Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1998-07-27
2004-09-28
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S308000, C348S241000, C348S243000, C348S302000, C358S463000, C358S461000, C250S2140AG
Reexamination Certificate
active
06798452
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a physical quantity distribution sensing semiconductor device for reading, as electric signals, the distribution of a physical quantity that has been externally input and transduced into the electric signals. More particularly, the present invention relates to an amplifying solid-state imaging device for reading externally incident light as electric signals while eliminating variations resulting from the sensitivity components of the signals, and also relates to a method for driving the same.
In recent years, semiconductor devices for sensing a physical quantity distribution have been increasingly demanded. In particular, an amplifying solid-state imaging device, that is, a solid-state image sensor that can sense light among various types of physical quantities, attracts a great deal of attention, because such a device can operate with lower electric power and can easily integrate a variety of circuits on a chip.
Hereinafter, a conventional amplifying solid-state imaging device will be described with reference to the drawings.
FIG. 5
schematically illustrates an equivalent circuit corresponding to one pixel of a conventional amplifying solid-state imaging device. In
FIG. 5
, the reference numeral
100
denotes one of a plurality of pixels arranged in matrix. The pixel
100
includes: a photoelectric transducer
101
, implemented as a photodiode to which an inverse bias voltage is applied, for transducing light into signal charges; a signal charge accumulator
102
, implemented as a capacitor, for accumulating the signal charges transduced by the photoelectric transducer
101
; a driving transistor
103
including an operation control section having the gate of a field effect transistor (hereinafter, abbreviated as “FET”) for controlling the drive current in accordance with the amount of the signal charges accumulated in the signal charge accumulator
102
; a row select transistor
104
for selecting a pixel
100
on a specific row from a plurality of pixels
100
; a column select transistor
105
for selecting a pixel
100
on a specific column from a plurality of pixels
100
; and a resetting transistor
106
for reading the potential in the signal charge accumulator
102
at a predetermined time and then resetting the potential at an initial potential V
DD
. Herein, the capacitor constituting the signal charge accumulating section
102
is, in actuality, capacitance formed by the photoelectric transducer
101
and the gate of the driving transistor
103
.
However, in the conventional amplifying solid-state imaging device, the driving transistor
103
is provided for each pixel
100
and the electrical characteristics of the respective driving transistors
103
are different from each other. Thus, an image of uniform quality is hard to obtain when the image is produced by using amplified signal current.
The noise components that has resulted from such a variation in electrical characteristics of the transistors and fixed in an image space, i.e., fixed pattern noise (FPN), can be roughly classified into the following two types.
The first type is noise components called “offset components” resulting from the variation in threshold voltages V
t
of the FETs, for example. In such a case, even when light is uniformly incident onto an input section, unevenness is caused in the resulting image, because variation is found in the current values of the output signals.
The second type is noise components called “dynamic sensitivity components” resulting from the variation in capacitance of the signal charge accumulator
102
or in gains obtained by using the driving transistor
103
as a source follower.
FIG. 6
illustrates these two types of FPNs. In
FIG. 6
, the axis of abscissas indicates the quantity of light incident onto a pixel, while the axis of ordinates indicates the output values of each pixel. Line
111
represents the output characteristics of a pixel, while Line
112
represents the characteristics of another pixel. As shown in
FIG. 6
, a potential difference corresponding to the difference
113
between the Y intercepts is the offset component of the FPN. The offset component is generated because a drive voltage applied to the gate of the driving transistor
103
is obtained from the difference between a power supply voltage V
DD
and the threshold voltage V
t
and because the threshold voltages V
t
of the respective driving transistors are different from each other. On the other hand, the sensitivity component of the FPN is a ratio associated with each characteristic line. Specifically, in Line
111
, the component is equal to: output/quantity of light=b
1
/a
0
, while in Line
112
, the component is equal to: output/quantity of light=b
2
/a
0
. In this case, as can be easily understood from
FIG. 6
, the absolute value of the ratio b
2
/a
0
of Line
112
is larger than that of the ratio b
1
/a
0
of Line
111
.
A solution for eliminating the offset component
113
is already disclosed in Japanese Laid-Open Publication No. 8181920.
However, no solution has ever been provided for the elimination of the FPN sensitivity component. As higher and higher image quality is sought after in the future, the adverse effects of the component on the image quality would presumably become more and more serious.
SUMMARY OF THE INVENTION
In view of the above-described conventional problems, the object of the present invention is to eliminate, from the FPN, the sensitivity components resulting from the variation in capacitance and the like of the charge accumulator.
In order to accomplish the object, according to the present invention, an output signal of each pixel is obtained by dividing a signal, which is output from a driving transistor in response to light incident upon a photoelectric transducer, by a reference output voltage, which is output from the driving transistor upon the application of a predetermined reference electric signal thereto.
A first amplifying solid-state imaging device according to the present invention includes: a plurality of photoelectric transducing means, each sensing externally incident light and transducing the sensed incident light into signal charges having a charge quantity corresponding to the incident light; a plurality of signal charge accumulating means, each accumulating the signal charges transduced by an associated one of the photoelectric transducing means; and signal reading means for sequentially reading out the signal charges, accumulated in the respective signal charge accumulating means, as electric signals. Each said electric signal is obtained by dividing an original electric signal by a reference electric signal and read out. The original electric signal is obtained by converting the signal charges accumulated in each said signal charge accumulating means. The reference electric signal is obtained by converting reference signal charges, which are output from each said signal charge accumulating means in response to predetermined reference light incident upon each said photoelectric transducing means, or by converting reference signal charges, which are output from each said signal charge accumulating means in response to a predetermined reference electric signal externally applied to each said signal charge accumulating means.
In the first amplifying solid-state imaging device, in the period of reading the electric signal, the original electric signal obtained by converting the signal charges accumulated in each said signal charge accumulating means that has received externally incident light is divided by the reference electric signal obtained by converting reference signal charges, which are output from each said signal charge accumulating means in response to predetermined reference light incident upon each said photoelectric transducing means, or by converting reference signal charges, which are output from each said signal charge accumulating means in response to a predetermined reference electric signal externally applied to each said signal
Kuroda Takao
Masuyama Masayuki
Hannett James
Ho Tuan
Matsushita Electric - Industrial Co., Ltd.
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