U.S. patent application number 15/438265 was filed with the patent office on 2017-06-08 for image pickup apparatus having photoelectric conversion function.
The applicant listed for this patent is RENESAS ELECTRONICS CORPORATION. Invention is credited to Katsumi DOSAKA, Zenzo SUZUKI.
Application Number | 20170162615 15/438265 |
Document ID | / |
Family ID | 42677924 |
Filed Date | 2017-06-08 |
United States Patent
Application |
20170162615 |
Kind Code |
A1 |
SUZUKI; Zenzo ; et
al. |
June 8, 2017 |
IMAGE PICKUP APPARATUS HAVING PHOTOELECTRIC CONVERSION FUNCTION
Abstract
An image pickup apparatus that makes it possible to achieve both
high picture quality and a wide dynamic range is provided. Each
pixel unit included in the image pickup apparatus includes: four
photodiodes; four transfer transistors; a charge storage portion
(four floating diffusions) for storing electric charges generated
at the photodiodes; an amplification transistor; a select
transistor; and a reset transistor. The image pickup apparatus
further includes multiple coupling transistors. Each coupling
transistor couples together the charge storage portions of two
pixel units of the pixel units. A scanning circuit switches on or
off the coupling transistors according to read mode.
Inventors: |
SUZUKI; Zenzo; (Tokyo,
JP) ; DOSAKA; Katsumi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RENESAS ELECTRONICS CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
42677924 |
Appl. No.: |
15/438265 |
Filed: |
February 21, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14591871 |
Jan 7, 2015 |
9615042 |
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15438265 |
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13864703 |
Apr 17, 2013 |
8947567 |
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14591871 |
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12700316 |
Feb 4, 2010 |
8441565 |
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13864703 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 27/14609 20130101;
H04N 5/37452 20130101; H04N 5/347 20130101; H04N 5/378 20130101;
H01L 27/14645 20130101; H04N 5/3559 20130101; H04N 5/335 20130101;
H01L 27/14603 20130101; H04N 5/37457 20130101; H01L 27/14641
20130101 |
International
Class: |
H01L 27/146 20060101
H01L027/146; H04N 5/378 20060101 H04N005/378; H04N 5/3745 20060101
H04N005/3745 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2009 |
JP |
2009-053724 |
Claims
1. An image pickup device comprising: a first pixel coupled with an
output signal node, a second pixel coupled with the output signal
node, wherein the first pixel includes: a first photoelectric
conversion element configured to produce first electric charges
corresponding to incident light; a first transfer transistor
configured to transfer the first electric charges; a first floating
diffusion portion configured to store the first electric charges; a
first amplification transistor configured to output a first image
signal based on the first electric charges stored in the first
floating diffusion portion; a first select transistor coupled with
the first amplification transistor, and configured to output the
first image signal to the output signal node; and a first coupling
transistor; wherein the second pixel includes: a second
photoelectric conversion element configured to produce second
electric charges corresponding to incident light; a second transfer
transistor configured to transfer the second electric charges; a
second floating diffusion portion configured to store the second
electric charges; a second amplification transistor configured to
output a second image signal based on the second electric charges
stored in the second floating diffusion portion; a second select
transistor coupled with the second amplification transistor, and
configured to output the second image signal to the output signal
node; and a second coupling transistor; wherein the first coupling
transistor is coupled with the first floating diffusion portion and
the second coupling transistor, wherein the second coupling
transistor is coupled with the second floating diffusion portion
and the first coupling transistor, wherein the first amplification
transistor and the first select transistor have a first impurity
region common thereto, and wherein the second amplification
transistor and the second select transistor have a second impurity
region common thereto.
2. An image pickup device according to claim 1, wherein the first
pixel further includes: a first reset transistor configured to
discharge the first electric charges stored in the first floating
diffusion portion, wherein the second pixel further includes: a
second reset transistor configured to discharge the second electric
charges stored in the second floating diffusion portion, wherein
the first coupling transistor and the first reset transistor have a
first impurity region common thereto, and wherein the second
coupling transistor and the second reset transistor have a second
impurity region common thereto.
3. An image pickup device comprising: a first pixel coupled with an
output signal node, a second pixel coupled with the output signal
node, wherein the first pixel includes: a first photoelectric
conversion element configured to produce first electric charges
corresponding to incident light; a first transfer transistor
configured to transfer the first electric charges; a first floating
diffusion portion configured to store the first electric charges; a
first amplification transistor configured to output a first image
signal based on the first electric charges stored in the first
floating diffusion portion; a first select transistor coupled with
the first amplification transistor, and configured to output the
first image signal to the output signal node; a first active region
over which the first amplification transistor and the first select
transistor are disposed; and a first coupling transistor; and
wherein the second pixel includes: a second photoelectric
conversion element configured to produce second electric charges
corresponding to incident light; a second transfer transistor
configured to transfer the second electric charges; a second
floating diffusion portion configured to store the second electric
charges; a second amplification transistor configured to output a
second image signal based on the second electric charges stored in
the second floating diffusion portion; a second select transistor
coupled with the second amplification transistor, and configured to
output the second image signal to the output signal node; a second
active region over which the second amplification transistor and
the second select transistor are disposed; and a second coupling
transistor; and wherein the first coupling transistor is coupled
with the first floating diffusion portion and the second coupling
transistor, wherein the second coupling transistor is coupled with
the second floating diffusion portion and the first coupling
transistor.
4. An image pickup device according to claim 3, wherein the first
pixel further includes: a first reset transistor configured to
discharge the first electric charges stored in the first floating
diffusion portion; a third active region over which the first
coupling transistor and the first reset transistor are disposed,
wherein the second pixel further includes: a second reset
transistor configured to discharge the second electric charges
stored in the second floating diffusion portion; a fourth active
region over which the second coupling transistor and the second
reset transistor are disposed.
5. An image pickup device according to claim 4, wherein the first
amplification transistor, the first select transistor, the first
coupling transistor and the first reset transistor are MOS
transistors each of which comprises a gate electrode, a source
region and a drain region, wherein the second amplification
transistor, the second select transistor, the second coupling
transistor and the second reset transistor are MOS transistors each
of which comprises a gate electrode, a source region and a drain
region.
6. An image pickup device according to claim 5, wherein the gate
electrode of the first amplification transistor and the gate
electrode of the first select transistor are disposed over the
first active region, wherein the gate electrode of the second
amplification transistor and the gate electrode of the second
select transistor are disposed over the second active region,
wherein the gate electrode of the first coupling transistor and the
gate electrode of the first reset transistor are disposed over the
third active region, and wherein the gate electrode of the second
coupling transistor and the gate electrode of the second reset
transistor are disposed over the fourth active region.
7. An image pickup device according to claim 6, wherein the gate
electrode of the first amplification transistor and the gate
electrode of the first select transistor are overlapped with the
first active region in a plan view, wherein the gate electrode of
the second amplification transistor and the gate electrode of the
second select transistor are overlapped with the second active
region in a plan view, wherein the gate electrode of the first
coupling transistor and the gate electrode of the first reset
transistor are overlapped with the third active region in the plan
view, and wherein the gate electrode of the second coupling
transistor and the gate electrode of the second reset transistor
are overlapped with the fourth active region in the plan view.
8. An image pickup device according to claim 7, wherein the gate
electrode of the first amplification transistor is arranged
separately from the gate electrode of the first select transistor
in the plan view, wherein the gate electrode of the first coupling
transistor is arranged separately from the gate electrode of the
first reset transistor in the plan view, wherein the gate electrode
of the second amplification transistor is arranged separately from
the gate electrode of the second select transistor in the plan
view, and wherein the gate electrode of the second coupling
transistor is arranged separately from the gate electrode of the
second reset transistor in the plan view.
9. An image pickup device according to claim 8, wherein no other
gate electrode of another MOS transistor is disposed between the
gate electrode of the first amplification transistor and the gate
electrode of the first select transistor, wherein no other gate
electrode of another MOS transistor is disposed between the gate
electrode of the first coupling transistor and the gate electrode
of the first reset transistor, wherein no other gate electrode of
another MOS transistor is disposed between the gate electrode of
the second amplification transistor and the gate electrode of the
second select transistor, and wherein no other gate electrode of
another MOS transistor is disposed between the gate electrode of
the second coupling transistor and the gate electrode of the second
reset transistor.
