U.S. patent application number 09/879615 was filed with the patent office on 2002-01-03 for image sensor.
This patent application is currently assigned to NEC Corporation. Invention is credited to Kurosawa, Susumu, Muramatsu, Yoshinori, Nagata, Tsuyoshi, Nakashiba, Yasutaka, Ohkubo, Hiroaki.
Application Number | 20020000508 09/879615 |
Document ID | / |
Family ID | 26593934 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000508 |
Kind Code |
A1 |
Muramatsu, Yoshinori ; et
al. |
January 3, 2002 |
Image sensor
Abstract
The image sensor of the present invention performs two exposures
of differing exposure times, holds the signal charge that is
generated in photodiode 1 in the first exposure period in pixel
interior capacitance 4 that is provided inside pixels and
integrates the signal charge that is generated in photodiode 1 in
the second exposure period with the first signal charge inside the
pixels and executes readout, whereby the white (overexposed)
portions that occur in the first exposure period are compensated by
information of the second exposure period, and black (underexposed)
portions that occur in the second exposure period are compensated
by information of the first exposure period, and an image is
obtained having wide dynamic range with respect to the amount of
light in which underexposure and overexposure are mitigated.
Inventors: |
Muramatsu, Yoshinori;
(Tokyo, JP) ; Kurosawa, Susumu; (Tokyo, JP)
; Ohkubo, Hiroaki; (Tokyo, JP) ; Nagata,
Tsuyoshi; (Tokyo, JP) ; Nakashiba, Yasutaka;
(Tokyo, JP) |
Correspondence
Address: |
Paul J. Esatto, Jr.
Scully, Scott, Murphy & Presser
400 Garden City Plaza
Garden City
NY
11530
US
|
Assignee: |
NEC Corporation
7-1, Shiba 5-chome
Tokyo
JP
|
Family ID: |
26593934 |
Appl. No.: |
09/879615 |
Filed: |
June 12, 2001 |
Current U.S.
Class: |
250/208.1 ;
348/E3.018 |
Current CPC
Class: |
H04N 5/374 20130101;
H04N 5/35581 20130101; H01L 27/14601 20130101; H04N 5/37452
20130101 |
Class at
Publication: |
250/208.1 |
International
Class: |
H01L 027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2000 |
JP |
2000-178666 |
Sep 5, 2000 |
JP |
2000-268824 |
Claims
What is claimed is:
1. An image sensor that includes a semiconductor device having a
semiconductor region and a diffusion layer formed within said
semiconductor region having the opposite conductivity of said
semiconductor region, that, after discharging carrier in said
diffusion layer of said semiconductor device, causes light to be
irradiated into said diffusion layer to generate carrier inside
said diffusion layer, that outputs a signal to an output section
based on the surface potential of the generated carrier, and
measures the amount of incidence of said light; comprising: a
timing generation means for creating: a first exposure period for,
when irradiating light into said diffusion layer and generating
carrier inside said diffusion layer, irradiating said light into
said diffusion layer and generating a first carrier inside said
diffusion layer; a storage period after said first exposure period
for moving said first carrier to a storage section; a second
exposure period after said storage period for irradiating said
light into said diffusion layer and generating a second carrier
inside said diffusion layer; and a readout period after said second
exposure period; and a carrier integration means for, when
outputting to an output section a signal based on the surface
potential of carrier that is generated by said timing generation
means and measuring the amount of incidence of said light,
integrating said first carrier and said second carrier in said
readout period and reading out the integrated carrier.
2. An image sensor according to claim 1 wherein: said first
exposure period generates said first carrier inside said diffusion
layer with said diffusion layer and said storage section in a
conducting state; and said storage period moves said first carrier
to said storage section by cutting off said diffusion layer and
said storage section.
3. An image sensor according to claim 1 wherein: said first
exposure period generates said first carrier inside said diffusion
layer with said diffusion layer and said storage section in a cut
off state; and said storage period moves said first carrier to said
storage section by conduction of said diffusion layer and said
storage section.
