JPS6130433B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a solid-state imaging device having a structure in which a photoconductor and a charge transfer element or a photoconductor and a switching element are combined. By eliminating the light sensitivity of the photoconductor during the period of reading into the transfer element or the switching element, a solid-state imaging device without blooming or afterimage is provided.

Conventionally, solid-state imaging devices (hereinafter referred to as charge transfer type) in which a photoconductive film is formed on a semiconductor substrate with a charge transfer function such as a CCD or BBD, or MOS switches arranged in a matrix, with X and In a solid-state imaging device (hereinafter referred to as an Since a reverse bias was applied to the body, the photoconductor remained light sensitive even during the signal reading period. Therefore, when there is strong incident light, the amount of photo-generated charge during the signal reading period becomes a non-negligible amount, and if this amount exceeds the maximum amount of charge that can be handled by the charge transfer element or switching element, the transfer stage will overflow. This caused afterimage development. When a camera is irradiated with strong spot light, the overflow of the transfer stage appears as a development in which the signal spreads above and below the spot on the television screen, which can be said to be a type of blooming phenomenon.

A conventional solid-state imaging device will be described below with reference to the drawings.

First, the conventional charge transfer type will be described.

Although there is a difference in whether charge is stored in a depletion layer or an impurity region, CCDs and BBDs operate essentially the same. Therefore, this will be explained in the future on BBD.

Figure 1 shows a unit of a solid-state imaging device that includes a BBD formed on a Si substrate, a transfer stage thereof as a drain, a gate electrode and a source region, and a photoconductor and electrodes formed on these circuit elements. be.

A diode is formed in an n ⁺ -type region 11 on a p-type semiconductor substrate 10 . 12 is a potential well in an n ⁺ type region. Reference numeral 13 denotes a gate electrode, which has a portion overlapping with the n ⁺ type region 11. 14 is an insulating film between the semiconductor substrate 10 and the gate electrode 13, which is a gate oxide film. 15 is a first electrode 16
This is an insulating layer for electrically separating the semiconductor substrate 10 and the gate electrode 13 from each other. A first electrode 16 is an electrode of a diode electrically connected to the n ⁺ type region 11 and also serves as an electrode of the hole blocking layer 17 . 18 is (Zn _1-x Cd _x Te) _1-y
It is a photoconductor made of (In ₂ Te ₃ ) _y , on which a transparent electrode 19 is formed, and incident light 21 is irradiated from the transparent electrode 19 side. Reference numeral 20 denotes a pulse voltage generator according to the present invention for zeroing out the bias applied to the photoconductor during the reading period, and the details will be described later.

Next, the case of the XY address type will be described.
FIG. 3 shows one unit of a solid-state imaging device in which a photoconductor and electrodes are formed on a MOS switch.

N ⁺ -type regions 31 and 32 are formed in a p-type semiconductor substrate 30 and serve as a source and a drain, respectively. 34
are gate electrodes of the source 31 and drain 32, which are insulated from the semiconductor substrate by a gate oxide film 33. The gate electrode 34 is also connected to the gate of each picture element in the column direction. The drain 32 is commonly connected to the drains of the picture elements in the column direction through an electrode 35. Reference numeral 36 denotes an insulator layer that electrically separates the first electrode 37 and the gate electrode 34. The first electrode 37 is electrically connected to the source region 31 and also serves as an electrode of the hole blocking layer 38 . 39 is (Zn _1-z Cd _x Te) _1-y・
(In ₂ Te ₃ ) _y is a photoconductor, on which a transparent electrode 40 is formed, and the transparent electrode 40
Incident light 42 is irradiated from the side. Reference numeral 41 denotes a pulse voltage generator for zeroing the bias applied to the photoconductor during the reading period according to the present invention, the details of which will be described later.

Next, the operations of the two types of solid-state imaging devices will be explained.

First, the optical information reading operation in a conventional charge transfer type device will be explained.