10. An image pickup device according to claim 9, wherein the drain
region of the first coupling transistor is coupled to the drain
region of the second coupling transistor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/591,871, filed Jan. 7, 2015, which is a
continuation of U.S. patent application Ser. No. 13/864,703, filed
Apr. 17, 2013, now U.S. Pat. No. 8,947,567 which issued on Feb. 3,
2015, which is a continuation of U.S. patent application Ser. No.
12/700,316, filed Feb. 4, 2010, now U.S. Pat. No. 8,441,565, which
issued on May 14, 2013, which in turn claims the benefit of
Japanese Patent Application No. 2009-53724 filed on Mar. 6, 2009
including the specification, drawings and Abstract is incorporated
herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to an image pickup apparatus
having a photoelectric conversion function.
[0003] Image sensors of CCD (Charge Coupled Device), CMOS
(Complementary Metal-Oxide Semiconductor), and the like are
required that their dynamic range should be widened with high
sensitivity and a high S/N (Signal-to-Noise) ratio maintained. As
image pickup apparatuses whose dynamic range can be widened, for
example, the following technologies are known.
[0004] In the technology disclosed in Japanese Unexamined Patent
Publication No. 2006-217410 (Patent Document 1), each pixel of an
image pickup apparatus includes the following: a photodiode, a
transfer transistor, a floating diffusion (floating area), an
overflow gate, a storage capacitor, a reset transistor, a storage
transistor, an amplification transistor, and a select transistor.
(Refer to FIG. 1 in the document.)
[0005] The photodiode receives light and generates and stores
photocharges. The transfer transistor is provided adjacently to the
photodiode and transfers photocharges generated at the photodiode.
The floating diffusion is coupled to the photodiode through the
transfer transistor. The overflow gate is provided adjacently to
the photodiode to transfer photocharges overflowing the photodiode
in storage operation. The storage capacitor stores photocharges
overflowing the photodiode in storage operation through the
overflow gate. The reset transistor is so formed that it is coupled
to the floating diffusion and discharges signal charges in the
storage capacitor and the floating diffusion. The storage
transistor is provided between the floating diffusion and the
storage capacitor. The amplification transistor reads as voltage
the signal charges of the floating diffusion or the signal charges
of the floating diffusion and storage capacitor. The select
transistor is coupled to the amplification transistor and selects a
pixel or a pixel block.
[0006] In the technology disclosed in Japanese Unexamined Patent
Publication No. 2006-245522 (Patent Document 2), each pixel of an
image pickup apparatus includes the following: a photodiode, a
transfer transistor, a floating diffusion, first and second storage
capacitors, a reset transistor, first and second storage
transistors, an amplification transistor, and a select transistor.
(Refer to FIG. 1 in the document.)
[0007] The photodiode receives light and generates photocharges.
The transfer transistor is provided adjacently to the photodiode
and transfers photocharges generated at the photodiode. The
floating diffusion is so provided that it is coupled to the
photodiode through the transfer transistor. The first and second
storage capacitors store photocharges overflowing the photodiode in
exposure storage operation through the transfer transistor. The
reset transistor is so formed that it is coupled to the first
storage capacitor and discharges signal charges in the first
storage capacitor, second storage capacitor, and floating
diffusion. The first storage transistor is provided between the
floating diffusion and the first storage capacitor. The second
storage transistor is provided between the first storage capacitor
and the second storage capacitor. The amplification transistor
reads as voltage the signal charges of the floating diffusion, the
signal charges of the floating diffusion and the first storage
capacitor, or the signal charges of the floating diffusion, first
storage capacitor, and second storage capacitor. The select
transistor is coupled to the amplification transistor and selects a
pixel or a pixel block.
PRIOR ART DOCUMENTS
Patent Documents
[Patent Document 1]
[0008] Japanese Unexamined Patent Publication No. 2006-217410
[Patent Document 2]
[0009] Japanese Unexamined Patent Publication No. 2006-245522
SUMMARY OF THE INVENTION
[0010] In the image pickup apparatuses described in the above
documents, it is switched by a transistor switch whether to couple
a capacitive element added to each pixel to a floating diffusion. A
dynamic range is adjusted based on whether or not a capacitive
element is coupled as mentioned above.
[0011] In these technologies, however, the percentage of the area
occupied by a photodiode in a pixel is reduced as compared with
conventional cases because of the addition of the capacitive
element and it is suspected to be difficult to maintain high
picture quality. Therefore, these technologies are unsuitable for
applications such as digital cameras though they may be suitable
for applications such as surveillance cameras and in-vehicle
cameras in which dynamic range takes precedence over picture
quality. In applications such as digital cameras, it is required to
maintain high picture quality and further widen a dynamic
range.
[0012] Therefore, it is an object of the invention to provide an
image pickup apparatus that makes it possible to achieve both high
picture quality and a wide dynamic range.
[0013] In sum, the invention is an image pickup apparatus including
multiple pixel units, multiple coupling transistors, and a scanning
circuit. Each of the pixel units includes multiple photoelectric
conversion elements, multiple transfer transistors, and a charge
storage portion.
[0014] In each of the pixel units, each photoelectric conversion
element generates electric charges corresponding to incident light.
The transfer transistors respectively correspond to the
photoelectric conversion elements. Each transfer transistor
transfers electric charges generated at the corresponding
photoelectric conversion element. The charge storage portion is
coupled with the photoelectric conversion elements through the
transfer transistors and stores electric charges generated at each
of the photoelectric conversion elements.
[0015] In the image pickup apparatus, each of the coupling
transistors couples together the charge storage portions in two
pixel unit of the multiple pixel units. In this case, each pixel
unit is coupled with at least one of the coupling transistors. The
scanning circuit switches on or off the transfer transistors and
coupling transistors included in each of the pixel units.
[0016] According to the invention, the following processing is
carried out when electric charges generated at each photoelectric
conversion element are read: a coupling transistor is switched on
or off according to light exposure. This makes it possible to vary
the capacity of each charge storage portion and thus the dynamic
range can be widened. The capacity of each charge storage portion
can be made variable by adding one or less junction transistors per
pixel unit; therefore, it is possible to maintain high picture
quality without scarifying the light receiving area of each
photoelectric conversion element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a block diagram illustrating the configuration of
an image pickup apparatus 1 in a first embodiment of the
invention;
[0018] FIG. 2 is an equivalent circuit diagram illustrating one
column in the pixel array portion 10 in FIG. 1;
[0019] FIG. 3 is a timing chart explaining the read operation of
the image pickup apparatus 1 in normal read mode;
[0020] FIG. 4 is a timing chart explaining the read operation of
the image pickup apparatus 1 in lower sensitivity read mode;
[0021] FIG. 5 is a timing chart explaining the read operation of
the image pickup apparatus 1 in two-pixel mixture read mode;
[0022] FIG. 6 is a timing chart explaining the read operation of
the image pickup apparatus 1 in three-pixel mixture read mode;
[0023] FIG. 7 is a plan view schematically illustrating the layout
of a pixel array portion 10;
[0024] FIG. 8 is a plan view schematically illustrating the layout
of a pixel array portion 10A; and
[0025] FIG. 9 is a block diagram schematically illustrating the
configuration of a digital still camera 200 using the image pickup
apparatus 1 in the first and second embodiments of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Hereafter, detailed description will be given to embodiments
of the invention with reference to the drawings. The same or
equivalent parts will be marked with the same reference numerals
and the description thereof will not be repeated.
First Embodiment
Configuration of Image Pickup Apparatus 1
[0027] FIG. 1 is a block diagram illustrating the configuration of
an image pickup apparatus 1 in a first embodiment of the
invention.
[0028] As illustrated in FIG. 1, the image pickup apparatus 1
includes a pixel array portion 10, a vertical scanning circuit 11,
a horizontal scanning circuit 12, multiple control signal lines tx,
rst, sel, sw, and multiple output signal lines vout.
[0029] The pixel array portion 10 includes a photodiode array PDA
(photoelectric conversion element array) comprised of multiple
photodiodes PD (photoelectric conversion elements) arranged in a
matrix pattern. In FIG. 1, a photodiode array PDA of 10 rows and
four columns is shown for the sake of simplicity. Specifically,
photodiodes PD in the first column to the fourth column are shown
from left to right in FIG. 1 and photodiodes PD in the first row to
the 10th row are shown from bottom up in FIG. 1. The left and right
direction in FIG. 1 will be designated as X direction, row
direction, or horizontal direction and the top and bottom direction
in FIG. 1 will be designated as Y direction, column direction, or
vertical direction. When the orientations (+side, -side) along each
direction are discriminated, they will be discriminated by affixing
a sign like +Y direction and -Y direction.
