U.S. patent application number 09/157655 was filed with the patent office on 2001-11-29 for pixel structure for imaging devices.
Invention is credited to DIERICKX, BART.
Application Number | 20010045508 09/157655 |
Document ID | / |
Family ID | 22564682 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010045508 |
Kind Code |
A1 |
DIERICKX, BART |
November 29, 2001 |
PIXEL STRUCTURE FOR IMAGING DEVICES
Abstract
Pixel structures and a read-out method of pixels are disclosed.
The pixel structures and the read-out method improve the image
quality of imaging devices or imaging sensors based on such pixels.
A pixel (10) comprises in a parallel circuit configuration a
radiation sensitive element (1) and an adjustable current source
(2), said current source (2) being adapted for delivering a high
current. A 4-transistor pixel structure is also disclosed. A method
of obtaining a calibrated read-out signal of a pixel having at
least a photosensitive element and a current source comprise a
number of steps. A photocurrent generated on the pixel added to a
current generated by a current source in parallel with the
photosensitive element is read to obtain a first signal. The pixel
is also read with the current source off to obtain a second signal.
The first signal is subtracted from the second signal, and the
resulting signal is amplified to obtain the read-out signal.
Inventors: |
DIERICKX, BART; (MORTSEL,
BE) |
Correspondence
Address: |
KNOBBE MARTENS OLSON AND BEAR
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
926608016
|
Family ID: |
22564682 |
Appl. No.: |
09/157655 |
Filed: |
September 21, 1998 |
Current U.S.
Class: |
250/208.1 ;
348/E3.021 |
Current CPC
Class: |
H04N 5/3651 20130101;
H04N 5/37452 20130101; H04N 5/35518 20130101; H04N 5/359 20130101;
H01L 27/14643 20130101; H04N 5/365 20130101; H04N 5/3745 20130101;
G01J 1/44 20130101; H04N 3/1568 20130101 |
Class at
Publication: |
250/208.1 |
International
Class: |
H01L 027/00 |
Claims
What is claimed is:
1. A pixel comprising a radiation sensitive element and an
adjustable current source in a parallel circuit configuration, said
current source adapted to deliver a high current.
2. A pixel as recited in claim 1, wherein said radiation is
electromagnetic radiation such as light.
3. A pixel as recited in claim 1, wherein said current source is a
transistor.
4. A pixel as recited in claim 1, wherein said current source
comprises a switched capacitor circuit, said circuit comprising a
capacitor, and a switch connected to the capacitor.
5. A pixel as recited in claim 1, wherein said circuit further
comprises at least one impedance element.
6. A pixel as recited in claim 5, wherein said impedance element
comprises a resistor.
7. A pixel as recited in claim 3, further comprising a second
switch in-between said capacitor and said radiation sensitive
element, and the first switch being in a parallel configuration
with said capacitor.
8. A pixel as recited in claim 1, further comprising a first
transistor in series with the photosensitive element and means for
reading out the signal acquired in said photosensitive element and
converted to a voltage drop across said first transistor, and
further comprising a switch between said current source and said
photosensitive element.
9. The pixel of claim 8, wherein said means comprises at least a
second transistor coupled to said photosensitive element and said
first transistor.
10. A method of obtaining a calibrated read-out signal of a pixel
having at least a photosensitive element, the method comprising the
steps of: adding a current generated by a current source in
parallel with said photosensitive element to said photocurrent to a
first signal; reading said first signal; reading said pixel with
said current source off to thereby obtain a second signal; and
subtracting said first signal from said second signal, the
resulting signal being amplified to obtain said read-out
signal.
11. The method as recited in claim 10, wherein the step of
subtracting is executed in a circuit external to said pixel.
12. A method as recited in claim 10, wherein said pixel is a CMOS
based pixel having a load transistor in series with said
photosensitive element.
13. A pixel for imaging applications fabricated in a MOS
technology, said pixel comprising: a photosensitive element and a
first transistor having a gate and a first and second electrode and
being in series with said photosensitive element, said first
transistor and said photosensitive element thereby forming a first
connection; a second transistor having a gate, said second
transistor being coupled to said first connection, thereby forming
a second connection, and said second transistor being part of an
amplifying circuit; and a third transistor having a gate and having
two electrodes, said third transistor being connected in said
second connection between said first connection and said second
transistor.
14. The pixel as recited in claim 13, wherein said gate of said
first transistor is at a first voltage and said first electrode of
said first transistor is at a second voltage, said second electrode
of said first transistor being connected to said photosensitive
element, said gate of said second transistor being connected to
said third transistor.
