JPH01214157A - Ultra-high sensitivity mos solid-state image sensing device - Google Patents

Abstract

PURPOSE:To improve the sensitivity of a solid-state image sensing device by endowing picture elements with amplification action, by using phtotransistors as the photodetector elements. CONSTITUTION:A photodetector 21 and a drain and source of a MOS transistor 22 are connected in series and this device is made by disposing and installing drain and gate wiring systems 23 and 24. The photodetector element 21 is composed by using a phototransistor where an NPN junction consisting of: an N-type region 33a which corresponding to a collector part formed on a P-type well 32 that is formed on an N-type substrate; a P-type region 33b which corresponds to a base part; and an N-type region 33c which corresponds to an emitter part operates as a transistor. As a result, the N-type region 33c is connected to the P-type well 32 by ohmic contact 41. Further, the N-type region 33a serves as even the source of the transistor 22 and then, the transistor 22 is composed of a read part drain 37, an isolated oxide film 35, and a transfer gate 36.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an ultra-high sensitivity MOS solid-state image sensor, and more particularly to an ultra-high-sensitivity MOS solid-state image sensor useful as a solid-state image sensor for video cameras and the like.

(Prior Art) FIG. 9 is an equivalent circuit diagram showing an example of a conventional solid-state image sensor. In the figure, 1 is a photodiode as a light receiving element, 2 is a MOS transistor, 3 is a drain wiring, and 4 is a gate wiring.As shown in FIG. A drain and a source are connected in series, and a source wiring 3 and a gate wiring 4 are arranged, and has the basic structure of a solid-state image sensor.

FIG. 10 is a sectional view showing the structure of a specific example corresponding to the circuit shown in FIG. 9. In FIG. , 14 is a light-receiving part protection film made of an oxide film, 15 is an isolated oxide film, 16 is a transfer gate, 17 is a continuous part drain, 18 is a continuous part electrode, 19 is a transfer gate drive electrode which is a gate wiring, and 20 is a drain wiring. is the drain drive electrode. This solid-state image sensor includes a P-type well 12 formed on an N-type substrate 11 and a light-receiving N-type region 1.
3, a photodiode 1 consisting of a PN junction, a light receiving part N-type region 13 (source), a MOS transistor 2 consisting of a continuous part drain 17, a transfer gate 16 formed in an isolated oxide film 15, and a transfer gate drive electrode 19. and a drain drive electrode 20 connected via a readout electrode 18. When light enters the PN junction, a charge is generated in the PN junction region, and by controlling this charge by the MOS)-run sister 2, images can be recorded and reproduced.

Incidentally, in response to the era of high-definition television, high resolution is also being required for solid-state image sensors.

In order to increase the resolution of a solid-state image sensor, it is necessary to increase the number of pixels per element, and the area occupied by the pixels must be further reduced. However, simply reducing the pixel area lowers the sensitivity of the pixel and lowers the S/N ratio.

However, since there is a limit to reducing the noise level,
It has been difficult to achieve high resolution with the solid-state imaging device shown in FIG. Therefore, a solid-state image sensor in which a pixel section is provided with an amplification function has been considered and proposed.

As a solid-state image sensor having an amplification function, for example, those whose equivalent circuits are shown in FIGS. 11 to 13 are known. In other words, a structure in which a MOS transistor is added after the photodiode of a conventional solid-state image sensor (Fig. 11), and a structure in which a MOS transistor has an amplification effect by utilizing the charge accumulation effect at the gate part (Fig. 11). 1st
Figure 2) [Nikkei Electronics, Vol. 428, No. 106
Page (1987)], 5IT (static 1ndu
Examples include those that have an amplification effect using the ction transistor system.

(Problems to be Solved by the Invention) However, conventional solid-state image sensors (hereinafter sometimes referred to as amplification type image sensors) in which an amplification function is added to the pixel section have variations in gain for each pixel, and S/ It is said to have drawbacks such as poor N ratio.

SUMMARY OF THE INVENTION An object of the present invention is to provide an amplification type image sensor that is provided with an amplification function using a method different from that of conventional amplification type image sensors.

(Means for Solving the Problems) The present invention provides a MOS solid-state image sensor in which a light receiving element and a MOS (MOS)-Ransistor drain and source are connected in series and a drain wiring and a gate wiring are arranged. This is an ultra-high-sensitivity MOS solid-state imaging device (hereinafter sometimes referred to as the imaging device of the present invention) characterized in that the device is a phototransistor.

(Function) The light-receiving element used in the image sensor of the present invention is a phototransistor and has an amplification effect.

(Example) Hereinafter, the present invention will be explained with reference to drawings showing embodiments.

FIG. 1 is a diagram showing an equivalent circuit of an image sensor according to the present invention. In the figure, 21 is a phototransistor as a light receiving element, 22 is a MOS transistor, 23 is a drain wiring, and 24 is a gate wiring. As shown in FIG. 2, the image sensor of the present invention has a configuration similar to that of the conventional solid-state image sensor shown in FIG. 9, in which the photodiode is replaced with a phototransistor 21 with a common emitter.

The phototransistor of the image sensor of the present invention is usually used as a two-terminal element in a base oven.

Next, embodiments of the image sensor of the present invention will be described with reference to the drawings.