4. An image sensor that includes a semiconductor device having a
semiconductor region and a diffusion layer formed within said
semiconductor region having the opposite conductivity of said
semiconductor region; that, after discharging carrier in said
diffusion layer of said semiconductor device, causes light to be
irradiated into said diffusion layer to generate carrier inside
said diffusion layer, that outputs a signal to an output section
based on the surface potential of the generated carrier, and that
measures the amount of incidence of said light; comprising: a
timing generation means for creating: a first exposure period for,
when irradiating light into said diffusion layer and generating
carrier inside said diffusion layer, irradiating said light into
said diffusion layer and generating a first carrier inside said
diffusion layer; a storage period after said first exposure period
for moving a portion of said first carrier to a storage section and
leaving said first carrier inside said diffusion layer; a second
exposure period after said storage period for irradiating said
light into said diffusion layer and generating a second carrier
inside said diffusion layer; and a readout period after said second
exposure period; and a carrier integration means for, when
outputting to an output section a signal based on the surface
potential of said generated carrier and measuring the amount of
incidence of said light, reading out carrier that is the sum of
said second carrier and said first carrier that is left in said
diffusion layer during a readout period.
5. An image sensor according to claim 1 wherein carrier that is
contained in said diffusion layer and said storage section is
discharged before said first exposure period by a reset transistor
that is included in said semiconductor region and connected to the
power supply.
6. An image sensor according to claim 4 wherein carrier that is
contained in said diffusion layer and said storage section is
discharged before said first exposure period by a reset transistor
that is included in said semiconductor region and connected to the
power supply.
7. An image sensor according to claim 1 wherein the period from
said first exposure period to said second exposure period is
positioned within a preceding readout period.
8. An image sensor according to claim 4 wherein the period from
said first exposure period to said second exposure period is
positioned within a preceding readout period.
9. An image sensor according to claim 1 wherein said first exposure
period is longer than said second exposure period.
10. An image sensor according to claim 4 wherein said first
exposure period is longer than said second exposure period.
11. An image sensor that includes a semiconductor device having a
semiconductor region and a diffusion layer formed inside said
semiconductor region and having the opposite conductivity of said
semiconductor region; that, after discharging carrier in said
diffusion layer of said semiconductor device, causes light to be
irradiated into said diffusion layer to generate carrier inside
said diffusion layer, that outputs a signal to an output section
based on the surface potential of generated carrier, and measures
the amount of incidence of said light is measured; comprising: a
timing generation means for creating: when irradiating light into
said-diffusion layer and generating carrier inside said diffusion
layer, a plurality of exposure periods that do not mutually overlap
for irradiating said light into said diffusion layer and generating
carriers corresponding to said plurality of exposure periods inside
said diffusion layer; a storage period for moving a preceding
carrier that was generated inside said diffusion layer in the one
preceding exposure period of said plurality of exposure periods
that relatively preceded to a storage section after said preceding
exposure period; a succeeding exposure period after said storage
period for irradiating said light into said diffusion layer after
said preceding exposure period and generating a succeeding carrier
in said diffusion layer; and a readout period after said succeeding
exposure period; and a carrier integration means for, when
outputting to an output section a signal based on the surface
potential of said generated carrier and measuring the amount of
incidence of said light, integrating, in the readout period after
the last exposure period of said plurality of exposure periods, the
carrier that was stored in said storage section up to the exposure
period immediately preceding said last exposure period and the
carrier that was generated inside said diffusion layer in said last
exposure period.
12. An image sensor that includes a semiconductor device having a
semiconductor region and a diffusion layer formed inside said
semiconductor region and having the opposite conductivity of said
semiconductor region; that, after discharging carrier in said
diffusion layer of said semiconductor device, causes light to be
irradiated into said diffusion layer to generate carrier inside
said diffusion layer, that outputs a signal that is based on the
surface potential of the generated carrier to an output section and
measures the amount of incidence of said light; comprising: a
timing generation means for creating: when irradiating light into
said diffusion layer and generating carrier in said diffusion
layer, a plurality of exposure periods that do not mutually overlap
for irradiating said light into said diffusion layer and generating
carriers that correspond to said plurality of exposure periods in
said diffusion layer; a storage period, which follows the
relatively preceding exposure period of said plurality of exposure
periods, for moving to a storage section a portion of preceding
carrier that was stored in said diffusion layer in exposure periods
up to said preceding exposure period; a succeeding exposure period
for simultaneously leaving said preceding carrier in said diffusion
layer and, after said storage period, irradiating said light into
said diffusion layer after said preceding exposure period and
generating a succeeding carrier in said diffusion layer; and a
readout period after said succeeding exposure period; and a carrier
integration means for, when outputting to an output section a
signal based on the surface potential of said generated carrier and
measuring the amount of incidence of said light, reading out
carrier, in the readout period that follows the last exposure
period of said plurality of exposure periods, said carrier being
the sum of preceding carrier that remained in said diffusion layer
until the exposure period immediately preceding said last exposure
period and the succeeding carrier that was generated in said
diffusion layer in said last exposure period.