15. The pixel as recited in claim 14, wherein said gate of said
first transistor is at said first voltage and wherein one of said
electrodes of said third transistor is connected to said gate of
said second transistor and the other of said electrodes is
connected to said first connection.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to solid state imaging devices
being manufactured in a CMOS- or MOS-technology. More particularly,
a novel pixel structure leading to an improved image quality for
the imaging devices is disclosed.
BACKGROUND OF THE INVENTION
[0002] Solid state image sensors are well known. Virtually all
solid-state imaging sensors have as a key element a photosensitive
element being a photoreceptor, a photo-diode, a photo-transistor, a
CCD gate, or alike. Typically, the signal of such a photosensitive
element is a current which is proportional to the amount of
electromagnetic radiation (light) striking the photosensitive
element.
[0003] A structure with a photosensitive element included in a
circuit having accompanying electronics is called a pixel. Such
pixel can be arranged in an array of pixels so as to build focal
plane arrays.
[0004] Commonly such solid state image sensors are implemented in a
CCD-technology or in a CMOS- or MOS-technology. Solid state image
sensors find widespread use in devices such as camera systems. In
this embodiment a matrix of pixels comprising light sensitive
elements constitutes an image sensor, which is mounted in the
camera system. The signal of the matrix is measured and multiplexed
to a so-called video signal.
[0005] Of the image sensors implemented in a CMOS- or
MOS-technology, CMOS or MOS image sensors with passive pixels and
CMOS or MOS image sensors with active pixels are distinguished. An
active pixel is configured with means integrated in the pixel to
amplify the charge that is collected on the light sensitive
element. Passive pixels do not have such means and require a charge
sensitive amplifier that is not integrated in the pixel. For this
reason, active pixel image sensors are potentially less sensitive
to noise fluctuations than passive pixels. Due to the additional
electronics in the active pixel, an active pixel image sensor may
be equipped to execute more sophisticated functions, which can be
advantageous for the performance of the camera system. The
functions can include filtering, operation at higher speed or
operation in more extreme illuminations conditions.
[0006] Examples of such imaging sensors are disclosed in
EP-A-0739039, in EP-A0632930 and in U.S. Pat. No. 5608204. The
imaging devices based on the pixel structures as disclosed in these
patent applications, however, are still subject to deficiencies in
the image quality of the devices.
[0007] A first problem in these CMOS based imaging devices appears
because material imperfections and technology variations have as
effect that there is a nonuniformity in the response of the pixels
in the array. This effect is caused by a nonuniformity or fixed
pattern noise (FPN) or by a photoresponse nonuniformity (PRNU).
Correction of the nonuniformity needs some type of calibration,
e.g., by multiplying or adding/subtracting the pixel's signals with
a correction amount that is pixel-dependent.
[0008] An example of such photoresponse nonuniformity correction
method is disclosed in EP-A-0354106. The method shown in
EP-A-0354106 is subtracting a current delivered by a current source
from the signal acquired in the photosensitive element and only
AC-currents are used in the further signal processing circuits.
[0009] A second problem in these CMOS based imaging devices appears
because the pixel structures as disclosed in EP-A-0739039,
EP-A-0632930 and U.S. Pat. No. 5608204 are sensitive to cross-talk
on the photosensitive element of the pixels. This cross-talk arises
from electronic components, for instance switches, in the
amplifying circuits or amplifying parts of the pixels or being
connected to the pixels. The pulses generated in such switches of
the amplifying circuits or amplifying parts of the pixels can be of
such magnitude that due to cross-talk of these pulses on the
photosensitive elements of the pixels the image quality of the
imaging devices based on this pixel can be significantly degraded.
Specifically the requirement for a direct connection of amplifying
transistor and photosensitive element in the pixel in EP-A-0632930
gives rise this problem.
[0010] Moreover the requirement for the short-circuiting of the
gate and one of the electrodes (the drain in a p-MOS configuration)
of the first transistor in EP-A-0632930, and the corresponding
connection of the gate and the drain electrode to one fixed
potential in order to achieve a logarithmic image conversion
characteristic takes away design freedom in making such pixels and
sensors. Specifically these latter requirements impede achieving
other improved characteristics of the imaging devices than the
logarithmic conversion characteristic of the imaging devices based
on the pixel in EP-A-0632930.
Aims of the Invention
[0011] The present invention aims to achieve pixel structures and a
read-out method of pixels which are able to improve the image
quality of imaging devices based on such pixels.
