Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-05-28
2001-02-06
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S208100, C250S2140AG
Reexamination Certificate
active
06184516
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photoelectric conversion device and an image sensor and, more particularly, to reduction of fixed pattern noise (FPN) in a photoelectric conversion device having a plurality of photodetectors and peripheral circuits, configured with MOS transistors, integrally formed on a semiconductor substrate, and to an image sensor.
Recently, a one- or two-dimensional photoelectric conversion device, having a plurality of photodetectors and peripheral circuits for processing and controlling signals, integrally formed on a semiconductor substrate, has been developed. For example, a photoelectric conversion device having an internal reference voltage generator configured with an operational amplifier is proposed in Japanese Patent Application Laid-Open No. 9-65215.
An example of a circuit configuration of a photoelectric conversion device having a plurality of photodetectors and peripheral circuits integrally formed on a semiconductor substrate is shown in FIG.
7
. In FIG.
7
, peripheral circuits include a CMOS operational amplifier
40
, and reference numeral
20
denotes a plurality of photodetectors;
22
, a connecting pad;
30
, a charge-voltage converters;
34
, a switch;
35
, a common output line; and
36
, a shift register. The photodetectors
20
are connected to the charge-voltage converters
30
where charges generated depending upon incident light on the photodetectors
20
are converted to voltage signals. The voltage signals are sequentially outputted onto the common output line
35
in accordance with signals provided from the shift register
36
to the charge-voltage converters
30
. The common output line
35
is connected to a positive terminal of the CMOS operational amplifier
40
, and, after the input voltage signals on the common output line
35
are processed with impedance transformation by the CMOS operational amplifier
40
, the voltage signals are outputted via the switch
34
and the pad
22
.
The CMOS operational amplifier
40
includes a differential section
50
and an output section
51
as shown in
FIG. 8
which is based on “Analog MOS Integrated Circuits for Signal Processing”, R. Gregorian, G. C. Temes, pp. 170,
FIG. 4.
59. Generally, larger current flows in the output section
51
than other block in order to drive an external load. As for the circuit configuration of the output section
51
, a source follower using an n-channel MOS (nMOS) transistor, whose mutual conductance g
m
is larger than that of a p-channel MOS (pMOS) transistor, an inversion amplifier and a push-pull circuit which are formed by combining nMOS and pMOS transistors are used.
In a MOS transistor, when a voltage is applied across a drain and a source while a channel is formed by applying a voltage to a gate, electric field becomes strong in the vicinity of the drain-side edge of the channel, which sometimes generates new electron-hole pairs due to impact ionization. Most of the carrier generated due to the impact ionization becomes substrate current and absorbed by a reference potential of the semiconductor substrate, however, a part of the carrier recombines. The recombination is accompanied by light emission, and the emitted light further generates new electron-hole pairs in the semiconductor substrate. The carrier generated in this manner becomes stray carrier which diffuses over the semiconductor substrate. When the stray carrier enters the photodetectors, ghost signals are generated in addition to essential signals generated in proportion to incident light. These ghost signals are a primary factor of fixed pattern noise in a photoelectric conversion device.
The measurement result, by the applicants of the present invention, of substrate current and drain current with respect to gate voltage Vg of nMOS and pMOS transistors is shown in FIG.
9
. In
FIG. 9
, an abscissa shows the absolute value of the gate voltage, and an ordinate shows substrate current and drain current. The substrate current flowing in the nMOS transistor is about 10
4
to 10
5
larger than that in the pMOS transistor, which indicates that more electron-hole pairs are generated due to impact ionization in the nMOS transistor the in the pMOS transistor. Thus, since the fact that more substrate current flows in the nMOS transistor than in the pMOS transistor, stray carrier is more easily generated in a semiconductor substrate of an nMOS transistor than a pMOS transistor.
Further, substrate current in a MOS transistor depends upon the drain-source voltage more than the gate voltage. Experimental results show that the substrate current increases logarithmically with respect to increase in the drain-source voltage. Accordingly, it is determined that generation of stray carrier can be reduced by lowering the drain-source voltage.
Fixed pattern noise, with respect to a signal level, caused by stray carrier entering a plurality of photodetectors is not ignorable as sensitivity of the photodetectors improves.
FIG. 10B
is a graph showing generated fixed pattern noise in a conventional one-dimensional image sensor (
FIG. 10A
) including the operational amplifier
40
using an nMOS transistor in an output section for driving a load. As shown in
FIG. 10A
, the photodetectors
20
and peripheral circuits
21
including the operational amplifier
40
having an nMOS transistor are formed on a semiconductor substrate
100
. With this configuration, as shown in
FIG. 10B
, dark current corresponding to the portion where the output section is laid out is larger than other portions, which shows that fixed pattern noise is caused by stray carrier entering the photodetectors
20
.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide a photoelectric conversion device, having a plurality of photodetectors and peripheral circuits integrally formed on a single semiconductor substrate, capable of reducing generation of stray carrier in the peripheral circuits, thereby reducing fixed pattern noise due to the stray carrier entering the photodetectors.
According to the present invention, the foregoing object is attained by providing a photoelectric conversion device comprising: a plurality of photodetectors which output electric signals in accordance with quantity of incident light; an operational amplifier having a MOS transistor; and an output circuit, wherein the plurality of photodetectors, the operational amplifier, and the output circuit are integrally formed on a single semiconductor substrate, and the operational amplifier and the output circuit perform impedance transformation for the electric signals outputted from the photodetectors.
According to the present invention, the foregoing object is also attained by providing an image sensor having a plurality of photoelectric conversion devices formed on a single semiconductor substrate, wherein each photoelectric conversion apparatus comprises: a plurality of photodetectors which output electric signals in accordance with quantity of incident light; an operational amplifier having a MOS transistor; and an output circuit, wherein the plurality of photodetectors, the operational amplifier, and the output circuit are integrally formed on a single semiconductor substrate, and the operational amplifier and the output circuit perform impedance transformation for the electric signals outputted from the photodetectors.
Preferably, the output circuit includes: a first source follower using an n-channel MOS transistor; and a second source follower using a p-channel MOS transistor for driving an external load to which an output of the first source follower is inputted and whose output enters an negative input terminal of the operational amplifier.
With the aforesaid configurations, an output section, which is the main source of stray carrier, of the photoelectric conversion device for driving a large load is configured with pMOS transistors without using nMOS transistors.
Preferably, in the output circuit, a voltage drop means is provided between
Kozuka Hiraku
Sawada Koji
Canon Kabushiki Kaisha
Lee John R.
Morgan & Finnegan L.L.P.
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