Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-10-24
2002-08-20
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C250S2140AG, C250S208100
Reexamination Certificate
active
06437309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a one-dimensional or two-dimensional photoelectric transducer in which a semiconductor circuit including a plurality of photosensors using, for example, pn junctions is formed in the same semiconductor substrate. More particularly, the invention relates to a photoelectric transducer having a circuit configuration effective for reducing noise, such as fixed-pattern noise or the like.
2. Description of the Related Art
Recently, photoelectric transducers in which photosensors and a peripheral circuit are formed on the same substrate have been actively developed.
For example, a linear sensor in which an operational amplifier and photosensors are formed on the same semiconductor substrate (The Journal of the Institute of Television Engineers of Japan, Vo. 47, No. 9 (1993), pp. 1180), an image sensor having a sample-and-hold circuit (Japanese Patent Laid-Open Application (Kokai) No. 4-223771 (1992), and a solid-state image pickup device having an internal reference-voltage generation circuit configured by an operational amplifier (Japanese Patent Laid-Open Application (Kokai) No. 9-65215 (1997) have been proposed.
In general, a bias current for an operational amplifier is generated using a constant-current-source circuit. When forming such a constant-current-source circuit using MOS (metal oxide semiconductor) transistors, for example, a CMOS (complementary MOS) constant-current-source circuit as shown in
FIG. 1
(R. Gregorian, G.C. Temes: Analog MOS Integrated Circuits for Signal Processing, p. 127, FIG. 4.5. ) is generally used. A CMOS constant-current-source circuit as disclosed in Japanese Patent Laid-Open Application (Kokai) No. 7-44254 (1995) has also been proposed.
The CMOS constant-current-source circuit shown in
FIG. 1
is obtained by performing tandem connection of an upper-stage current mirror circuit and a lower-stage current mirror circuit between a power supply V
DD
and the ground. A constant-voltage-source output V
01
is obtained from a connection point of the upper-stage current mirror circuit and the lower-stage current mirror circuit. That is, the upper-stage current mirror circuit includes a pMOS transistor Q
4
, whose source is connected to the power supply V
DD
, and a pMOS transistor Q
3
. The gates of the pMOS transistor Q
4
and the pMOS transistor Q
3
are connected to each other, and the gate and the drain of the pMOS transistor Q
3
are directly connected, so that each drain supplies the load side with the same current I
bias
. The lower-stage current mirror circuit includes an nMOS transistor Q
1
, whose source is connected to the ground, and an nMOS transistor Q
2
whose source is connected to the ground via a resistor R. The gates of the nMOS transistor Q
1
and the nMOS transistor Q
2
are connected to each other, and the gate and the drain of the nMOS transistor Q
1
are directly connected, so that each drain obtains the same current from the load side. A constant current by the current mirror circuit passes through the resistor R. An output voltage provided by a voltage generated in the resistor R and the source-drain voltage of the nMOS transistor Q
2
is output as the constant voltage V
01
of this voltage source.
However, the inventors of the present invention have found that in conventional photoelectric transducers in which a CMOS constant-current-source circuit and photosensors are formed in the same semiconductor substrate, the CMOS constant-current-source circuit generates fixed-pattern noise.
When a voltage is applied to the gate of a MOS transistor, and a voltage is applied between the source and the drain in a state in which a channel is formed, carriers are accelerated by the concentration of the electric field near the end of the drain in the channel, to generate avalanche multiplication. Most of the carriers generated by the avalanche multiplication result in a substrate current. Light emission occurs in the process of recombination of such hot carriers. New electron-hole pairs are generated in the same semiconductor substrate by the emitted light. A part of the generated carriers diffuses through the semiconductor substrate and is mixed in the photosensors. Since the degree of mixture is not constant for all of the photosensors, fixed-pattern noise is generated.
FIG. 2
is a schematic diagram illustrating the manner of generation of fixed-pattern noise in a one-dimensional photoelectric transducer having a conventional CMOS constant-current source, together with a plan view of the semiconductor substrate. In
FIG. 2
, a constant-current-source circuit having the same configuration as the current mirror circuits shown in
FIG. 1
is provided. The upper-stage current mirror circuit includes pMOS transistors, and the lower-stage current mirror circuit includes nMOS transistors. These circuits are disposed in the vicinity of the fourth to the sixth photosensor elements of a photosensor array. It can be understood that a dark output at a portion where the constant-current source is disposed is larger than other bit outputs and results in fixed-pattern noise.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a photoelectric transducer formed on the same semiconductor substrate which can reduce noise.
It is another object of the present invention to provide a photoelectric transducer having a CMOS constant-current circuit which can reduce fixed-pattern noise when the CMOS constant-current circuit and photosensors are formed on the same semiconductor substrate.
According to one aspect, the present invention which achieves these objectives relates to a photoelectric transducer including a current mirror circuit which includes a first pMOS transistor whose source is connected to a positive power supply, a second pMOS transistor whose source is connected to the positive power supply, and whose gate and drain are connected to the gate of the first pmOS transistor, a first nMOS transistor whose source is connected to a reference potential, and whose gate and drain are connected to the drain of the first pMOS transistor, and a second nMOS transistor whose source is connected to the reference potential via a resistor, and whose gate is connected to the gate of the first nMOS transistor, and whose drain is connected to the drain of the second pMOS transistor, and a plurality of photosensors including a first-conduction-type semiconductor substrate and a plurality of second-conduction-type semiconductor regions formed near a surface of the first-conduction-type semiconductor substrate. The current mirror circuit and the plurality of photosensors are formed on the same semiconductor substrate. Voltage drop means is provided between the drain of the second nMOS transistor and the drain of the second pMOS transistor.
According to another aspect, the present invention which achieves these objectives relates to a photoelectric transducer including a first current mirror circuit which includes pMOS transistors connected to a positive power supply, a second current mirror circuit which includes nMOS transistors connected to a reference potential so as to receive respective currents of the first current mirror circuit, a constant-voltage output circuit for outputting a constant voltage from a connection point of the first current mirror circuit and the second current mirror circuit, a plurality of photosensors including a first-conduction-type semiconductor substrate and a plurality of second-conduction-type second semiconductor regions formed near a surface of the first-conduction-type semiconductor substrate, a source follower circuit for converting photoelectric charges of the photosensors into a voltage and outputting the obtained voltage, and a switching MOS transistor provided between the output of the source follower circuit and the positive power supply. These circuits and components are formed on the same semiconductor substrate. The gate of the switching MOS transistor is connected to the output of the constant-voltage output circuit
Kozuka Hiraku
Sugawa Shigetoshi
Allen Stephone
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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