SUMMARY OF THE INVENTION
[0012] In a first aspect, the present invention is related to a
pixel comprising in a parallel circuit configuration, a radiation
sensitive element and an adjustable current source. In the pixel,
the current source is adapted for delivering a high current. A high
current is a current that is higher than or as high as the current
being generated by radiation, preferably light, impinging on the
radiation sensitive element for standard imaging applications.
Thus, the current source is able to be on in a condition very
similar to the condition of an illumination of the pixel with a
high light intensity thereby perform a calibration for instance of
the FPN or PRNU of the pixel. With the term `in an illumination
condition of the pixel` it is meant that a photocurrent is
generated on the radiation sensitive element.
[0013] In a second aspect, the present invention is also related to
a method of obtaining a calibrated read-out signal of a pixel
having at least a radiation sensitive element, the method
comprising the steps of reading-out a photocurrent generated on the
pixel while adding a current generated by a current source in
parallel with the photosensitive element to the photocurrent to
thereby obtain a first signal, reading the pixel with the current
source off to thereby obtain a second signal, and subtracting the
first signal from the second signal, the resulting signal being
amplified to obtain a read-out signal.
[0014] A method is suggested of calibrating a photosensitive
element such as a photoreceptor or a photodiode in a pixel having a
structure which comprises at least a photosensitive element, a
first transistor in series with the photosensitive element and
means comprising at least a second transistor coupled to the
photosensitive element and the first transistor for reading out the
signal acquired in the photosensitive element and converted to a
voltage drop across the first transistor.
[0015] In this method, a current source is connected in parallel,
possibly along with a switch in series with the current source,
with the photosensitive element. The current source is active in a
condition very similar to the condition of an illumination of the
pixel with a high light intensity thereby performing a calibration
of pixel nonuniformity, for instance of the FPN or PRNU of the
pixel.
[0016] In a third aspect, the present invention is related to a
pixel for imaging applications that is fabricated in a MOS
technology. The pixel comprises a photosensitive element and a
first transistor having a gate and a first and a second electrode
and being in series with the photosensitive element. The first
transistor and the photosensitive element form a first connection.
The pixel further comprises a second transistor having a gate. The
second transistor is coupled to the first connection, thereby
forming a second connection. The second transistor is part of an
amplifying circuit. The pixel further comprises a third transistor
having a gate and two electrodes. The third transistor is in the
second connection between the first connection and the second
transistor. The electrodes referred to above are the drain and
source contacts of the transistors. The gate of the first
transistor can be at a first voltage and the first electrode
(source or drain) of the first transistor can be at a second
voltage.
[0017] In a preferred embodiment of the invention, the second
electrode (drain or source) of the first transistor is connected to
the photosensitive element, and the gate of the second transistor
is connected to one of the electrodes of the third transistor.
According to this embodiment of the invention, the gate of the
third transistor can be at the first voltage and the other
electrode of the electrodes of the third transistor is connected to
the first connection. The first voltage and the second voltage can
be fixed voltages or predetermined voltages or variable voltages.
One of the voltages can be the supply voltage of the imaging device
of which the pixel according to this aspect of the invention can
form part.
[0018] Yet in another embodiment of this aspect of the invention,
the pixel can further comprise an adjustable current source adapted
to deliver a high current. The current source can be in a parallel
configuration to the pixel. BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1a represents an embodiment of a pixel according to a
first aspect of the present invention which permits a calibration
of the photosensitive element present in the pixel structure.
[0020] FIG. 1b represents a alternative embodiment for the pixel
depicted in FIG. 1.
[0021] FIG. 2 represents a graph of a logarithmic pixel output
voltage versus the light intensity when using the method of
calibration of the photosensitive element of the pixel according to
the present invention.
[0022] FIG. 3a represents another embodiment of the pixel according
to the first aspect of the present invention where the calibration
current is given by the discharge of a capacitor.
[0023] FIG. 3b represents a graph of the pixel current versus time
the when performing the method of calibration of the photosensitive
element according to a specific embodiment of the present invention
and using the pixel structure of FIG. 3a.
[0024] FIG. 4 illustrates a pixel structure according to a
preferred embodiment of the third aspect of the invention wherein a
pixel has a four-components (photosensitive element and three
transistors) base structure and wherein the gates of two of the
transistors are at the same voltage. The symbols X and Y refer to
the row and column connections. VDD1 and VDD2 are the voltages
applied to the first electrode of the first transistor and to the
gates of the second and third transistors respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0025] FIG. 1 represents a pixel (10) where the photosensitive
element(s) consist of a photoreceptor (1) which yields a current
proportional to the light intensity. Such a photosensitive element
can also be a photodiode, a photo BJT, a photogate, or a CCD-cell.