[0030] When the image pickup apparatus 1 is used for color images,
a color filter is provided over the light receiving surface of each
photodiode PD. In the common Bayer arrangement of color filters,
the color filters are arranged as illustrated in FIG. 1. That is,
the color filters in green color (G) that are large in the
percentage of contribution to luminance signals are arranged in a
checkered pattern; and the color filters in red color (R) and blue
color (B) are arranged in a checkered pattern in the remaining
areas.
[0031] In the photodiode array PDA, every four photodiodes
successively arranged in each column comprise one group. The four
photodiodes PD in each group comprise a pixel unit PU together with
multiple transistors. (These transistors are the transfer
transistors TX, amplification transistors AMI, select transistors
SEL, and reset transistors RST in FIG. 2.) The pixel array portion
10 operates by on a pixel unit PU-by-pixel unit PU basis.
[0032] The pixel array portion 10 further includes multiple
coupling transistors SW. Each coupling transistor SW is provided
between pixel units PU adjoining to each other in the column
direction. When electric charges generated in each photodiode PD
are read, each coupling transistor SW is switched on or off
according to the read mode of the image pickup apparatus 1.
Hereafter, description will be given to the configuration and
operation of each pixel unit PU.
[0033] FIG. 2 is an equivalent circuit diagram of a circuit
equivalent to one column of the pixel array portion 10 in FIG. 1.
The pixel units PU coupled to the output signal lines vout1 to
vout4 are identical in electrical circuitry. In FIG. 2, therefore,
the pixel units PU1, PU2 are coupled to an arbitrary output signal
line vout. In this specification, components of the same kind are
discriminated by suffixing a number to their reference numerals,
like the photodiodes PD1 to PD8. When the components of the same
kind are generically referred to or any unspecified thing is
referred to, their reference numerals are indicated without
suffixing a number.
[0034] The pixel unit PU1 includes: four photodiodes PD1 to PD4,
four transfer transistors TX1 to TX4, four floating diffusions
(also referred to as floating diffusion portions) FD1 to FD4, one
reset transistor RST1, one amplification transistor AMI1, and one
select transistor SEL1. The pixel unit PU1 further includes a metal
wiring FDL1 that electrically couples the floating diffusions FD1
to FD4 together. The floating diffusions FD1 to FD4 and the metal
wiring FDL1 forma charge storage portion FDU1 that stores electric
charges generated in each photodiode PD.
[0035] The configuration of the pixel unit PU2 is identical. That
is, the pixel unit PU2 includes: four photodiodes PD5 to PD8, four
transfer transistors TX5 to TX8, four floating diffusions FD5 to
FD8, one reset transistor RST2, one amplification transistor AMI2,
one select transistor SEL2, and a metal wiring FDL2. The floating
diffusions FD5 to FD8 and the metal wiring FDL2 form a charge
storage portion FDU2.
[0036] FIG. 2 depicts an arbitrary column. When a column is
discriminated from the others, a character representing a column
number is added. In this case, characters a, b, c, . . .
respectively represent first column, second column, third column, .
. . For example, the metal wiring layers FDL in the first column
are described as FDL1a, FDL2a, FDL3a, and the metal wiring layers
FDL in the second column are described as FDL1b, FDL2b, FDL3b, . .
. (Refer to FIG. 7.)
[0037] The photodiodes PD1 to PD4 are photoelectric conversion
elements that generate electric charges (electrons) according to
received light. The generated electric charges are stored in the
n-type impurity regions (diffusion regions) of the p-n junction
diodes. The p-type impurity regions as the anodes of the
photodiodes PD are grounded.
[0038] The floating diffusions FD1 to FD4 are n-type impurity
regions respectively provided in correspondence with the
photodiodes PD1 to PD4. The impurity concentration of the floating
diffusions FD is higher than that of the n-type impurity regions
(also referred to as n layers) of the photodiodes PD. The floating
diffusions FD1 to FD4 are respectively coupled with the cathodes (n
layers) of the corresponding photodiodes PD through the transfer
transistors TX1 to TX4 as NMOS (N-channel Metal-Oxide
Semiconductor) transistors.
[0039] The floating diffusions FD1 to FD4 operate as charge storage
portions that store electric charges generated in the photodiodes
PD1 to PD4. In this case, first, high potential (supply voltage) is
applied to each floating diffusion FD and the electric charges are
thereby all drawn and they are depleted. When the transfer
transistors TX between them and the photodiodes PD are thereafter
turned on, the electric charges stored in the n layers of the
photodiodes PD are transferred to the floating diffusions FD. At
this time, the floating diffusions FD having a higher impurity
concentration are higher in depletion potential than the
photodiodes PD. Therefore, the electric charges generated in the n
layers of the photodiodes PD are all transferred to the floating
diffusions FD.
[0040] In case of the example in FIG. 2, the floating diffusions
FD1 to FD4 are coupled with one another through the metal wiring
FDL1. Therefore, the floating diffusions FD1 to FD4 are brought to
substantially the same potential. The whole of the floating
diffusions FD1 to FD4 and the metal wiring FDL1 forms the charge
storage portion FDU1 common to the individual photodiodes PD.
[0041] The amplification transistor AMI1 as an NMOS transistor
functions as a source follower circuit. The gate of the
amplification transistor AMI1 is coupled to the floating diffusions
FD1 to FD4 through the metal wiring FDL1 and its drain is coupled
to a power supply wiring VDD. The source of the amplification
transistor AMI1 is coupled with an output signal line vout through
the select transistor SEL1 as an NMOS transistor.
[0042] When the select transistor SEL1 is on, the amplification
transistor AMI1 outputs signal voltage corresponding to the
potential of the floating diffusions FD1 to FD4 to the output
signal line Vout. The potential of the floating diffusions FD1 to
FD4 is determined according to the number of electric charges
transferred from the respective photodiodes PD. At the above time,
therefore, the output voltage of the amplification transistor AMI1
is linearly varied according to the amount of light received of
each photodiode PD.
[0043] The reset transistor RST1 as an NMOS transistor is coupled
between the power supply wiring VDD and the metal wiring FDL1. The
reset transistor RST1 is provided to discharge the electric charges
stored in the floating diffusions FD1 to FD4.
[0044] As mentioned above, each pixel unit PU includes the four
photodiodes PD and the seven transistors comprised of transfer TX,
amplification AMI, select SEL, and reset RST. Therefore, 1.75
transistors are provided per photodiode PD and thus the above
configuration of the pixel unit PU is generally designated as 1.75
transistor architecture. The transistors of amplification AMI,
select SEL, and reset RST are shared among the four photodiodes
PD.
[0045] The pixel array portion 10 in FIG. 1 further includes
multiple coupling transistors SW as NMOS transistors. Each coupling
transistor SW is provided between pixel units PU adjoining to each
other in the column direction and couples together the respective
charge storage portions FDU of these pixel units PU. In FIG. 2,
specifically, a coupling transistor SW1 coupling together the
charge storage portion FDU1 of the pixel unit PU1 and the charge
storage portion FDU2 of the pixel unit PU2 is depicted. In FIG. 2,
further, a coupling transistor SW2 coupling together the charge
storage portion FDU2 of the pixel unit PU2 and the charge storage
portion of the pixel unit next to the pixel unit PU2 in the +Y
direction is depicted.
[0046] When electric charges generated in each photodiode PD are
read, the coupling transistors SW are switched on or off according
to the read mode of the image pickup apparatus 1. As described
later, the read modes include, in addition to normal read mode,
lower sensitivity read mode in which read operation is carried out
with reduced sensitivity and mixture read mode in which the pixels
in the same color are mixed when read operation is carried out.
[0047] It is generally thought that the sensitivity of each pixel
corresponding to one photodiode PD is determined by the product of
three values. A first value is quantum efficiency indicating to how
many electrons one photon incident upon a photodiode PD is
converted. A second value is conversion gain that refers to the
ratio of variation in the potential of a photodiode PD to variation
in the potential of a floating diffusion FD. A third value is the
gain of an amplification transistor AMI. In general, when the
structure of a pixel is determined, these values become constant.
In the image pickup apparatus 1 in the first embodiment, the
conversion gain, or the second value, is made variable by providing
the coupling transistors SW that couple together charge storage
portions FDU adjoining to each other.
