.2 are p-channel TFTs.
[0084] In FIG. 12, the source of the p-type drive TFT is connected
to V.sub.supply. Therefore, Vgs, gate-to-source voltage, and hence
the drive current of the p-type TFT is independent of OLED
characteristics. In other words, the driver shown in FIG. 12
performs as a good current source. Hence, bottom emissive OLEDs are
suitable for use with p-channel drive TFTs, and top emissive OLEDs
are suitable for use with n-channel TFTs.
[0085] The trade-off with using polysilicon is that the process of
making polysilicon TFTs requires much higher temperatures than that
of amorphous silicon. This high temperature-processing requirement
greatly increases the cost, and is not amenable to plastic
substrates. Moreover, polysilicon technology is not as mature and
widely available as amorphous silicon. In contrast, amorphous
silicon is a well-established technology currently used in liquid
crystal displays (LCDs). It is due to these reasons that amorphous
silicon combined with top emissive OLED based circuit designs is
most promising for AMOLED displays.
[0086] Compared to polysilicon TFTs, amorphous silicon TFTs are
n-type and thus are more suitable for top emission circuits as
shown in FIG. 2, and doesn't preclude their use in full colour
bottom emission circuits either. However, amorphous silicon TFTs
have inherent stability problems due to the material structure. In
amorphous silicon circuit design, the biggest hurdle is the
increase in threshold voltage V.sub.th after prolonged gate bias.
This shift is particularly evident in the drive TFT of an OLED
display pixel. This drive TFT is always in the `ON` state, in which
there is a positive voltage at its gate. As a result, its V.sub.th
increases and the drive current decreases based on the
current-voltage equation below:
Ids=(.mu.C.sub.oxW/2L)(V.sub.gs-V.sub.th).sup.2(in Saturation
region)
where Ids represents drain to source current; .mu. represents
mobility; C.sub.ox represents gate capacitance; W represents
channel width; L represents channel length; V.sub.gs represents
gate to source voltage; and V.sub.th represents threshold
voltage.
[0087] In the display, this would mean that the brightness of the
OLED would decrease over time, which is unacceptable. Hence, the
2-T driver circuits as described above are not practical for OLED
displays, as they do not compensate for any increase in
V.sub.th.
[0088] By contrast, the current mirror based pixel current driver
circuit illustrated in FIG. 7A automatically compensates for shifts
in the V.sub.th of the drive TFT in a pixel.
[0089] FIGS. 13-17 illustrate pixel circuits having pixel current
driver circuits in accordance with further embodiments of the
present invention. Each of the pixel circuits shown in FIGS. 13-16
includes a 4-T pixel current driver circuit, an OLED and a
capacitor C.sub.s. The pixel circuit shown in FIG. 17 includes a
pixel current driver circuit, OLEDs, and capacitors C.sub.s. While
the pixel current driver circuits of FIGS. 13-16 are presented for
a monochrome OLED display, the pixel current driver circuits of
FIGS. 13-16 are, however, applicable to a fill color display. The
pixel current driver circuit of FIG. 17 is provided for a full
colour, OLED display.
[0090] The pixel driver circuits of FIGS. 13-17 are current mirror
based pixel driver circuits. All these circuits illustrated in
FIGS. 13-17 have mechanisms that automatically compensate for the
V.sub.th shift of a drive TFT.
[0091] The pixel current driver circuit of FIG. 13 is a
modification of the 5-T pixel driver circuit of FIG. 7A. The 4-T
pixel current driver circuit of FIG. 13 has four TFTs,
T.sub.1-T.sub.4. The 4-T pixel current driver circuit of FIG. 13
compensates for the shift of V.sub.th of T.sub.4. The 4-T pixel
current driver circuit of FIG. 13 occupies a smaller area than that
of the 5-T pixel current driver circuit, and provides a higher
dynamic range. The higher dynamic range allows for a larger signal
swing at the input, which means that the OLED brightness can be
adjusted over a larger range.
[0092] The 4-T pixel current driver circuit of FIG. 14 has four
TFTs, T.sub.1-T.sub.4, and has a lower discharge time. The 4-T
pixel current driver circuit of FIG. 14 compensates for the shift
of V.sub.th of T.sub.4. The advantage of this circuit is that the
discharge time of the capacitor C.sub.s is substantially reduced.
This is because the discharge path has two TFTs (as compared to
three TFTs in the circuit of FIG. 13). The charging time remains
the same. The other advantage is that there is an additional gain
provided by this circuit because T.sub.3 and T.sub.4 do not have
the same source voltages. However, this gain is non-linear and may
not be desirable in some cases.
[0093] The 4-T pixel current driver circuit of FIG. 15 has four
TFTs, T.sub.1-T.sub.4. The 4-T pixel current driver circuit of FIG.
15 compensates for the shift of V.sub.th of T.sub.4. This circuit
does not have the non-linear gain present in the pixel driver
circuit of FIG. 14, since the source terminals of T.sub.3 and
T.sub.4 are at the same voltage. It still maintains the lower
capacitance discharge time, along with the other features of the
circuit of FIG. 9.