In the configuration shown in FIG. 1, a drive pulse as shown in FIG. 2a is applied to the gate electrode 13. Further, at time t ₁ when the ground potential or DC voltage is applied to the transparent electrode 19, the electrode 16 is set to (V _CH -V _T ) as shown in FIG. 2b. Here, V _T is a field effect transistor composed of n ⁺ regions 11, 12 and gate electrode 13.
This is the threshold voltage of the FET. When the incident light 21 is present, electron and hole pairs are generated in the photoconductor 18 and reach the electrodes 16 and 19, respectively.
potential decreases. Furthermore, when V _CH is applied to the gate electrode 13 at time t ₂ , from the n ⁺ region 11
Signal charges are transferred to n ⁺ region 12. As a result, the potential of n ⁺ region 11 rises again (V _CH -
V _T ). The signal charge transferred to the n ⁺ region 12 is then transferred to the output section by the transfer pulse V shown in FIG. 2a. The above is the conventional charge transfer type optical information reading operation.

Next, the optical information reading operation in the conventional XY address type will be explained with reference to FIGS. 4 and 5.

FIG. 4 shows a circuit configuration in which a plurality of unit elements shown in FIG. 3 are formed in each of the X and Y directions. 51 to 62 are FETs, and Q _on is a diode formed of a semiconductor substrate and an n ⁺ region. _Q'on is a diode made of a photoconductor, 63 is an output terminal R _L which is a load resistor, and V _O is a power supply.

FIG. 5 shows the time change of the pulse applied to the FET from the X/Y scanning circuit.
Further, a ground potential or a DC voltage is applied to the transparent electrode 40. When a pulse is applied to line _Y1 from the Y scanning circuit at time _t0 , FETs 54-56 are turned on. At this time, pulses are applied from the X scanning circuit in order from X ₁ , so _{the diodes Q 1n and Q 1n are
1} and _Q11 are reverse biased in order. At time t ₁ a pulse is applied to line Y ₂ and Q _2n ,
Q′ _2n is sequentially reverse biased. Similarly, all diodes are reverse biased.

Now, when light is incident _on the diode Q'on, electron-hole pairs are generated in the photoconductor, the electrons are collected at the first electrode 37, and the potentials of the diodes _Q'on and _Qon decrease. This potential drop is proportional to the amount of incident light, and optical information is accumulated for one field or one frame period. Next, when the diodes Q' _on and Q _on are reverse biased by a pulse from the X/Y scanning circuit, a potential reduced in proportion to the amount of light is supplied from the power source V _O through the load resistor R _L . Therefore, the output terminal 63
A potential proportional to the amount of light is output. The above is X
- This is an optical information reading operation in a configuration in which a photoconductor and an electrode are provided on a circuit element having a Y address function.

The above is the operation of the solid-state imaging device within the illuminance range where the charge due to light generation during the reading period can be ignored. Next, we will discuss the case where there is strong incident light such that the charges generated during the reading period cannot be ignored.
As is clear from the above, the potential difference V _N applied to the photoconductor during the signal reading period is equal to
In the case of the charge transfer type shown in the figure, the following equation is obtained.

V _N =V _CH -V _T -V'...(1) Also, in the case of the XY address type shown in Figure 3,
The potential difference V _N is expressed by the following formula.

V _N =V _O -V' (2) In any of the above cases, a reverse bias is applied to the photoconductor and it has photosensitivity. In the case of the conventional example, the signal reading period is approximately 200 μs, and the saturation illuminance of the photoconductor corresponds to the first electrode potential becoming equal to the second electrode potential V′ in one frame period (33.3 ms). . Therefore, if light with an illuminance 33.3/0.2=167 times the saturation illuminance is incident, the charge generated during the reading period will be equal to the charge generated at the saturation illuminance, and the total amount of charge will be doubled. The part indicated by the solid line in Fig. 6 shows the measurement results of the amount of overflow of a conventional charge transfer type solid- _state imaging device. It shows the vertical extension L of the television screen. The reason why the illuminance at 167 times the saturation illuminance does not match twice is because flare occurs due to diffuse reflection from the glass plate for protecting the element. Further, when the total amount of charge becomes extremely large, afterimage development becomes significant.

The present invention provides a solid-state imaging device having the above configuration,
By eliminating the photosensitivity of the photoconductor during the reading period, we eliminate the photo-generated charge during this period, making it possible to create a solid-state imaging device that eliminates transfer stage overflow and afterimage phenomena even in the presence of strong incident light. This is what we provide.

Figures 7 and 8 show how to eliminate the photosensitivity of the photoconductor during the signal reading period, which is a feature of the present invention.
This will be explained below using figures.