[0048] To control the turn-on/off of the transfer transistors TX,
select transistors SEL, reset transistors RST, and coupling
transistors SW, the control signal lines tx, sel, rst, sw are
coupled to the gate electrode of each transistor. More specific
description will be given. The control signal lines tx1 to tx8 are
respectively coupled to the gates of the transfer transistors TX1
to TX8. The control signal lines sell, sell are respectively
coupled to the gates of the select transistors SEL1, SEL2. The
control signal lines rst1, rst2 are respectively coupled to the
gates of the reset transistors RST1, RST2. The control signal lines
sw1, sw2 are respectively coupled to the gates of the coupling
transistors SW1, SW2.
[0049] Supplementary explanation will be given to the overall
configuration of the image pickup apparatus 1 with reference to
FIG. 1 again. The image pickup apparatus 1 includes the control
signal lines tx, rst, sel, sw extended in the row direction and the
output signal lines vout extended in the column direction. The
control signal lines tx, rst, sel, sw couple together the vertical
scanning circuit 11 and the pixel array portion 10 and the output
signal lines vout couple together the horizontal scanning circuit
12 and the pixel array portion 10.
[0050] The control signal lines tx for the transfer transistors TX
are provided individually in correspondence with the rows of the
photodiode array PDA. Each control signal line tx is provided in
common to the multiple photodiodes PD comprising the corresponding
row. In FIG. 1, the control signal lines tx1 to tx10 respectively
provided in correspondence with first row to the 10th row of the
pixel array portion 10 are depicted.
[0051] The output signal lines vout are provided to read output
voltage corresponding to signal charges generated at each
photodiode PD. The output signal lines vout are provided in the
order of the columns individually in correspondence with the
columns of the photodiode array PDA. Each output signal line vout
is coupled to the select transistor SEL of each pixel unit PU
provided in the corresponding column. In FIG. 1, the output signal
lines vout1 to vout4 provided in correspondence with the first
column to the fourth column are depicted.
[0052] The control signal lines sel for the select transistors SEL
include control signals line sel1o, sel2o, . . . for odd-numbered
columns and control signal lines sel1e, sel2e, . . . for
even-numbered columns. (The odd-numbered columns and the
even-numbered columns are discriminated from each other by
suffixing a character of o or e to their reference numerals .) Each
of the control signal lines sel1o, sel2o, . . . for odd-numbered
columns is coupled with the select transistor SEL of the pixel unit
PU provided in the corresponding odd-numbered column. Each of the
control signal lines sel1o, sel2o, . . . for even-numbered columns
is coupled with the select transistor SEL of the pixel unit PU
provided in the corresponding even-numbered column. The pixel units
PU arranged in the row direction share a control signal line sel
among them.
[0053] The control signal lines rst for the reset transistors RST
include control signal lines rst1o, rst2o, . . . for odd-numbered
columns and control signal lines rst1e, rst2e, . . . for
even-numbered columns. The control signal lines rst1o, rst2o, . . .
for odd-numbered columns are coupled with the reset transistors RST
of the pixel units PU provided in the respective odd-numbered
columns. The control signal lines rst1e, rst2e, . . . for
even-numbered columns are coupled with the reset transistors RST of
the pixel units PU provided in the respective even-numbered
columns. The pixel units PU arranged in the row direction share a
control signal line rst among them.
[0054] The control signal lines sw for the coupling transistors SW
include control signal lines sw1o, sw2o, odd-numbered columns and
control signal line sw1e, sw2e, . . . for even-numbered columns.
Each of the control signal lines sw1o, sw2o, . . . for odd-numbered
columns is coupled with the coupling transistor SW provided between
pixel units PU adjoining to each other in the corresponding
odd-numbered column. Each of the control signal lines sw1e, sw2e,
for even-numbered columns is coupled with the coupling transistor
SW provided between pixel units PU adjoining to each other in the
corresponding even-numbered column. The coupling transistors SW
arranged in the row direction share a control signal line sw among
them.
[0055] The vertical scanning circuit 11 sequentially switches the
voltage of the control signal lines tx, rst, sel to the H level or
the L level. At this time, the vertical scanning circuit 11
switches the control signal lines sw to the H level or the L level
according to the read mode. As a result, the voltage corresponding
to the amount of electric charges stored in each row of the
photodiode array PDA is outputted to an output signal line vout
with respect to each row of the photodiode array PDA. The
horizontal scanning circuit 12 sequentially reads voltages
outputted to the output signal lines vout and thereby detects the
amount of light received of each photodiode PD.
Read Operation of Image Pickup Apparatus 1--Normal Read Mode)
[0056] Concrete description will be given to the procedure for
reading the signal charges of each photodiode PD in the image
pickup apparatus 1. First, description will be given to normal read
mode in which the electric charges generated in each photodiode PD
are individually read on a photodiode PD-by-photodiode PD
basis.
[0057] FIG. 3 is a timing chart explaining the read operation of
the image pickup apparatus 1 in normal read mode. FIG. 3 indicates
the following from top down: the opened/closed state of a
mechanical shutter, the voltage waveforms of the control signal
lines tx1 to tx8, rst1, rst2, sw1, sw2, sell, sel2 in FIG. 2, and
the voltage waveform of an output signal line vout. In normal read
mode, the voltages of the control signal lines sw1, sw2 are at the
L level in every period of time and the coupling transistors SW1,
SW2 are kept off. Hereafter, description will be given to the
procedure for reading the signal charges of each photodiode PD in
the pixel units PU1, PU2 in order of time with reference to FIG. 2
and FIG. 3.
[0058] At time t1, the voltages of the control signal lines tx1 to
tx8, rst1, rst2 are at the H level and the voltages of the control
signal lines sell, sel2 are at the L level. Therefore, the transfer
transistors TX1 to TX8 and the reset transistors RST1, RST2 are on.
As a result, the electric charges in the n layers of the
photodiodes PD1 to PD8 and the floating diffusions FD1 to F8 are
all drawn and they are depleted. When the voltages of the control
signal lines tx1 to tx8 are thereafter returned to the L level, the
transfer transistors TX1 to TX8 are turned off.
[0059] At time t2, subsequently, the mechanical shutter is opened.
While the shutter is open, electric charges are generated at the
photodiodes PD1 to PD8 by incident light and stored in the n layers
of the photodiodes PD1 to PD8.
[0060] At time t3 after the shutter is closed, the voltages of the
control signal lines rst1, rst2 are at the L level; the voltage of
the control signal line sel1 is at the H level; and the voltage of
the control signal line sel2 is at the L level. As a result, the
reset transistors RST1, RST2 are turned off and the select
transistor SEL1 is turned on. Therefore, output voltage
corresponding to the depletion potential of the floating diffusions
FD1 to FD4 (the potential of the charge storage portion FDU1) is
outputted to an output signal line vout. The horizontal scanning
circuit 12 in the FIG. 1 detects the voltage of the output signal
line vout at this time as dark level output.
[0061] At time t4, subsequently, the voltage of the control signal
line tx1 is brought to the H level and thus the transfer transistor
TX1 is turned on. As a result, the electric charges of the
photodiode PD1 are transferred to the floating diffusions FD1 to
FD4. The potential of the floating diffusions FD1 to FD4 is changed
to a value corresponding to the number of electric charges
transferred from the photodiode PD1 and in conjunction therewith,
the voltage of the output signal line vout is varied.
[0062] At time t5 after the voltage of the control signal line tx1
is returned to the L level, the horizontal scanning circuit 12 in
FIG. 1 detects the voltage of the output signal line vout at this
time. The difference from the dark level output detected at time t3
becomes the light reception signal of the photodiode PD1.
[0063] At time t6, subsequently, the voltage of the control signal
line rst1 is at the H level and the voltage of the control signal
line sel1 is at the L level. As a result, the electric charges of
the floating diffusions FD1 to F4 are all drawn and they are
initialized. This terminates the readout of signal charges obtained
by receiving light at the photodiode PD1 and preparation for the
readout of signal charges of the next photodiode PD2 is
completed.
[0064] During the period from time t6 to time t7, subsequently, the
same process as during the period from time t3 to time t6 is
carried out on the photodiode PD2 and the signal charges generated
at the photodiode PD2 are read. During the period from time t7 to
time t8, similarly, the signal charges generated at the photodiode
PD3 are read and during the period from time t8 to time t9, the
signal charges obtained by receiving light at the photodiode PD4
are read. This completes the readout of the pixel unit PU1.