[0094] The 4-T pixel current driver circuit of FIG. 16 has four
TFTs, T.sub.1-T.sub.4. The 4-T pixel current driver circuit of FIG.
16 compensates for the shift of V.sub.th of T.sub.4. This circuit
forms the building block for the 3-colour RGB circuit shown in FIG.
17. It also has a low capacitance discharge time and high dynamic
range.
[0095] The full colour circuit shown in FIG. 17 minimizes the area
required by an RGB pixel on a display, while maintaining the
desirable features like threshold voltage shift compensation,
in-pixel current gain, low capacitance discharge time, and high
dynamic range. In FIG. 17, V.sub.blue, V.sub.Green, V.sub.Red
represent control signals for programming the blue, green, and red
pixels, respectively. The pixel current driver circuit of FIG. 17
compensates for the shift of V.sub.th of T.sub.6.
[0096] The circuits described above may be fabricated using normal
inverted staggered TFT structures. The length and width of the thin
film transistors may change depending on the maximum drive current
required by the circuit and the fabrication technology used.
[0097] The pixel current driver circuits of FIGS. 7 and 13-17 use
n-type amorphous silicon TFTs. With the above structure on the
a-Si:H current driver according to the embodiments of the present
invention, the charge induced in the top channel of the TFT is
minimized, and the leakage currents in the TFT is minimized so as
to enhance circuit performance.
[0098] However, polysilicon technology may be applied to the pixel
current driver circuits using p-type or n-type TFTs. These
circuits, when made in polysilicon, can compensate for the spatial
non-uniformity of the threshold voltage. The p-type circuits are
conjugates of the above-mentioned circuits and are suitable for the
bottom emissive pixels.
[0099] In FIGS. 6A, 7A, and 12-17, the TFT having dual gates is
shown, where the dual gate includes a top gate and a bottom gate.
The top gate may be grounded (for example, in FIGS. 6A, 7A and
12-17), or electrically tied to a bottom gate (FIG. 18).
[0100] The dual-gate TFTs are used in the above-mentioned circuits
to enable vertical integration of the OLED layers with minimum
parasitic effects. However, the above-mentioned circuits compensate
for the V.sub.th shift when the circuits comprise single-gate
TFTs.
[0101] FIGS. 19-24 illustrate pixel current driver circuits having
single-gate TFTs. FIGS. 19-24 correspond to FIGS. 7A and 13-17,
respectively.
[0102] For example, the pixel current driver circuit of FIG. 19
contains single-gate TFTs having a switch TFT T.sub.1, a feedback
TFT T.sub.2, a reference TFT T.sub.3, a diode-use TFT T.sub.4, and
a drive TFT T.sub.5. The pixel current driver circuit of FIG. 20
contains single-gate TFTs having a switch TFT T.sub.1, a feedback
TFT T.sub.2, a reference TFT T.sub.3, and a drive TFT T.sub.4. The
pixel current driver circuit of FIG. 22 contains single-gate TFTs
having a feedback TFT T.sub.1, a switch TFT T.sub.2, a reference
TFT T.sub.3, and a drive TFT T.sub.4. The pixel current driver
circuit of FIG. 24 contains single-gate TFTs having switch TFTs
T.sub.1, T.sub.3, T.sub.4, T.sub.5, a feedback TFT T.sub.2, and
drive TFT T.sub.6, T.sub.7, T.sub.8.
[0103] The driving scheme and operation of the pixel driver
circuits of FIGS. 19-24 are same as those of FIGS. 7A and 13-17.
The major difference between the pixel current driver circuit
having dual-gate TFTs and the pixel current driver circuit having
single-gate TFTs is that the pixel current driver circuit having
the dual-gate TFTs utilize a better TFT design which minimizes the
leakage currents in the TFTs, thus enhancing circuit performance.
However, the single-gate TFTs are the standard and preferred design
in industry.
[0104] In FIGS. 19-24, n-type TFTs are shown. However, the pixel
current driver circuits having single-gate TFTs may include p-type
TFTs. Pixel driver circuits with p-type TFTs are shown in FIG.
25-30, where the circuits for FIGS. 25-30 are analogous to those of
FIG. 19-24, respectively.
[0105] With regard to the current driver circuits of FIGS. 19-30
the OLEDs can be either non-inverted or inverted. The four possible
cases are presented in Table 1.
TABLE-US-00001 TABLE 1 Possible OLED connections. Bottom Emission
Top Emission OLED Reduced aperture ratio Large aperture ratio
connected at Regular OLED - Regular OLED - source of transparent
anode transparent cathode. drive TFT OLED current depends on OLED
current depends OLED voltage which on OLED voltage which changes
with aging - changes with aging - undesirable location undesirable
location Safeguards against Safeguards against small small
variation in drive variation in drive current by source current by
source degeneration degeneration OLED Reduced aperture ratio Large
aperture ratio connected at Inverted OLED - Inverted OLED - drain
of drive transparent cathode transparent anode TFT OLED current
independent OLED current of OLED voltage independent of OLED
voltage
[0106] The present invention has been described with regard to one
or more embodiments. However, it will be apparent to persons
skilled in the art that a number of variations and modifications
can be made without departing from the scope of the invention as
defined in the claims.
* * * * *