13. An image sensor according to claim 11 wherein a preceding
exposure period among said plurality of exposure periods is a
longer period than exposure periods that are positioned later.
14. An image sensor according to claim 12 wherein a preceding
exposure period among said plurality of exposure periods is a
longer period than exposure periods that are positioned later.
15. An image sensor according to claim 11 wherein the period that
extends over said plurality of exposure periods is positioned
within a preceding readout period.
16. An image sensor according to claim 12 wherein the period that
extends over said plurality of exposure periods is positioned
within a preceding readout period.
17. An image sensor according to claim 1 wherein said diffusion
layer constitutes pixels of an image sensor and said storage
section is provided inside said pixels corresponding to said
diffusion layer.
18. An image sensor according to claim 4 wherein said diffusion
layer constitutes pixels of an image sensor and said storage
section is provided inside said pixels corresponding to said
diffusion layer.
19. An image sensor according to claim 11 wherein said diffusion
layer constitutes pixels of an image sensor and said storage
section is provided inside said pixels corresponding to said
diffusion layer.
20. An image sensor according to claim 12 wherein said diffusion
layer constitutes pixels of an image sensor and said storage
section is provided inside said pixels corresponding to said
diffusion layer.
21. An image sensor having unit pixels comprising: a photodiode of
a structure that converts irradiated light to electrons, has an
anode connected to ground, and extracts said electrons from a
cathode; an amplification transistor having gate connected to the
cathode of said photodiode, drain connected to a power supply line,
and source connected to the drain of a readout transistor; a reset
transistor having source connected to the cathode of said
photodiode, gate connected to a reset line, and drain connected to
said power supply line, a pixel interior capacitance selection
transistor having drain connected to the cathode of said
photodiode, gate connected to a pixel interior capacitance
selection line, and source connected to pixel interior capacitance;
pixel interior capacitance having one end grounded and the other
end connected to the source of said pixel interior capacitance
selection transistor; and a readout transistor having drain
connected to the source of said amplification transistor, gate
connected to a horizontal selection line, and source connected to a
vertical readout line.
22. An image sensor according to claim 21 having a construction
wherein said pixel interior capacitance is constituted by an MOS
transistor, the source and drain of said MOS transistor are
short-circuited and grounded, and the gate is connected to the
source of said pixel interior capacitance selection transistor.
23. An image sensor according to claim 21 wherein said reset
transistor and said pixel interior capacitance selection transistor
are both depletion-type MOS transistors.
24. An image sensor according to claim 23 wherein the potential of
said reset transistor when OFF is higher than the potential of said
pixel interior capacitance selection transistor when OFF.
Description
BACKGROUND OF THE INVENTION:
[0001] 1. Field of the Invention
[0002] The present invention relates to an image sensor, and
particularly to an MOS image sensor that extends the dynamic range
with respect to the amount of incident light.
[0003] 2. Description of the Related Art
[0004] In contrast to a CCD image sensor that requires a dedicated
process, MOS image sensors, such as the sensor of the present
invention, have received considerable attention in recent years
because they can be fabricated by standard MOS processes and
therefore enable the advantages of low power consumption by means
of a low-voltage and single-power supply and because they allow
incorporation of peripheral logic and macros on a single chip.
[0005] FIG. 1 shows an example of a prior-art method of extending
the dynamic range with respect to the amount of light by O.
Yadid-Pecht and E. Fossum as reported in "Image sensor with
ultra-high-linear-dynamic range utilizing dual output CMOS active
pixel sensors" (IEEE Trans. Elec. Dev., special issue on solid
state image sensors, Vol. 44, No. 10, pp. 1721-1724, October
1997).