[0065] At time t9, the voltage of the control signal line rst1 is
at the L level and the voltage of the control signal line rst2 is
at the H level. Further, the voltages of the control signal lines
sel1, sel2 are at the L level. As a result, the reset transistor
RST2 is turned on and the electric charges of the floating
diffusions FD5 to FD8 are all drawn and they are initialized. This
completes preparation for reading the signal charges of the
photodiode PD5 of the next pixel unit PU2.
[0066] At time t10, subsequently, the voltage of the control signal
line sel2 is brought to the H level and a dark level signal of the
floating diffusions FD5 to FD8 (charge storage portion FDU2) is
thereby outputted. At time t11, thereafter, the control signal line
tx5 is brought to the H level and the transfer transistor TX5 is
thereby turned on. As a result, the electric charges of the
photodiode PD5 are transferred to the floating diffusions FD5 to
FD8. At time t12, subsequently, voltage corresponding to the
transferred electric charges is read through the output signal line
vout.
[0067] The same read operation is performed during the period from
t13 to time t14, during the period from time t14 to time t15, and
during the period from time t15 to time t16. The signal charges
generated in the photodiodes PD6 to PD8 are thereby sequentially
read. This completes the readout of the pixel unit PU2.
Read Operation of Image Pickup Apparatus 1--Lower Sensitivity Read
Mode
[0068] Description will be given to lower sensitivity read mode in
which readout is carried out with the sensitivity of the image
pickup apparatus 1 reduced.
[0069] In general, image sensors for digital cameras are required
to have a wide range of ISO sensitivity. At this time, it is
required to obtain high-resolution picture quality in a low ISO
sensitivity range within which light exposure is high. Further, it
is required to increase an S/N ratio as much as possible to obtain
an image with less noise in a high ISO sensitivity range within
which light exposure is low.
[0070] To improve an S/N ratio in a high ISO sensitivity range,
increasing photoelectric conversion efficiency is effective. If
this is done, however, the signal level is early saturated in a low
ISO sensitivity range (overexposure becomes excessive in images)
and the dynamic range is narrowed.
[0071] Consequently, the following measure is taken in the image
pickup apparatus 1 in lower sensitivity read mode: in a low ISO
sensitivity range within which light exposure is high, the coupling
transistor SW provided between a pixel unit PU provided with a
photodiode PD to be read and the pixel unit PU adjacent thereto is
turned on. This increases the capacity of the charge storage
portion FDU; therefore, the saturation of signal level in the low
ISO sensitivity range can be prevented.
[0072] FIG. 4 is a timing chart explaining the read operation of
the image pickup apparatus 1 in lower sensitivity read mode. The
vertical axis in FIG. 4 indicates the same items as that in FIG. 3.
Hereafter, description will be given to the procedure for reading
the signal charges of each photodiode PD in the pixel units PU1,
PU2 in order of time with reference to FIG. 2 and FIG. 4. The
operations at time t1 and time t2 are the same as those in FIG. 3
and the description thereof will not be repeated.
[0073] At time t3 after the shutter is closed, the voltages of the
control signal lines rst1, rst2 are at the L level; the voltages of
the control signal lines sw1, sell are at the H level; and the
voltages of the control signal lines sw2, sel2 are at the L level.
Therefore, the reset transistors RST1, RST2 are off and the
coupling transistor SW1 and the select transistor SEL1 are on. As a
result, the charge storage portions FDU1 and FDU2 are coupled
together by the coupling transistor SW1 in the on state. Therefore,
output voltage corresponding to the depletion potential of the
floating diffusions FD1 to FD8 (the potential of the entire charge
storage portions FDU1 and FDU2) is outputted to the output signal
line vout. The horizontal scanning circuit 12 in FIG. 1 detects the
voltage of the output signal line vout at this time as dark level
output.
[0074] At time t4, subsequently, the voltage of the control signal
line tx1 is brought to the H level and thus the transfer transistor
TX1 is turned on. As a result, the electric charges of the
photodiode PD1 are transferred to the floating diffusion FD1 to FD8
(the entire charge storage portions FDU1 and FDU2). The potential
of the floating diffusions FD1 to FD8 is changed to a value
corresponding to the number of electric charges transferred from
the photodiode PD1 and in conjunction therewith, the voltage of the
output signal line vout is varied. At this time, the capacity of
the charge storage portions FDU is approximately twice that in
normal read mode; therefore, the potential of the charge storage
portions FDU is substantially half of that in normal read mode.
Consequently, the saturation of the output voltage vout can be
prevented.
[0075] At time t5 after the voltage of the control signal line tx1
is returned to the L level, the horizontal scanning circuit 12 in
FIG. 1 detects the voltage of the output signal line vout at this
time. The difference from the dark level output detected at time t3
becomes the light reception signal of the photodiode PD1.
[0076] At time t6, subsequently, the voltages of the control signal
lines rst1, rst2 are at the H level and the voltages of the control
signal lines sw1, sw2, sell, sel2 are at the L level. As a result,
the electric charges of the floating diffusions FD1 to FD8 are all
drawn and they are initialized. This terminates the readout of
signal charges obtained by receiving light at the photodiode PD1 in
lower sensitivity read mode and preparation for the readout of
signal charges of the next photodiode PD2 in lower sensitivity read
mode is completed.
[0077] During the period from time t6 to time t7, subsequently, the
same process as during the period from time t3 to time t6 is
carried out on the photodiode PD2 and the signal charges generated
at the photodiode PD2 are read. During the period from time t7 to
time t8, similarly, the signal charges generated at the photodiode
PD3 are read and during the period from time t8 to time t9, the
signal charges obtained by receiving light at the photodiode PD4
are read. This completes the readout of the pixel unit PU1 in lower
sensitivity read mode.
[0078] Also with respect to the pixel unit PU2, the signal charges
of the photodiodes PD5 to PD8 are similarly read when the voltage
of the control signal line sw1 is at the H level (the coupling
transistor SW1 is on). That is, the same read operation as in the
pixel unit PU1 is carried out during the following periods: the
period from time t9 to time t10, the period from time t10 to time
t11, the period from time t11 to time t12, and the period from time
t12 to time t13. As a result, the signal charges of the photodiodes
PD5 to PD8 are read. This completes the readout of the pixel unit
PU2 in lower sensitivity read mode.
Read Operation of Image Pickup Apparatus 1--Two-Pixel Mixture Read
Mode
[0079] Description will be given to two-pixel mixture read mode in
which two pixels in the same color are mixed when read operation is
performed.
[0080] The number of pixels of each recent digital still camera has
been significantly increased and that of some cameras exceeds 10
million (10 M) pixels. When a digital camera is used to take not
only freeze-frame pictures but also moving video pictures, however,
a problem arises. When all the 10 million (10 M) pixels are used to
take a moving video picture, the number of pixels is too large and
this is not practical. This is apparent from the fact that even the
full-HD (Full High Definition) digital television is approximately
two million (2 M) pixels in the number of pixels.
[0081] When it is required to carry out high-speed readout as in
taking a moving video picture as mentioned above, pixel mixture
readout is an effective means. In the pixel mixture readout, the
electric charges generated in multiple pixels (photodiodes PD) in
the same color are mixed when they are read. The pixel mixture
readout is an effective technique also to enhance an S/N ratio to
obtain an image with less noise in a high ISO sensitivity range
within which the readout light exposure is low.
[0082] Hereafter, description will be given to two-pixel mixture
read mode. In this case, the following operation is performed, for
example, at each pixel unit PU in the first column in FIG. 1: two
pixels in green color (G) or red color (R) are mixed and read. At
each pixel unit PU in the second column, two pixels in green color
(G) or blue color (B) are mixed and read. Therefore, when two-pixel
mixture readout is carried out with the 1.75 transistor
architecture, it can be carried out with each coupling transistor
SW kept off.
[0083] FIG. 5 is a timing chart explaining the read operation of
the image pickup apparatus 1 in two-pixel mixture read mode. The
vertical axis in FIG. 5 indicates the same items as that in FIG. 3.
In two-pixel mixture read mode, the voltages of the control signal
lines sw1, sw2 are at the L level in every period of time and the
coupling transistors SW1, SW2 are kept off. Hereafter, description
will be given to the procedure for reading the signal charges of
each photodiode PD in the pixel units PU1, PU2 in order of time
with reference to FIG. 2 and FIG. 5. The operation during the
period from time t1 to time t3 is the same as that in FIG. 3 and
the description thereof will not be repeated.