The reading of such pixels for a certain light intensity is in fact
the reading of a moderate photo current or charge of the
photoreceptor (1). Such pixels (10) when forming an array often
exhibit a relatively large nonuniformity over the arrays. This
nonuniformity is typically an offset in the output voltage, as
shown in FIG. 2 for a logarithmic response pixel (10) as shown in
FIG. 1. The transfer curves for each pixel do not coincide.
[0026] FIG. 2 represents the output voltage versus the input flux
for a set of logarithmic pixels (10). The curves are parallel, but
have an offset relative to each other. The offset can be determined
by imposing a high current on the photoreceptor (1) while reading
out the photocurrent of the pixels. The signal obtained for each
pixel in this way must be distinguished from the "normal" reading
of the pixel.
[0027] In order to calibrate the pixel (10) nonuniformities, and to
be able to restore the proper value of the photocurrent, a second
reading of the same pixel is done with a known or predetermined
current. The photocurrent is added with a current that originates
from a current source (2). This is an advantageous method as it
does not involve illumination of the device.
[0028] The current source (2) can be of several kinds. For example,
FIG. 1A illustrates an embodiment which provides a known current
with an impedance element (5), such as a resistor, connected to a
known supply voltage. Of course, it is advantageous that this
current source is small in size and precise. Other possible
advantageous implementations are
[0029] a fixed current source, outside the pixel, and common for
part of the imaging array. The source can be connected to several
pixels in turn by switches.
[0030] a MOSFET transistor connected as current source, to be
placed inside each pixel. The current source can be turned on by
applying a certain DC voltage between source and gate. The current
source can be turned off by turning off the gate voltage.
[0031] The current source may be composed of a "switched capacitor"
circuit (see FIG. 3), where the current source is not stable, but
composed of the discharge of at least one capacitor (33). In the
simplest implementation, the current source in the figure is a
capacitor (33) that is discharged on the photodetector node, which
yields a high current during a short time.
[0032] The pixel can further comprise a further transistor (7) in
series with the photosensitive element (1) and means comprising at
least a second transistor (8) coupled to the photosensitive element
(1) and the first transistor (7) for reading out the signal
acquired in the photosensitive element and converted to a voltage
drop across the first transistor (7), and further comprising a
switch (4) between the current source (2) and the photosensitive
element (1).
[0033] FIG. 3 is a schematic view of the implementation of the
current source in a preferred embodiment as a switched capacitor
network. The current is a transient of a discharge of the capacitor
onto the photo diode node (36). Two samples are taken from the
diode node voltage: A1, being the normal signal, and A2 taken
during or after the transient of the discharge. The signal level of
A2 depends only on the height of the discharge current, and not on
the photo current which is smaller. The difference (.DELTA.)
between A1 and A2 is then a measure of the normal sign level which
is free of offset or of PRNU.
[0034] FIG. 4 shows another aspect of the present invention
involving a pixel for imaging applications that is fabricated in a
MOS technology. the pixel comprises a photosensitive element (41)
and a first transistor (47) having a gate and a first and a second
electrode and being in series with the photosensitive element (41).
The first transistor (47) and the photosensitive element (41) form
a first connection or a first node. The pixel further comprises a
second transistor (48) having a gate. The second transistor (48) is
coupled to the first connection, thereby forming a second
connection or a second node. The second transistor is part of an
amplifying circuit. The amplifying circuit can be in the pixel or
can be external to the pixel. The pixel further comprises a third
transistor (49) having a gate and having two electrodes. The third
transistor (49) is in the second connection between the first
connection and the second transistor (48). The electrodes referred
to above are the drain and source contacts of the transistors. The
gate of the first transistor (47) can be at a first voltage VDD2
and the first electrode (source or drain) of the first transistor
can be at a second voltage VDD1. In this embodiment of the
invention, the second electrode (drain or source) of the first
transistor (47) is connected to the photosensitive element (41),
and the gate of the second transistor (48) is connected to one of
the electrodes of the third transistor (49). In a preferred
embodiment the gate of the third transistor (49) can be at the
first voltage and the other of the electrodes of the third
transistor (49) is connected to the first connection. The first
voltage and the second voltage can be fixed voltages or
predetermined voltages or variable voltages. One of the voltages
can be the supply voltage of the imaging device of which the pixel
according to this aspect of the invention can form part. Yet in
another embodiment of this aspect of the invention, the pixel can
further comprise an adjustable current source adapted for
delivering a high current. The current source can be in a parallel
configuration to the pixel.
* * * * *