First, a charge transfer type case will be explained. As is clear from equations (1) and (2) above, the signal reading period is such that the second electrode has a signal as shown in Figure 7b in synchronization with the timing of the read pulse of the drive pulses in Figure 7a. A pulse is applied. The second electrode potential at this time is the reset potential VR due to the read operation.
(=V _CH -V _T ), the potential difference applied to the photoconductor becomes OV during the read period, and there is no photosensitivity. When the read operation is completed, the second electrode potential is held at VC. The first electrode potential V at this time is determined using the equivalent circuit of one unit of the solid-state imaging device of this embodiment shown in FIG. 8 (charge transfer section and switching element section are omitted) and the initial charge conservation law. From V _R C _S = V C _S + (V-V _C ) C _N ... (3), V = (V _R C _S + _V C _N )/(C _S + C _N ) ... (4). where C _N and C _S are the photoconductor and Si
It represents the capacitance of the diode. The potential difference V _N applied to the photoconductor is V _N = V - V _C = (V _R - _{V C} ) C _S / (C _S + C _N ) > O ... (5), and the photo conductor has A reverse bias is applied, and it has photosensitivity only during periods other than the reading period.
Therefore, in principle, no more charge than the saturated illuminance of the photoconductor is generated.

Next, the case of the XY address type will be explained. In the case of the X-Y address type, a signal reading operation is performed for the picture element whose Y line selection matches the X line selection, so pulses are uniformly applied to the transparent electrode 40 in the same way as the charge transfer type. I can't. In this case, it is preferable to divide the transparent electrode 40 along the Y line and apply pulses of height V _R (=V _O ) sequentially from y ₁ in synchronization with the Y scanning pulse as shown in FIG. 7d.

FIG. 6 shows the effect when the present invention is applied to a charge transfer type solid-state imaging device. As is clear from the figure, almost no overflow is seen, and the slight overflow that remains is considered to be the flare component caused by the glass plate mentioned above.

As is clear from what has been described above, if the present invention is used, even if a solid-state imaging device is exposed to strong spot light, the signal will spread above and below the spot portion, which is unique to solid-state imaging devices, as seen in the past. (blooming) and afterimage phenomena are completely eliminated, and its industrial value is great.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of one unit of a solid-state imaging device according to an embodiment of the present invention having a structure in which a photoconductor and a charge transfer element are combined, and FIG. 2 is an explanation of the operation of the solid-state imaging device shown in FIG. 1. FIG. 3 is a sectional view of one unit of an XY address type solid-state imaging device according to another embodiment of the present invention, which combines a photoconductor and a switching element, and FIG. A circuit diagram when a plurality of unit elements of each are formed in the XY direction, Figure 5 is a drive pulse waveform diagram of the solid-state imaging device of Figure 3, and Figure 6 is a diagram of the drive pulse waveforms caused by charges generated during the reading period. A diagram for comparing the signal spread by using the present invention and the effect of suppressing the signal spread when using the present invention,
7a and 7b are pulse waveform diagrams applied to the second electrode to bring the photoconductor into a zero bias state during the reading period, which is a feature of the present invention, and FIG. The photoconductor of the solid-state imaging device in Figure 3
FIG. 3 is an equivalent circuit diagram of a Si diode section. 11, 12, 31, 32...n ⁺ type area, 1
3, 34... Electrode, 14, 33... Insulator film, 1
5, 36... Insulator layer, 16, 37... First electrode, 17, 38... Hole blocking layer, 18, 39...
Photoconductor, 19, 40...Transparent electrode, 20, 41
...Pulse voltage generator.

Claims (1)

[Claims] 1. A semiconductor substrate having a first conductivity type and including at least one region having a second conductivity type opposite to the first conductivity type; an insulator layer formed on the semiconductor substrate except for a part of the region having a second conductivity type; and a region selected on the insulator layer so as to be electrically coupled to the region having the second conductivity type. a first electrode formed on the semiconductor substrate; a photoconductor formed to cover the first electrode and the insulating layer; a second electrode formed on the photoconductor; In a solid-state imaging device consisting of a charge transfer element or bus line wiring provided in a portion other than the region having the second conductivity type and a gate electrode formed in the insulator layer, at least optical information reading is possible. A solid-state imaging device characterized by having means for eliminating the photosensitivity of the photoconductor during the period. 2. The solid-state imaging device according to claim 1, wherein the means for eliminating photosensitivity of the photoconductor is voltage application means for eliminating photosensitivity by applying a voltage to the photoconductor.