[0084] At time t4, the voltages of the control signal lines tx1,
tx3 are brought to the H level and thus the transfer transistors
TX1, tx3 are turned on. As a result, the mixed electric charges of
the photodiodes PD1, PD3 are transferred to the floating diffusions
FD1 to FD4. (In case of odd-numbered columns in FIG. 1, the
photodiodes PD1, PD3 correspond to red color (R); and in case of
even-numbered columns, the photodiodes PD1, PD3 correspond to green
color (G).) The potential of the floating diffusions FD1 to FD4 is
changed to a value corresponding to the number of electric charges
transferred from the photodiodes PD1, PD3 and in conjunction
therewith, the voltage of the output signal line vout is
varied.
[0085] At time t5 after the voltages of the control signal lines
tx1, tx3 are returned to the L level, the horizontal scanning
circuit 12 in FIG. 1 detects the voltage of the output signal line
vout. The difference from the dark level output detected at time t3
becomes a signals arising from the mixed electric charges of the
photodiodes PD1, PD3.
[0086] At time t6, subsequently, the voltages of the control signal
lines rst1, rst2 are at the H level and the voltages of the control
signal lines sell, sell are at the L level. As a result, the
electric charges of the floating diffusions FD1 to FD8 are all
drawn and they are initialized. This terminates the readout of
signal charges obtained by receiving light at the photodiodes PD1,
PD3 and preparation for the readout of signal charges of the next
photodiodes PD2, PD4 is completed. (In case of odd-numbered columns
in FIG. 1, the photodiodes PD2, PD4 correspond to green color (G);
and in case of even-numbered columns, the photodiodes PD2, PD4
correspond to blue color (B).)
[0087] During the period from time t6 to time t10, subsequently,
the same process as during the period from time t3 to time t6 is
carried out on the photodiodes PD2, PD4 and the signal charges
generated at the photodiodes PD2, PD4 are read. More specific
description will be given. At time t7, the voltage of the control
signal line sell is at the H level and a dark level voltage signal
is outputted to the output signal line vout. At time t8, the
control signal lines tx2, tx4 are brought to the H level.
Therefore, the mixed electric charges of the electric charges
generated at the photodiodes PD2, PD4 are transferred to the
floating diffusions FD1 to FD4 (charge storage portion FDU1). At
time t9 after the control signal lines tx2, tx4 are returned to the
L level, the voltage of the output signal line vout corresponding
to the mixed electric charges of the photodiodes PD2, PD4 is
detected by the horizontal scanning circuit 12 in FIG. 1. The
difference from the dark level output detected at time t7 becomes a
signal arising from the mixed electric charges of the photodiodes
PD2, PD4. This completes the readout of the signal charges of the
pixel unit PU1 in two-pixel mixture mode.
[0088] Subsequently, the similar operation is performed. That is,
during the period from time t10 to time t11, the voltage of the
control signal line sel2 is at the H level and the voltages of the
control signal lines tx5, tx7 are brought to the H level. As a
result, voltage corresponding to the mixed electric charges of the
photodiodes PD5, PD7 is outputted to the output signal line vout.
During the period from time t11 to time t12, voltage corresponding
to the mixed electric charges of the photodiodes PD6, PD8 is
outputted to the output signal line vout. This completes the
readout of the signal charges of the pixel unit PU2 in two-pixel
mixture mode.
Read Operation of Image Pickup Apparatus 1--Three-Pixel Mixture
Read Mode
[0089] Description will be given to three-pixel mixture read mode
in which three pixels in the same color are mixed when read
operation is performed.
[0090] In case of three-pixel mixture readout, the following
operation is performed, for example, at the pixel units PU in the
first column in FIG. 1: three pixels in green color (G) or red
color (R) are mixed and read. At the pixel units PU in the second
column, three pixels in green color (G) or blue color (B) are mixed
and read. Therefore, when three-pixel mixture readout is carried
out with the 1.75 transistor architecture, it is required to turn
on the coupling transistor SW between pixel units PU adjoining to
each other.
[0091] FIG. 6 is a timing chart explaining the read operation of
the image pickup apparatus 1 in three-pixel mixture read mode. The
vertical axis in FIG. 6 indicates the same items as that in FIG. 3.
Hereafter, description will be given to the procedure for reading
the signal charges of each photodiode PD in the pixel units PU1,
PU2 in order of time with reference to FIG. 2 and FIG. 6. The
operation during the period from time t1 to time t3 is the same as
that in FIG. 4 and the description thereof will not be
repeated.
[0092] At time t4, the voltages of the control signal lines tx1,
tx3, tx5 are brought to the H level and thus the transfer
transistors TX1, TX3, TX5 are turned on. Since the coupling
transistor SW1 is on, the mixed electric charges of the photodiodes
PD1, PD3, PD5 are transferred to the floating diffusions FD1 to FD8
(the entire charge storage portions FDU1 and FDU2). (In case of
odd-numbered columns in FIG. 1, the photodiodes PD1, PD3, PD5
correspond to red color (R); and in case of even-numbered columns,
the photodiodes PD1, PD3, PD5 correspond to green color (G).) The
potential of the floating diffusions FD1 to FD8 is changed to a
value corresponding to the number of electric charges transferred
from the photodiodes PD1, PD3, PD5 and in conjunction therewith,
the voltage of the output signal line vout is varied.
[0093] At time t5 after the voltages of the control signal lines
tx1, tx3, tx5 are returned to the L level, the horizontal scanning
circuit 12 in FIG. 1 detects the voltage of the output signal line
vout at this time. The difference from the dark level output
detected at time t3 becomes a signal arising from the mixed
electric charges of the photodiodes PD1, PD3, PD5.
[0094] At time t6, subsequently, the voltages of the control signal
lines rst1, rst2 are at the H level and the voltages of the control
signal lines sw1, sw2, sel1, sel2 are at the L level. As a result,
the electric charges of the floating diffusions FD1 to FD8 are all
drawn and they are initialized. This terminates the readout of
signal charges obtained by receiving light at the photodiodes PD1,
PD3, PD5 and preparation for the readout of signal charges of the
next photodiodes PD2, PD4, PD6 is completed. (In case of
odd-numbered columns in FIG. 1, the photodiodes PD2, PD4, PD6
correspond to green color (G); and in case of even-numbered
columns, the photodiodes PD2, PD4, PD6 correspond to blue color
(B).)
[0095] During the period from time t6 to time t10, subsequently,
the same process as during the period from time t3 to time t6 is
carried out on the photodiodes PD2, PD4, PD6 and signal charges
generated at the photodiodes PD2, PD4, PD6 are read. More specific
description will be given. At time t7, the voltages of the control
signal lines sw1, sell are at the H level and a dark level voltage
signal is outputted to the output signal line vout. At time t8, the
control signal lines tx2, tx4, tx6 are brought to the H level.
Therefore, the mixed electric charges of the electric charges
generated at the photodiodes PD2, PD4, PD6 are transferred to the
floating diffusions FD1 to FD8 (the entire charge storage portions
FDU1 and FDU2). At time t9 after the control signal lines tx2, tx4,
tx6 are returned to the L level, the voltage of the output signal
line vout corresponding to the mixed electric charges of the
photodiodes PD2, PD4, PD6 is detected by the horizontal scanning
circuit 12 in FIG. 1. The difference from the dark level output
detected at time t7 becomes a signal arising from the mixed
electric charges of the photodiodes PD2, PD4, PD6.
[0096] At time t10, subsequently, the voltages of the control
signal lines rst1, rst2 are at the H level and the voltages of the
control signal lines sw1, sw2, sell, sel2 are at the L level. As a
result, the electric charges of the floating diffusions FD1 to FD8
are all drawn and they are initialized. This terminates the readout
of signal charges obtained by receiving light at the photodiodes
PD2, PD4, PD6 and preparation for the readout of signal charges of
three pixels of the next photodiode PD7 and the following diodes.
(In case of odd-numbered columns in FIG. 1, the three pixels
correspond to red color (R); and in case of even-numbered columns,
the three pixels correspond to green color (G).)
[0097] During the period from time t10 to time t11, the mixed
electric charges of three pixels of the photodiode PD7 and the
following diodes are read. At this time, the voltage of the control
signal line tx2, in place of the control signal line tx1, is
brought to the H level to carry out three-pixel mixture. As a
result, the coupling transistor SW2 is turned on.
[0098] The above mentioned mixture readout of pixels in the same
color is not limited to two-pixel or three-pixel and more pixels
can be read.
Layout of Pixel Array Portion 10
[0099] Description will be given to the layout of the photodiodes
PD, floating diffusions FD, and transistors AMI, SEL, RST, SW in
the pixel array portion 10.