[0006] According to this prior-art example for extending the
dynamic range with respect to amount of light, the signal charge of
pixel 21 for row n and row (n-.DELTA.), which have different
exposure times, is read out separately to each of first horizontal
transfer register 22 above and second horizontal transfer register
23 below, and these are integrated off-chip.
[0007] The above-described method, however, results in an increase
in circuit scale because it necessitates both upper and lower
horizontal scan circuits. There is the additional drawback that
system scale increases because the integration of two screens
having different exposure times is realized by off-chip
processing.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide an image
sensor that can realize an image having wider dynamic range with
respect to the amount of light in which overexposure and
underexposure are mitigated without an accompanying increase in
circuit scale.
[0009] The first image sensor of the present invention is an image
sensor that includes a semiconductor device having a semiconductor
region and a diffusion layer formed within the semiconductor region
having the opposite conductivity of the semiconductor region; that,
after discharging carrier in the diffusion layer of the
semiconductor device, causes light to be irradiated into the
diffusion layer to generate carrier in the diffusion layer, outputs
a signal to an output section based on the surface potential of the
generated carrier, and measures the amount of incident light; and
that includes:
[0010] a timing generation means for creating: a first exposure
period for, when irradiating light into the diffusion layer and
generating carrier inside the diffusion layer, irradiating the
light into the diffusion layer and generating a first carrier
inside the diffusion layer; a storage period after the first
exposure period for moving the first carrier to a storage section;
a second exposure period after the storage period for irradiating
the light into the diffusion layer and generating a second carrier
inside the diffusion layer; and a readout period after the second
exposure period; and
[0011] a carrier integration means for, when outputting to an
output section a signal based on the surface potential of carrier
that is generated by the timing generation means and measuring the
amount of incidence of light, integrating the first carrier and the
second carrier in the readout period and reading out the integrated
carrier.
[0012] Furthermore, in a first mode of application of the first
image sensor, the operation of irradiating light into the diffusion
layer during the first exposure period and generating the first
carrier in the diffusion layer is carried out in a state in which
the diffusion layer and storage section conduct, and the operation
of moving the first carrier to the storage section during the
storage period that follows the first exposure period is carried
out in a state in which the diffusion layer and storage section are
cut off. In addition, carrier in the first image sensor that is
contained in the diffusion layer and storage section is discharged
before the second exposure period by means of a reset transistor
that is connected to the power supply.
[0013] In a second mode of application of the first image sensor of
the present invention, the operation of irradiating light into the
diffusion layer during the first exposure period and generating the
first carrier in the diffusion layer is carried out in a state in
which the diffusion layer and storage section are cut off, and the
operation of moving the first carrier to the storage section during
the storage period that follows the first exposure period is
carried out in a state in which the diffusion layer and storage
section conduct.
[0014] The second image sensor of the present invention is an image
sensor that includes a semiconductor device having a semiconductor
region and a diffusion layer formed inside the semiconductor region
having the opposite conductivity of the semiconductor region; that,
after discharging carrier in the diffusion layer of the
semiconductor device, causes light to be irradiated into the
diffusion layer to generate carrier in the diffusion layer, outputs
a signal to an output section based on the surface potential of the
generated carrier, and measures the amount of incident light; and
that includes:
[0015] a timing generation means for creating: a first exposure
period for, when irradiating light into the diffusion layer and
generating a first carrier, irradiating the light into the
diffusion layer and generating carrier in the diffusion layer; a
storage period after the first exposure period for moving a portion
of the first carrier to a storage section ahd leaving first carrier
in the diffusion layer; a second exposure period after the storage
period for irradiating light into the diffusion layer and
generating the second carrier inside the diffusion layer, and a
readout period after the second exposure period; and
[0016] a carrier integration means for, when outputting to an
output section a signal based on the surface potential of the
generated carrier and measuring the amount of incidence of light,
reading out carrier that is the sum of the second carrier and the
first carrier that is left in the diffusion layer during a readout
period.
[0017] In the above-described first and second image sensors of the
present invention, a modification is possible in which carrier that
is contained in the diffusion layer and storage section is
discharged before the first exposure period by means of a reset
transistor that is connected to the power supply; the period that
extends from the first exposure period to the second exposure
period is positioned within the preceding readout period; and the
first exposure period is longer than the second exposure
period.
