JPH0377712B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a charge transfer device. Imaging devices using charge transfer devices have been developed using methods called frame transfer method and interline transfer method, and these devices are rapidly developing based on the characteristics of solid-state devices: compact size, low power consumption, and high reliability. . However, considering the advantages and disadvantages of imaging devices, in addition to the advantages of solid-state devices mentioned above, they are superior to currently used image pickup tubes in terms of noise, afterimages, burn-in, etc., but they still have problems with blooming and smearing. There is. Further, when such a charge transfer device is used as a system such as a camera, a mechanical mechanism, that is, an aperture is required to adjust the amount of incident light.

As shown in FIG. 1, a conventional charge transfer imaging device using an interline transfer method has multiple columns of vertical shift registers 11 driven by the same charge transfer electrode group.
, a photoelectric conversion unit 12 adjacent to one side of each vertical shift register and electrically isolated from each other, a transfer gate electrode 13 for controlling signal charge transfer between the vertical shift register and the photoelectric conversion unit, and each vertical shift register. A charge transfer horizontal shift register 14 is provided electrically coupled to one end of the register, and a signal charge sensing device 15 is provided at one end of the horizontal shift register.

In a charge transfer imaging device using such an interline transfer method, a photoelectric conversion unit 12 converts a signal charge into a transfer gate 13 according to the amount of incident light.
The signals are transferred to the corresponding vertical shift registers 11 via the respective vertical shift registers 11. After transferring the signal charge to the vertical shift register, the transfer gate is closed,
The photoelectric conversion unit 11 accumulates signal charges for the next cycle. On the other hand, the signal charges transferred to the vertical shift register 11 are transferred vertically in parallel, and transferred to the horizontal shift register 14 for each horizontal line of each vertical shift register. The charge sent to the horizontal shift register is taken out as a signal from a device 15 that transfers and detects the signal charge in the horizontal direction while a signal is transferred from the next vertical shift register.

Figure 2 shows A- of the charge transfer imaging device shown in Figure 1.
This is a schematic diagram showing a cross section along line A'. In FIG. 2, there are two P-type regions 2 that form a P-N junction with the substrate semiconductor 16 and have different junction depths.
2 and 23 are formed, an N-type photoelectric conversion region 24 is formed on the P-type region 22 with a shallow junction, and a vertical N-type buried channel is formed on the P-type region 23 with a deep junction. A shift register 25 is formed.

FIG. 3 shows a potential distribution diagram on line B-B' of the photoelectric conversion section in FIG. 2, where the horizontal axis represents the distance in the depth direction and the vertical axis represents the potential. Let us now assume that the potential of the channel stop region 20 shown in FIG. 2 is a reference potential (in this case, 0 volts). N-type photoelectric conversion region 24
is set at a potential of V TG -V T, where the voltage of the transfer gate 13 is V _TG and the threshold voltage of the transfer gate _{is V T.} In addition, the P-type region 22 and the substrate 1
The direct current reverse bias voltage V _SUb applied to curve 2
From the lower voltage shown at 6 to the higher reverse bias voltage
When _VSUb is set, the P-type region 22 is completely depleted as shown by a curve 27. When the photoelectric conversion region 24 is irradiated with light and signal charges are accumulated, the potential of the photoelectric conversion region 24 decreases from the curve 28 to the curve 29, and finally the potential of the photoelectric conversion region 24 decreases as shown by the curve 30. The junction of the P-type region 22 is in the forward direction, and any more charges generated in the photoelectric conversion region 24 flow into the substrate semiconductor 16 via the P-type region 22. In other words, the junction potential between the photoelectric conversion region 24 and the P-type region 22 is higher than the surface potential of the areas directly below the transfer gate 13, directly below the channel stop region 20, and all regions (not shown) surrounding the photoelectric conversion region 24 shown in FIG. The substrate semiconductor and the P-type region 22
By applying a DC reverse bias voltage to
Excess charges generated in the photoelectric conversion region 24 can be completely swept out to the substrate semiconductor 16. With this structure and operation, the blooming phenomenon can be completely suppressed.

However, when such a charge transfer imaging device is used as a system such as a camera, there is a drawback that the amount of incident light must be adjusted by a mechanical mechanism, that is, an aperture.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a new method for driving a charge transfer device capable of suppressing the blooming described above.

According to the present invention, in the junction region formed on the principal surface of a semiconductor substrate and having a conductivity type opposite to that of the substrate, the first region has a shallow junction depth and the second region has a deep junction depth. A charge transfer imaging device comprising a region, a group of photoelectric conversion elements is formed on the main surface of the first region, and a device for reading out signals from the group of photoelectric conversion elements is formed on the main surface of the second region. In this step, a voltage higher than a normal reverse bias voltage is applied between the first region and the second region and the semiconductor substrate so that the first region and the photoelectric conversion element group are in the forward direction during one field period. and applying a normal reverse bias voltage necessary for the first region to be completely depleted after a certain period of time to the first region, the second region, and the semiconductor substrate. A method for driving a charge transfer imaging device is obtained.