FIG. 2 is a sectional view showing the structure of an embodiment of the image sensor of the present invention (hereinafter referred to as a ground type). In the figure, 31 is an N-type substrate, 32 is a P-type well, 33a is an N-type region corresponding to a part of the collector, 33b is a P-wall region corresponding to a base part, 33c is an N-type region corresponding to a part of an emitter, 34 is a light-receiving part protection film made of an oxide film, 35 is an isolated oxide film, 36 is a transfer gate, 37 is a continuous part drain, 38 is a continuous part electrode, 39 is a transfer gate drive electrode which is a gate wiring, and 40 is a drain wiring. A certain drain drive electrode 41 is an ohmic contact that connects the N type region 33c corresponding to a part of the emitter and the P type well 32. In the image sensor of the present invention shown in FIG.
In order to operate the N junction as a transistor, an ohmic contact 41 is made between the N type region 33c and the P type well 32.
Connect them so that they have the same voltage.

The N type region 33a corresponding to a part of the collector is MO
It also serves as the source of the S transistor 22, and the MOS transistor 22 is constituted by this, the continuous portion drain 37, the isolated oxide film 35, and the transfer gate 36. The wiring arrangement is similar to that of a conventional solid-state image sensor that uses a photodiode as a light receiving element. Note that FIG. 3 shows a perspective view corresponding to FIG. 2.

FIG. 4 is a sectional view showing the structure of another embodiment (hereinafter referred to as bias type) of the image sensor of the present invention. In the figure,
42 is an emitter electrode for bias application, and the numbers indicating other parts are the same as in FIG. 2. In the image sensor of the present invention shown in FIG. 4, the emitter electrode 4 for bias application is
2 is provided, and the emitter electrode 42 is shared by all elements so that a uniform bias can be applied. Furthermore, the fifth
The figure shows a perspective view corresponding to FIG. 4.

Next, the operation of the image sensor of the present invention will be explained.

6 to 8 (a) are conductor energy level diagrams of the image sensor of the present invention, and FIG. 6 (a) shows the emitter of the phototransistor 21, the gate and drain of the MOS transistor 22, and FIG. Figure 7(a) shows the case where all the voltages are the same; Figure 7(a) shows the case of the grounded type (or the bias type with bias O) and positive voltage applied to the gate and drain; A) is the case where a negative voltage is applied to the emitter of the phototransistor 21 and a positive voltage is applied to the gate and drain of the bias type. Moreover, (b) of FIGS. 6 to 8 all show equivalent circuits corresponding to FIG. 6 (a).

Under the conditions shown in FIG. 6(b), the conduction band energy level (hereinafter referred to as energy level) of the N-type region is located slightly lower than the energy level of the P-wall region.
6(a)], then, as shown in FIG. 7(b), when the emitter is grounded, the gate is made to have a positive voltage, and the drain is made to a higher voltage, the collector junction of the phototransistor 21 is reversed. It is biased, and the energy level of the N-type region (source) 33a corresponding to the drain 37 and part of the collector is lowered [FIG. 7(a)]. When the collector junction of the phototransistor 21 receives light in a reverse biased state, pairs of electrons and holes are formed.
a, and if the MOS transistor is in the OFF state, charges are accumulated and an image is recorded. In addition, if the MOS transistor 22 is in the ON state, the charge flows to the drain through the channel of the MOS transistor 22, and an image can be reproduced.

Furthermore, as shown in FIG. 8(b), when both the gate and drain are set to positive voltage while the emitter is set to a negative voltage, the N-type region, especially the readout region drain 37
is lower, and the P wall region, especially the P region 33b corresponding to the base portion, is higher [FIG. 8(a)]. In this case as well, since the collector junction of the phototransistor 21 is reverse biased, a photocurrent flows when light is received.

Under the conditions shown in FIG. 8(b), since the emitter is set to a negative voltage, the output voltage can be operated in the same manner as in a conventional photodiode using a photodiode as a light receiving element. Further, as can be visually understood by comparing FIG. 7(a) and FIG. 8(b), there is an advantage that even if the continuous voltage during signal transmission is small, there is less unread signal.

(Effects of the Invention) As explained in detail above, the ultra-high sensitivity MOS solid-state image sensor of the present invention has a light receiving section of a pixel having a transistor structure (bipolar type), and this transistor is configured so that its emitter is grounded. Therefore, according to the present invention, it is possible to impart an amplification effect to the pixel, and the light receiving sensitivity of the solid-state image sensor can be greatly improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an equivalent circuit of the image sensor of the present invention, FIG. 2 is a cross-sectional view showing the structure of an embodiment of the present invention, and FIG.
4 is a sectional view showing the structure of another embodiment of the present invention, FIG. 5 is a perspective view of FIG. 4, and FIGS. 6 to 8 (a)
) is a semiconductor energy level diagram, FIG. 9 is an equivalent circuit diagram showing an example of a conventional solid-state image sensor, and FIG. The cross-sectional views shown in FIGS. 11 to 13 are equivalent circuit diagrams of a conventional amplification type image sensor. 21...Phototransistor, 22.-MOS)-
Run sister, 23...drain wiring, 24... gate wiring. Figure 4 Figure 5

Claims (1)

[Claims] An ultra-high-sensitivity MOS in which the light-receiving element is a phototransistor, in a MOS solid-state image sensor in which a light-receiving element and the drain and source of a MOS transistor are connected in series and a drain wiring and a gate wiring are arranged.
Solid-state image sensor.