[0018] The third image sensor, which expands on the first image
sensor of the present invention, is an image sensor that includes:
a semiconductor device having a semiconductor region and a
diffusion layer formed inside the semiconductor region having the
opposite conductivity of the semiconductor region; that, after
discharging carrier in the diffusion layer of the semiconductor
device, causes light to be irradiated into the diffusion layer to
generate carrier in the diffusion layer, outputs a signal to an
output section based on the surface potential of the generated
carrier, and measures the amount of incidence of light;
including:
[0019] a timing generation means for creating: when irradiating
light into the diffusion layer and generating carrier inside the
diffusion layer, a plurality of exposure periods that do not
mutually overlap for irradiating light into the diffusion layer and
generating carriers that correspond to the plurality of exposure
periods inside the diffusion layer; a storage period for moving a
preceding carrier that was generated inside the diffusion layer in
the one preceding exposure period of the plurality of exposure
periods that relatively preceded to a storage section after the
preceding exposure period; a succeeding exposure period after the
storage period for irradiating the light into the diffusion layer
after the preceding exposure period and generating a succeeding
carrier inside the diffusion layer; and a readout period after the
succeeding exposure period; and
[0020] a carrier integration means for, when outputting to an
output section a signal based on the surface potential of the
generated carrier and measuring the amount of incidence of the
light, integrating, in the readout period following the last
exposure period of the plurality of exposure periods, the carrier
that was stored in the storage section up to the exposure period
immediately preceding the last exposure period and the carrier that
was generated inside the diffusion layer in the last exposure
period.
[0021] Next, the fourth image sensor, which expands on the second
image sensor of the present invention, is an image sensor that
includes a semiconductor device having a semiconductor region and a
diffusion layer formed inside the semiconductor region having the
opposite conductivity of the semiconductor region; that, after
discharging carrier in the diffusion layer of the semiconductor
device, causes light to be irradiated into the diffusion layer and
carrier to be generated inside the diffusion layer, that outputs a
signal that is based on the surface potential of the generated
carrier to an output section and measures the amount of incident
light; and that includes:
[0022] a timing generation means for creating: when irradiating
light into the diffusion layer and generating carrier in the
diffusion layer, a plurality of exposure periods that do not
mutually overlap for irradiating light into the diffusion layer and
generating carriers in the diffusion layer that correspond to the
plurality of exposure periods; a storage period, which follows the
relatively preceding exposure period of the plurality of exposure
periods, for moving to a storage section a portion of preceding
carrier that was stored in the diffusion layer in exposure periods
up to the preceding exposure period; a succeeding exposure period
for simultaneously leaving preceding carrier in the diffusion layer
and, after the storage period, irradiating light into the diffusion
layer after the preceding exposure period and generating a
succeeding carrier in the diffusion layer; and a readout period
after the succeeding exposure period; and
[0023] a carrier integration means for, when outputting to an
output section a signal based on the surface potential of generated
carrier and measuring the amount of incidence of light, reading out
carrier that is the sum of the preceding carrier that remained in
the diffusion layer until the exposure period immediately preceding
the last exposure period and the succeeding carrier that was
generated in the diffusion layer in the last exposure period in the
readout period following the last exposure period of the plurality
of exposure periods.
[0024] In the above-described third and fourth image sensors of the
present invention, a modification can be adopted in which the
preceding exposure period of the plurality of exposure periods is a
longer period than an exposure period that is positioned later; and
in which a period that extends over a plurality of exposure periods
is positioned within a preceding readout period.
[0025] A modification is adopted in common to each of the
above-described first, second, third, and fourth image sensors of
the present invention in which the diffusion layer constitutes the
pixels of the image sensor and the storage section is provided
inside pixels in correspondence to the diffusion layer.