Next, embodiments of the present invention will be described using the drawings. Hereinafter, in the embodiments of the present invention, an N-channel semiconductor device will be described to simplify the explanation.

FIG. 4 is a drive pulse waveform diagram for explaining one embodiment of the present invention. The difference from the conventional example is that the reverse bias voltage _sub applied between the semiconductor substrate and the two P-type regions 22 and 23 that form a P-N junction and have different junction depths as shown in FIG. 2 has two levels. It means having “V _s ” and “V _p ”.

Similar to FIG. 3, FIG. 5 shows the potential in the depth direction on the line B-B' of the charge transfer imaging device shown in FIG.

At time t ₁ in FIG. 4, the substrate semiconductor 16
The reverse bias voltage _sub applied between the P-type region 22 and the P-type region 22 forming the P-N junction is "V _p ". Fifth
In the figure, this reverse bias voltage _sub “V _p ” is N
The direction between the photoelectric conversion region 24 of the mold and the P-type region 22 must be in the forward direction. In FIG. 5, the potential distribution curve created by this reverse bias voltage _sub "V _p " is shown at 31. During the period when the reverse bias voltage _sub "V _p " is at the level, the charges generated by entering the photoelectric conversion region 24 are transferred to the substrate semiconductor 16 because the N-type photoelectric conversion region 24 and the P-type region 22 are in the forward direction. It does not become a signal charge because it is swept out.

At time t ₂ , the reverse bias voltage _sub changes from “V _p ” to “V _s ”, so the curve indicating the depth direction potential of the photoelectric conversion section changes from 31 to curve 27 . During the period when the reverse bias voltage _sub is at the level of “V _s ”, charges generated by light incident on the N-type photoelectric conversion region 24 become signal charges, are accumulated in the N-type photoelectric conversion region 24, and the amount of incident light changes. Correspondingly, the potential of the N-type photoelectric conversion region 24 decreases from the curve 28.

At time t ₃ , the voltage at the transfer gate is
V _TG is applied, and the signal charges accumulated in the N-type photoelectric conversion region 24 are transferred to the vertical shift register 11 . At this time, the N-type photoelectric conversion region 24 is
It is set at a potential of V _TG - _VT .

At time _t4 , before the vertical shift register starts charge transfer, the reverse bias voltage _Vsub " _Vp " is again applied to the N-type photoelectric conversion region 24, and the operation of the next field is repeated.

Due to this operation, the light accumulation time in one field is a period _T1 from time _t2 to time _t3 . Period T ₁
The amount of signal charge can be controlled by making it short when the amount of incident light is large and lengthening it when the amount of incident light is small depending on the amount of incident light. Therefore, the diaphragm mechanism used in conventional cameras and the like is no longer necessary.

Further, the reverse bias voltage _sub "V _p " needs to be greater than "V _s " and at least a voltage that makes the connection between the N-type photoelectric conversion region and the P-type region 22 in the forward direction. Do not apply a voltage high enough to deplete the capacitor.

Although the driving method of the embodiment of the present invention has been described above for an N-channel, it goes without saying that it can be applied to a P-channel semiconductor device by reversing the conductivity type of each region.

[Brief explanation of drawings]

Fig. 1 is a plan view of an imaging device using a conventional charge transfer device, Fig. 2 is a schematic cross-sectional view taken along line A-A' shown in Fig. 1, and Fig. 3 is a schematic cross-sectional view taken along line B-B shown in Fig. 2. ’ shows the potential distribution on the line. FIG. 4 shows a driving pulse waveform for explaining an embodiment of the present invention, and FIG. 5 shows a potential distribution on the line B-B' shown in FIG. 2 according to the driving method of the present invention. In the figure, 11...vertical shift register, 1
2... Photoelectric conversion unit, 13... Transfer gate, 14... Horizontal shift register, 15... Charge detection device, 16... Semiconductor substrate, 17... Insulating film, 18... Vertical shift register electrode, 19...
Transfer gate electrode, 20...channel stopper, 21...light shielding, 22, 23...P
Type region, 24... N-type photoelectric conversion region, 25...
N-type vertical shift register, 26, 27, 28,
29, 30, 31...Curves showing potential distribution.

Claims (1)

[Claims] 1. A first region having a shallow junction depth and a second region having a deep junction depth are provided on a main surface of a semiconductor substrate, the junction region having a conductivity type opposite to that of the substrate, and the second region having a shallow junction depth. In a charge transfer imaging device, a group of photoelectric conversion elements is formed on the main surface of the first region, and a device for reading out signals from the group of photoelectric conversion elements is formed on the main surface of the second region. Applying a voltage higher than a normal reverse bias voltage between the first region and the second region and the semiconductor substrate so that the first region and the photoelectric conversion element group are in the forward direction during the period, Charge transfer imaging characterized in that a normal reverse bias voltage necessary for the first region to be completely depleted after a certain period of time is applied to the first region, the second region, and the semiconductor substrate. How to drive the device.