[0100] FIG. 7 is a plan view schematically illustrating the layout
of the pixel array portion 10. FIG. 7 depicts the arrangement of
the photodiodes PD in the first columns to the third column (C1 to
C3) and in the fourth row to the ninth row (R4 to R9) in the pixel
array portion 10 in FIG. 1. In FIG. 7, a numeral representing a row
number and a character representing a column number are suffixed to
the reference numeral of each photodiode PD. The characters a, b, c
. . . respectively represent first column, second column, third
column . . . .
[0101] Further, FIG. 7 depicts the arrangement of the floating
diffusions FD, transfer transistors TX, amplification transistors
AMI, select transistors SEL, reset transistor RST, and coupling
transistors SW related to these photodiodes PD. The areas of the
floating diffusions FD and the areas of the gate electrodes of the
transfer transistors TX are hatched for the sake of simplicity of
the drawing.
[0102] Further, the FIG. 7 also depicts the relation of coupling
between each transistor TX, SEL, RST, SW and the control signal
lines tx, sel, rst and output signal line vout. In the drawing, the
position of a contact hole CH at each point of coupling is
indicated. The position of a contact hole (indicated by reference
mark VDD) coupled with a power supply wiring is also indicated.
[0103] In FIG. 7, further, the areas where a pixel unit PU is
provided are defined by broken line. As already described, each
pixel unit PU includes four photodiodes PD successively arranged in
the column direction. In FIG. 7, the positions of three pixel units
PU2a, PU2b, PU2c are indicated.
[0104] The pixel units PU are discriminated from one another by
suffixing a character representing a column and a numeral
representing their order in the respective columns to their
reference numerals. (The characters a, b, c, . . . respectively
correspond to first column, second column, third column, For
example, the pixel units PU in the first column are discriminated
from one another by sequentially adding reference marks from the
lower side (-Y direction side) of the drawing, like PU1a, PU2a,
PU3a, . . . .
[0105] Hereafter, description will be sequentially given to the
arrangement of each component in FIG. 7. The photodiodes PD are
arranged over a semiconductor substrate at equal intervals both in
the column direction and in the row direction. This is intended to
match the optical center of each pixel with the center of the
corresponding photodiode PD.
[0106] The floating diffusions FD individually correspond to the
photodiodes PD and are provided in a position adjacent to the
corresponding photodiode PD in the +Y direction. The transfer
transistors TX are provided between a photodiode PD and a floating
diffusion FD corresponding to each other.
[0107] In each row of the photodiode array PDA, the following
measure is taken: in every two photodiodes PD, the corresponding
floating diffusions FD comprise a pair (reference mark FDP)
(referred to as floating diffusion pair, floating diffusion portion
pair, or FD pair). For example, the floating diffusions FD
corresponding to the photodiodes PD4a, PD4b comprise an FD pair
(FDP). In FIG. 7, similarly, the floating diffusions FD
corresponding to each of sets of (PD5b, PD5c), (PD6a, PD6b), (PD7b,
PD7c), (PD8a, PD8b), and (PD9b, PD9c) comprise an FD pair
(FDP).
[0108] As illustrated in FIG. 7, the floating diffusions FD
comprising an FD pair (FDP) are placed in proximity to each other.
In other words, the interval between two floating diffusions FD
respectively corresponding two photodiodes PD adjoining to each
other in the row direction is shorter in cases where they comprise
an FD pair (FDP) than in cases where they do not comprise an FD
pair (FDP).
[0109] The interval between floating diffusions FD that do not
comprise an FD pair (FDP) in the row direction can be lengthened by
the above configuration. In the pixel array portion 10, the areas
between floating diffusions FD that do not comprise an FD pair
(FDP) are used as transistor areas TRA (TRA1, TRA2) for placing the
transistors AMI, SEL, RST, SW. The width of each transistor area
TRA in the row direction is equivalent to the interval between
adjoining floating diffusions FD that do not comprise an FD pair
(FDP). Its width in the column direction is equivalent to the
interval between rows of the photodiode array PDA adjoining to each
other.
[0110] An area for the photodiodes PD enough to take in a
sufficient amount of light received can be ensured by increasing
the empty areas to place the transistors AMI, SEL, RST as mentioned
above. As a result, the light receiving efficiency of the image
pickup apparatus can be enhanced.
[0111] Further, the pixel array portion 10 in FIG. 7 is
characterized in that as viewed in the column direction, the
transfer transistors TX in each column are arranged in a staggered
pattern. In other words, the FD pairs (FDPs) are arranged in a
staggered pattern. That is, in adjoining rows of the photodiode
array PDA, corresponding floating diffusions FD comprise an FD pair
(FDP) every two photodiodes PD shifted by one column. In case of
the example in FIG. 7, specifically, the FD pairs are comprised as
described below. In the fourth, sixth, and eighth rows (R4, R6,
R8), an FD pair (FDP) is comprised of floating diffusions FD
corresponding to photodiodes PD in the first and second columns
(C1, C2). In the fifth, seventh, and ninth rows (R5, R7, R9),
meanwhile, an FD pair (FDP) is comprised of floating diffusions FD
corresponding to photodiodes PD in the second and third columns
(C2, C3).
[0112] As a result, it is possible to enhance the symmetrical
property of the shape of the active region of each pixel including
a photodiode PD and a floating diffusion FD corresponding thereto.
In case of the pixel array portion 10 in FIG. 7, that is, the
following is understood: the active regions of pixels adjoining to
each other in an oblique direction are in the relation of
translational symmetry; and the active regions of pixels adjoining
to each other in the row direction are in the relation of line
symmetry.
[0113] As already described, the following measure is taken in the
Bayer arrangement of color filters: the color filters in green
color (G) are arranged in a checkered pattern and the color filters
in red color (R) and in blue color (B) are arranged in a checkered
pattern in the remaining areas. In case of the example in FIG. 7,
the shape of the active regions of pixels in green color (G)
obliquely adjoining to each other is in the relation of
translational symmetry. Therefore, it is possible to minimize the
signal output difference between photodiodes PD for green color
obliquely adjoining to each other. As a result, it is possible to
reduce variation in characteristics from pixel to pixel.
[0114] Concrete description will be given to the arrangement of
transistors AMI, SEL, RST, XT in transistor areas TRA (TRA1, TRA2).
In each transistor area TRA, transistors AMI, SEL, RST, XT are
placed two by two. At this time, the amplification transistor AMI
and select transistor SEL corresponding to an identical pixel unit
PU are placed in an identical first transistor area TRA1. The
amplification transistor AMI and the select transistor SEL are
coupled in series with the respective gate length directions
substantially matched with the row direction and provided in a
first transistor area TRA1. The reset transistor RST in each pixel
unit PU is combined with the coupling transistor SW provided
between it and the adjacent pixel unit PU and placed in a second
transistor area TRA2. (The transistor areas are classified into
first and second transistor areas TRA1, TRA2 according to the type
of a transistor placed there.)
[0115] Description will be given with, for example, the pixel unit
PU2a taken as representative. The amplification transistor AMI and
select transistor SEL corresponding to the pixel unit PU2a are
provided in the transistor area TRA1 adjoining to the photodiode
PD7a, which is one of the photodiodes PD comprising the pixel unit
PU2a, in the +Y direction. The amplification transistor AMI is
provided on the side closer to the floating diffusion FD
corresponding to the photodiode PD7a and the select transistor SEL
is provided on the side farther from the same. The source region of
the amplification transistor AMI and the drain region of the select
transistor SEL are integrated with each other. This makes it
possible to reduce a space required for arranging the
transistors.
[0116] In case of the pixel unit PU2a, the reset transistor RST is
provided in the transistor area TRA2 adjoining to the photodiode
PD9a in the +Y direction. When it is provided, the reset transistor
RST is combined with the coupling transistor SW coupling together
the metal wiring FDL2a and the adjacent metal wiring FDL3a. The
reset transistor RST and the coupling transistor SW are also series
coupled and placed with their gate length directions substantially
matched with the row direction. The source region of the reset
transistor RST and the drain region of the coupling transistor SW
are integrated with each other. This makes it possible to reduce a
space required for arranging the transistors. The shape of the
active regions of the reset transistor RST and coupling transistor
SW is substantially identical with the shape of the active regions
of the amplification transistor AMI and select transistor SEL.