[0026] The first, second, third, and fourth image sensors of the
above-described present invention have the following circuit
configuration:
[0027] The circuit configuration of the first, second, third, and
fourth image sensors include unit pixels that are each composed
of:
[0028] a photodiode of a structure that converts irradiated light
to electrons, has its anode connected to ground, and extracts
electrons from its cathode;
[0029] an amplification transistor having its gate connected to the
cathode of the photodiode, its drain connected to a power supply
line, and its source connected to the drain of a readout
transistor;
[0030] a reset transistor having its source connected to the
cathode of the photodiode, its gate connected to a reset line, and
its drain connected to a power supply line,
[0031] a pixel interior capacitance selection transistor having its
drain connected to the cathode of the photodiode, its gate
connected to the pixel interior capacitance selection line, and its
source connected to pixel interior capacitance;
[0032] pixel interior capacitance having one end grounded and the
other end connected to the source of the pixel interior capacitance
selection transistor; and
[0033] a readout transistor having its drain connected to the
source of the amplification transistor, its gate connected to a
horizontal selection line, and its source connected to a vertical
readout line.
[0034] The pixel interior capacitance is composed of a MOS
transistor and is of a construction in which the source and drain
of the MOS transistor are short-circuited and connected to ground
and the gate is connected to the source of the pixel interior
capacitance selection transistor. The reset transistor and pixel
capacitance selection transistor are both depletion-type MOS
transistors, and in this case, the potential of the reset
transistor when OFF is higher than the potential of the pixel
capacitance selection transistor when OFF.
[0035] As described hereinabove, by performing two exposures of
different exposure periods, mixing the signals that are generated
in each exposure period in pixels (holding the signal charge that
is generated in the first exposure period in capacitance that is
provided in pixels and mixing the signal charge that is generated
in the second exposure period with the first signal charge in the
pixels), and reading out, the image sensors of the present
invention can obtain an image of wider dynamic range with respect
to the amount of light with mitigated overexposure and
underexposure because overexposed portions in the first exposure
period are compensated by information of the second exposure period
and underexposed portions in the second exposure period are
compensated by information of the first exposure period.
[0036] In addition, the invention has the advantage of enabling an
extension of dynamic range without reducing the frame readout speed
because the above-described two exposures can be performed during
readout of one frame.
[0037] The above and other objects, features, and advantages of the
present invention will become apparent from the following
description based on the accompanying drawings which illustrate
examples of preferred embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a schematic plan view of the vicinity of a pixel
for explaining one method of extending the dynamic range of an
image sensor of the prior art.
[0039] FIG. 2 is an equivalent circuit diagram for explaining an
embodiment of the present invention.
[0040] FIG. 3 is a timing chart showing the operation of the first
embodiment of the present invention.
[0041] FIG. 4 is a timing chart showing the operation of the second
embodiment of the present invention.
[0042] FIG. 5 is a timing chart showing the operation of the third
embodiment of the present invention.
[0043] FIG. 6 is a graph showing the relation between the amount of
light and output that is obtained by a method of the prior art, and
the relation between the amount of light and the output obtained by
the first, second and third embodiments of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] Before entering into an explanation of the embodiments of
the present invention, the feature of the present invention will
first be described.
[0045] The feature of the present invention resides in the
execution of a plurality of light exposures of different durations
during the readout of one frame in an MOS-type image sensor, the
separate storage of the light charges that are stored in the memory
regions that are provided in pixels in these different exposure
periods, and during readout, the readout of these stored charges
after mixing in the pixels.
[0046] According to the present invention, as shown by the circuit
diagram of a pixel according to the present invention in FIG. 2,
TGB (capacitance selection line) is held at high level and
photodiode 1 and pixel interior capacitance 4 are caused to
conduct, and RST (reset line) is activated to initialize photodiode
1, and the first exposure is started. In this first exposure
period, the exposure time is set long to reduce the underexposure
of the dark portions in the screen. The light charge is therefore
saturated in bright portions within the screen, and overexposure
occurs in some cases.
[0047] After the first exposure, TGB is set to a low level and the
result of the first exposure is stored in pixel interior
capacitance 4.
[0048] RST is then again activated to begin the second exposure. In
this second exposure period, the exposure time is set shorter than
in the first exposure period to reduce overexposure in the bright
portions of the screen. After completing the second exposure, TGB
is again set to high level, the result of the second exposure is
mixed inside the pixels with the result of the first exposure
result that is stored in pixel interior capacitance 4, and VL is
activated for readout.
[0049] By means of this series of operations, regions in which
overexposure occurred during the first exposure are compensated by
the charges generated during the second exposure, and
simultaneously, regions in which underexposure occurred during the
second exposure are compensated by the stored charges of the first
exposure, whereby overexposure and underexposure within a screen
having a large contrast can be mitigated and the dynamic range with
respect to light intensity can be extended.
