Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1998-09-28
2003-12-30
Vu, Ngoc-Yen (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S304000, C348S307000, C250S208100, C257S233000, C257S292000
Reexamination Certificate
active
06670990
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photoelectric conversion device having a plurality of photoelectric conversion elements and, more particularly, to a photoelectric conversion device capable of improving linearity of the photoelectric conversion, further, widening a dynamic range by utilizing characteristics of transistors used for outputting signals corresponding to charges generated by photoelectric conversion elements and characteristics of metal-oxide semiconductor (MOS) transistor switches, thereby improving signal
oise (S/N) ratio.
Conventionally, in a solid-state image sensing device, charge-coupled-device (CCD) type photoelectric conversion elements are used in most cases; however, MOS type photoelectric conversion elements have been recently developed for commercial products. It has been said that a MOS type photoelectric conversion device provide an image of inferior quality compared to an image sensed by a CCD type photoelectric conversion device. However, if noise is reduced, there are advantages in the MOS type photoelectric conversion device in that it is possible to drive the MOS type photoelectric conversion device by the same power source with lower energy compared to the CCD type photoelectric conversion device, and photo-receiving unit and its peripheral circuits are manufactured in the same MOS manufacturing processes, thus it is easier to integrate the photo-receiving unit and the peripheral circuits. Accordingly, these merits of the MOS type photoelectric conversion device start attracting attentions recently. Currently, it is possible to reduce random noise and fixed noise for improving quality of an image provided by the MOS type photoelectric conversion device, and there is a new demand for widening the dynamic range of each MOS type photoelectric conversion element in order to obtain image signals of a higher S/N ratio.
Note, in the following explanation, a MOS type a photoelectric conversion element and a MOS type photoelectric conversion device are simply referred to as a photoelectric conversion element and a photoelectric conversion device.
FIG. 1
is a circuit diagram illustrating a brief configuration of a conventional photoelectric conversion device. In
FIG. 1
, photoelectric conversion elements
1
(e.g., photodiodes), arranged in two dimensions, generate charges corresponding to the received quantity of light. In
FIG. 1
, only 16 (=4×4) photoelectric conversion elements are shown for the sake of illustrative convenience, however, a large number of photoelectric conversion elements are usually used in practice. One end of each photoelectric conversion element is connected to the gate of a MOS transistor
2
; the drain of the MOS transistor
2
is connected to the source of a MOS transistor
3
which configures a row selection switch, and the source of the MOS transistor
2
is connected to a constant current source
7
via a vertical output line
6
; and the drain of each MOS transistor
3
is connected to a power supply terminal
5
via a power supply line
4
. The foregoing elements collectively form the source follower. Reference
14
denotes a MOS transistor configuring a reset switch, and its source is connected to the gate of the MOS transistor
2
and its drain is connected to the power supply terminal
5
via the power supply line
4
.
In this circuit, a signal corresponding to the gate voltage of the MOS transistor
2
which changes depending upon charge generated by the photoelectric conversion element
1
of each pixel, is amplified and outputted by the source follower which performs current amplification.
The gate of each MOS transistor
3
is connected to a vertical scanning circuit
9
via a vertical gate line
8
. The gate of each reset switch
14
is also connected to the vertical scanning circuit
9
via a reset gate line
15
. Further, an output signal from the source follower is outputted via the vertical output line
6
, a MOS transistor
10
which configures a switch for horizontal transference, a horizontal output line
11
, and an output amplifier
12
. The gate of each MOS transistor
10
is connected to a horizontal scanning circuit
13
.
An operation of this circuit is as follows. First, the photoelectric conversion elements
1
are reset by the reset switches
14
, thereafter, charges are stored. Note, since the photoelectric conversion elements
1
generate electrons depending upon the amount of light received, the gates of the MOS transistor
2
are charged to a reset potential during the reset operation, and the potentials at the gates of the MOS transistors
2
drop in response to the generation of the electrons. Accordingly, a potential corresponding to the generated charges appears at the gate of each MOS transistor
2
. After the charging period is over, a signal of a pixel selected by the vertical scanning circuit
9
and the horizontal scanning circuit
13
is amplified by the source follower, and outputted via the output amplifier
12
.
In the above configuration, since the source follower and the reset switch
14
share the same power supply line
4
, it is possible to down-size the circuit.
Further, by arranging the row selection switch
3
on the side of the power supply with respect to the MOS transistor
2
, impedance of the selection switch
3
does not exist between the source of the MOS transistor
2
and the constant current source
7
; accordingly, an output of good linearity is obtained from the source follower.
Below, output characteristics of the source follower as described above is explained.
In order to simplify the explanation, one photoelectric conversion element
1
and its peripheral circuit corresponding to a single pixel are shown in FIG.
2
. In
FIG. 2
, the same elements as those shown in
FIG. 1
are referred to by the same reference numerals. Generally, for the source follower to operate linearly, i.e., to output a voltage in proportion to an input voltage, a MOS transistor, forming the source follower, needs to operate in the saturation region; thus, the following condition should be satisfied.
V
ds
>V
gs
−V
th
(1)
where V
ds
is the voltage difference between the drain and the source, V
gs
is the voltage difference between the gate and the source, and V
th
is a threshold voltage.
In a case of the source follower having a configuration as shown in
FIG. 2
, let the ON-state impedance of the row selection switch
3
be R
on
, and current flowing through the source follower be I
a
, then the drain voltage of the MOS transistor
2
is
Power supply voltage−
R
on
×I
a
(2)
due to a voltage drop in the row selection switch
3
. Accordingly, V
ds
in the equation (1) decreases, thereby a region for the source follower to operate linearly (called“linear operation region” hereinafter) is narrowed. As a result, the source follower does not operate within the linear operation region for every voltage, applied to the gate of the MOS
2
, which depends upon the charge generated by the photoelectric conversion element
1
, and the following two problems arise:
(a) Input-output linearity in low luminosity region deteriorates.
(b) Saturation voltage becomes small, thus the dynamic range is narrowed.
Further, when the current flowing through the source follower is reduced in order to reduce the voltage drop in the row selection switch
3
, it takes considerable time to charge a capacitance with a small current. Accordingly, it takes a considerable time to transfer signals, thus the number of pixels in the photoelectric conversion device is limited when charges should be transferred in a predetermined period. Consequently, the conventional circuit is not suitable for operating a great number of pixels.
Another example of a conventional photoelectric conversion device is explained below.
FIG. 3
is a circuit diagram illustrating a brief configuration of a conventional CMOS area sensor. In
FIG. 3
, a two-dimensional area sensor having 2×2 pixels is shown, however, the number of pixels is not limited to
Hiyama Hiroki
Kochi Tetsunobu
Koizumi Toru
Ogawa Katsuhisa
Sakurai Katsuhito
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Vu Ngoc-Yen
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