[0117] When the whole of FIG. 7 is viewed, the first transistor
area TRA1 where the amplification transistor AMI and the select
transistor SEL are provided is placed in the following position: a
position adjoining to the photodiodes PD in the sixth row (R6) and
in the seventh row (R7) in the +Y direction. Meanwhile, the second
transistor area TRA2 where the reset transistor RST and the
coupling transistor SW are provided is placed in the following
position: a position adjoining to the photodiodes PD in the fourth
row (R4), fifth row (R5), eighth row (R8), and ninth row (R9) in
the +Y direction. That is, a row in which the first transistor area
TRA1 is placed in a position adjoining in the +Y direction and a
row in which the second transistor area TRA2 is placed alternately
occur every two rows of the pixel array portion 10.
[0118] Wiring of the control signal lines sel, rst, sw is
facilitated by placing the transistors AMI, SEL, RST, SW as
mentioned above. More specific description will be given. With
respect to the fourth row (R4), fifth row (R5), eighth row (R8),
and ninth row (R9) in which the second transistor area TRA2 is
placed adjacently in the +Y direction, the following measure is
taken: the control signal lines sw, rst are so wired that they run
in proximity to the second transistor area TRA2. With respect to
the sixth row (R6) and the seventh row (R7) in which the first
transistor area TRA1 is placed adjacently in the +Y direction, the
following measure is taken: the control signal line sel is so wired
that it runs in proximity to the first transistor area TRA1.
[0119] The control signal lines tx (tx4 to tx9) are wired along in
the row direction so that they run in proximity to the gate
electrodes of the transfer transistors TX corresponding to
photodiodes PD in each row. Each control signal line sel is coupled
with the gate electrode of a select transistor SEL through a
contact hole CH.
[0120] The output signal lines vout (vout1 to vout3), except the
endmost output signal line vout, are provided in intercolumn areas.
The intercolumn areas are areas where a photodiode is not provided
between a column and a column adjoining to each other in the
photodiode array PDA. Each output signal line vout is coupled with
the source region of the select transistor SEL comprising each
pixel unit PU provided in a corresponding column.
[0121] Each pixel unit PU is further provided with a metal wiring
FDL for coupling floating diffusions FD. In case of the pixel unit
PU2a in FIG. 7, for example, the metal wiring FDL2a for coupling
the floating diffusions FD respectively corresponding to the
photodiodes PD5a, PD6a, PD7a, PD8a is provided. The metal wiring
FDL2a is also coupled with the source region of the corresponding
reset transistor RST and the gate electrode layer of the
amplification transistor AMI through a contact hole CH. (The above
source region also functions as the drain region of a coupling
transistor SW coupling adjoining pixel units PU together.)
[0122] Similarly, the pixel unit PU1a in the first column is
provided with the metal wiring FDL1a and the pixel unit PU3a is
provided with the metal wiring FDL3a. The pixel units PU1b, PU2b, .
. . in the second column are respectively provided with the metal
wirings FDL1b, FDL2b, . . . and the pixel units PU1c, PU2c, . . .
in the third column are respectively provided with the metal
wirings FDL1c, FDL2c, . . . .
[0123] According to the image pickup apparatus 1 in the first
embodiment, as described up to this point, the following can be
implemented by adding a twist to the transistor arrangement in the
pixel array portion 10: it is possible to place coupling
transistors SW coupling together the charge storage portions FDU of
pixel units PU adjoining to each other in the column direction with
substantially no area penalty. Use of this coupling transistor SW
makes it possible to obtain images with high picture quality over a
wide ISO sensitivity range. More specific description will be
given. In a low ISO sensitivity range within which light exposure
is high, the saturation of signal level can be prevented by lower
sensitivity readout in which a coupling transistor SW provided
between adjacent pixel units PU is turned on. In a high ISO
sensitivity range within which light exposure is low, an S/N ratio
can be enhanced by two-pixel mixture readout. Further, when a
moving image is recorded, the readout speed can be enhanced by
three or more-pixel mixture readout.
Second Embodiment
[0124] FIG. 8 is a plan view schematically illustrating the layout
of a pixel array portion 10A. The layout of the pixel array portion
10A in FIG. 8 is a modification to the layout of the pixel array
portion 10 in the first embodiment illustrated in FIG. 7. FIG. 8
depicts photodiodes PD in the first and second columns (C1, C2) and
in the fifth row to the eighth row (R5 to R8) of the photodiode
array PDA. The photodiodes PD are arranged over a semiconductor
substrate at equal intervals both in the column direction and in
the row direction. In the following description, the same or
equivalent parts as in FIG. 7 will be marked with the same
reference marks and the description thereof may not be
repeated.
[0125] In the pixel array portion 10A, four photodiodes PD in each
column of the photodiode array PDA comprise a group and each group
corresponds to a pixel unit PU. In case of the example in FIG. 8,
specifically, the pixel unit PU2a includes the photodiodes PD5a,
PD6a, PD7a, PD8a and the pixel unit PU2b includes the photodiodes
PD5b, PD6b, PD7b, PD8b.
[0126] The floating diffusions FD individually correspond to the
photodiodes PD and each floating diffusion FD is provided in a
position adjacent to the corresponding photodiode PD in the +X
direction. Each transfer transistor TX is provided between a
photodiode PD and a floating diffusion FD corresponding to each
other.
[0127] The transistors AMI, SEL, RST, SW are provided in an
interrow area GPA that is an area where a photodiode PD is not
provided between a row and a row of the photodiode array PDA
adjoining to each other. At this time, the amplification transistor
AMI and the select transistor SEL included in an identical pixel
unit PU are arranged in an identical interrow area GPA so that they
share an impurity region between them. It is more desirable that
the directions of the respective gate lengths of the amplification
transistor AMI and select transistor SEL should be substantially
matched with the row direction.
[0128] The coupling transistors SW are provided in interrow areas
GPA where neither the amplification transistor AMI nor the select
transistor SEL is provided at intervals of four rows. The reset
transistors RST are provided in interrow areas GPA where the
amplification transistor AMI, select transistor SEL, or coupling
transistor SW is not provided.
[0129] The control signal lines rst, sel, sw extended in the row
direction are wired in an interrow area GPA so that they run
substantially over the respective corresponding transistors RST,
SEL, SW. The control signal lines tx are provided in correspondence
with the individual rows of the photodiode array PDA and one
control signal line tx is provided for one interrow area GPA.
[0130] The output signal lines vout extended in the column
direction, except the endmost output signal line vout, are provided
one by one in proximity to intercolumn areas GPB. The intercolumn
areas GPB are areas where a photodiode PD is not provided between a
column and a column of the photodiode array PDA adjoining to each
other.
[0131] Each pixel unit PU is further provided with a metal wiring
FDL for coupling floating diffusions FD. Each metal wiring FDL is
coupled with the source region of the corresponding reset
transistor RST and the gate electrode layer of the amplification
transistor AMI through a contact hole CH.
[0132] In case of the pixel array portion 10 in the first
embodiment, there is the following difference between the pixel
units PU provided in the odd-numbered columns of the photodiode
array PDA and the pixel units PU provided in the even-numbered
columns: they are different in the control signal lines to which
the respective transistors SEL, RST, SW are coupled. In case of the
pixel array portion 10A in the second embodiment, meanwhile, the
following measure is taken with respect to the pixel units PU
arranged in the row direction: only one control signal line is
coupled to each of the transistors SEL, RST, SW. Therefore, the
number of the control signal lines rst, sel, sw can be reduced as
compared with the first embodiment.
Example of Application of Image Pickup Apparatus in First or Second
Embodiment to Camera
[0133] FIG. 9 is a block diagram schematically illustrating the
configuration of a digital still camera 200 using an image pickup
apparatus 1 in the first or second embodiment of the invention.
[0134] As illustrated in FIG. 9, the digital still camera 200
includes: the above image pickup apparatus 1; an image pickup lens
201 as an imaging optical system for providing an image of a
subject on the pixel array portion 10, 10A of this image pickup
apparatus 1; and a signal processing circuit 202 that processes the
output signals of the image pickup apparatus 1. The digital still
camera 200 can obtain picture signals with high picture quality
over a wide dynamic range by using the above image pickup apparatus
1. The above application is not limited to the digital still camera
200 and the same effect can be obtained by using the above image
pickup apparatus 1 for any other image pickup system such as a
digital video camera.
[0135] It should be understood that the embodiments disclosed in
this application are just examples in every respect and are not
limitative. The scope of this invention is indicated not by the
above description but by claims. It is intended to include all the
modifications within the meaning and scope equivalent to
claims.
* * * * *