[0050] Referring now to FIGS. 2 and 3, the first embodiment of the
present invention is next explained. FIG. 2 shows the circuit
configuration of a pixel of a CMOS image sensor as the first
embodiment of the present invention.
[0051] This CMOS image sensor includes: photodiode 1 that receives
and converts light 10 to an electric signal and that has its anode
side grounded; transistor 2 that operates as an amplifier having
its gate connected to the cathode side that extracts electrons
resulting from the photoelectric conversion from photodiode 1 and
its drain connected to power supply line VDD; reset transistor 3
having its source connected to the cathode side of photodiode 1,
its gate connected to reset line RST, and its drain connected to
the power supply line VDD; transistor 5 having its drain connected
to the cathode side of photodiode 1, its gate connected to pixel
interior capacitance selection line TGB, and its source connected
to pixel interior capacitance 4; pixel interior capacitance 4
having one end grounded; and readout transistor 6 having its drain
connected to the source of transistor 2 that functions as an
amplifier, its gate connected to horizontal selection line HL, and
its source connected to vertical readout line VL. The operation of
this embodiment is next explained.
[0052] The operation of the CMOS image sensor of the first
embodiment of the present invention is next explained using the
timing chart of FIG. 3.
[0053] Selection line TGB for selecting pixel interior capacitance
4 is first fixed to high level, and horizontal selection line HL is
fixed to low level. In this state, a high-level pulse is applied to
reset line RST, and the cathode of photodiode 1 and pixel interior
capacitance 4 are reset to the power supply line level.
[0054] After the high-level pulse is applied to reset line RST,
photodiode 1 enters the first exposure period, and electrons
generated by the light signal accumulate on the cathode of
photodiode 1. Upon completion of the first exposure period,
selection line TGB of pixel interior capacitance 4 is set to low
level and the signal of the first exposure period is stored in
pixel interior capacitance 4.
[0055] Next, after continuously applying a high-level pulse to
reset line RST and resetting the cathode of photodiode 1 to the
level of power supply line VDD, photodiode 1 enters the second
exposure period, and electrons generated by an optical signal are
again accumulated on the cathode of photodiode 1.
[0056] After completion of the first and second exposure periods,
selection line TGB of pixel interior capacitance 4 is set to high
level and the signals of the first exposure period and the second
exposure period are mixed to produce a mixed signal. Horizontal
selection line HL is then set to high level and the mixed signal is
read out to vertical readout line VL.
[0057] In this mixed signal, overexposed portions that occurred in
the first exposure period are compensated by the information of the
second exposure period, and underexposed portions that occurred in
the second exposure period are compensated by the information of
the first exposure period, whereby an image is obtained that has
mitigated overexposure and underexposure, and moreover, that has
wide dynamic range with respect to the amount of light.
[0058] The exposure time of the first exposure period is preferably
set long to prevent underexposure of the dark portions of the
imaged subject. Similarly, the exposure time of the second exposure
period is preferably set shorter to prevent overexposure of the
bright portions of the imaged section. This is because setting the
exposure time longer in the first exposure period allows excess
charge to be extracted to power supply line VDD by way of
transistor 3 in cases in which the accumulated charge of photodiode
1 is saturated. Conversely, if a long exposure is performed in the
second exposure time such that the accumulated charge of photodiode
1 may reach a saturated state, there is the danger that the excess
charge will destroy the signal of the first exposure time that is
stored in pixel interior capacitance 4.
[0059] Thus, by performing two exposures of different exposure
times, mixing the signals that are generated in these exposure
periods inside the pixels and than reading out the mixed signal,
the overexposed portions that occur in the first exposure period
are compensated by information of the second exposure period, and
underexposed portions that occur in the second exposure period are
compensated by the information of the first exposure period, and an
image can be obtained having wide dynamic range with respect to the
amount of light and in which the occurrence of overexposure and
underexposure has been mitigated.
[0060] In addition, the above-described two exposures can be
performed during the readout of one frame, and the present
invention therefore has the advantage of allowing an expansion of
the dynamic range without reducing the frame readout speed.
[0061] Although a case has been described in the above-described
embodiment in which the transistors inside the pixel were of the
n-channel type, exactly the same results can be obtained in the
case of the p-channel type. In such a case, the polarity of the
input signal and photodiode is obviously reversed.
[0062] A further extension of dynamic range can be achieved by
increasing the pixel interior capacitance and pixel interior
capacitance selection transistors to provide more than two exposure
periods during the interval of one frame and then mixing inside the
pixels by the same operation as in the foregoing explanation.
[0063] In addition, the layout area can be reduced by using
transistors having both sources and drains grounded in pixel
interior capacitance 4.
[0064] Further, the use of depletion-type transistors for both
reset transistor 3 and pixel interior capacitance selection
transistor 5 can prevent drops in the signal threshold value
without boosting the high level of the transistor gate.
[0065] Still further, the use of depletion-type transistors such
that the potential of reset transistor 3 when OFF is higher than
the potential of pixel interior capacitance selection transistor
when OFF enables control of blooming in which excessive charge is
discharged to power supply line through reset transistor 3.
[0066] Although the configuration of the second embodiment of the
present invention is basically the same as the first embodiment,
the second embodiment is distinguished by differences in the method
of operation from that of the first embodiment. The operation of
the CMOS image sensor of the second embodiment of the present
invention is next described using the timing chart of FIG. 4.
[0067] A high-level pulse is first applied to pixel interior
capacitance 4 selection line TGB and reset line RST with horizontal
selection line HL fixed to low level, whereby the cathode of
photodiode 1 and pixel interior capacitance 4 are reset to the
level of the power supply line. After applying the high-level pulse
to pixel interior capacitance 4 selection line TGB and reset line
RST, photodiode 1 enters the first exposure period, and electrons
generated by an optical signal accumulate on the cathode of
photodiode 1.
[0068] Upon the completion of the first exposure period, a
high-level pulse is applied to pixel interior capacitance 4
selection line TGB and the signal of the first exposure period is
stored in pixel interior capacitance 4. Photodiode 1 then enters
the second exposure period and electrons generated by an optical
signal continue to accumulate on the cathode of photodiode 1.
[0069] After the first and second exposure periods have been
completed, horizontal selection line HL is set to high level and a
signal is read out to vertical readout line VL. Since overexposed
portions that occurred during the first exposure period are
compensated in this signal, an image having wide dynamic range for
the amount of light and mitigated overexposure is obtained.
[0070] The operation of the CMOS image sensor of the third
embodiment of the present invention is next explained using the
timing chart of FIG. 5.
[0071] A high-level pulse is first applied to pixel interior
capacitance 4 selection line TGB and reset line RST with horizontal
selection line HL fixed to a low level, whereby the cathode of
photodiode 1 and pixel interior capacitance 4 are reset to the
power supply line level. After the high-level pulse has been
applied to pixel interior capacitance 4 selection line TGB and
reset line RST, photodiode 1 enters the first exposure period, and
electrons that are generated by an optical signal accumulate on the
cathode of photodiode 1.
[0072] When the first exposure period has been completed, a
high-level pulse is applied to pixel interior capacitance 4
selection line TGB and the signal of the first exposure period is
stored in pixel interior capacitance 4. Photodiode 1 then enters
the second exposure period, and electrons that are generated by an
optical signal continue to accumulate on the cathode of photodiode
1.
[0073] After the first and second exposure periods have been
completed, pixel interior capacitance 4 selection line TGB is set
to high level, the signals of the first exposure period and the
second exposure period are mixed, following which horizontal
selection line HL is set to high level and the mixed signal is read
out to vertical readout line VL. Because overexposed and
underexposed portions that occurred during the first exposure
period and second exposure period are mutually compensated in this
mixed signal, an image having wide dynamic range with respect to
the amount of light and having mitigated overexposure and
underexposure can be obtained.
[0074] The effects of the first, second, and third embodiments are
shown in FIG. 6. From these results, it can be seen that, compared
to the prior art in which dynamic range is not extended, all
embodiments tend to eliminate saturation of output with respect to
the amount of light and the output varies over a wide range of
amount of light, i.e., the dynamic range with respect to the amount
of light has been extended.
[0075] While preferred embodiments of the present invention have
been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the following claims.
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