JPS6258595B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sensitivity adjustment method in a solid-state imaging device.

In solid-state imaging devices, it is known that photoelectric conversion is performed not with a conventional silicon (Si) single crystal substrate but with a photoconductive layer made of an amorphous semiconductor layer, for example. This sensor uses a conventional Si single-crystal substrate for the signal readout section, but because the amorphous semiconductor layer is provided on the signal readout section using this Si single-crystal substrate, it is called a two-story sensor and is different from the conventional Si single-crystal substrate. It is distinguished from a solid-state imaging device that uses only a Si single-crystal substrate.

An object of the present invention is to provide a driving method for performing automatic sensitivity adjustment in this two-story sensor.

FIG. 1 shows the basic configuration of the present invention. Here, an explanation will be given using a device using a CCD (Charge Coupled Device) in the reading section. For ^example , as shown in FIG.
An N ⁺ layer 5 is formed. and said first N ⁺ layer 2
P ⁺ layers 6-1 and 6-2, which are channel stoppers, are provided adjacent to the second N ⁺ layer 5. Further, a transparent electrode 7 is formed on the amorphous semiconductor layer 3. The signal charges (electrons) photoelectrically converted in the amorphous semiconductor layer 3 are transferred to the second N ⁺ layer 5.
By applying a voltage to the readout gate electrode 8 adjacent to the readout gate electrode 8, the data is transferred to the first N+ layer 2 and read out. Here, a CCD transfer electrode 9 is formed on the first N ⁺ layer 2, and an insulating film 10 made of an oxide film is formed around the readout gate electrode 8 and the transfer electrode 9.
-1 and 10-2 exist. Let the voltage applied to this transparent electrode 7 be _VG , and let the surface potential of the second N ⁺ layer be _VN . The amorphous semiconductor layer 3 sandwiched between the metal electrode 4 and the transparent electrode 7 can be equivalently described as one diode. Then, V _G and V _N are applied to both ends of this amorphous semiconductor layer 3 as shown in FIG. 1b.

In FIG. 1a, V _N immediately after a voltage is applied to the read gate electrode 8 and signal charges are read out is assumed to be V _NO . Here, as shown in FIG. 1c, the voltage V _G applied to the transparent electrode 7 is changed over time. In other words, V _G is held at the V _NO for an arbitrary period that is continuous with the signal read period in an imaging operation in which one cycle consists of a signal read period for reading signal charges and an accumulation period for accumulating signal charges. , the voltage V _I is set lower than the V _NO for the remaining period. Automatic sensitivity adjustment is performed by changing this low voltage period T _I according to the amount of incident light. Here, V _I is the signal charge accumulated in the diode capacitance formed by the amorphous semiconductor layer 3 and the second N ⁺ layer in FIG ^.
The voltage is such that it does not overflow into the layer. By adopting such a method, no voltage is applied to the diode formed by the amorphous semiconductor layer 3 during the V _NO holding period of the storage period, so no signal charge is accumulated, and during the low voltage V _I holding period, no signal charge is accumulated. Only signal charges are accumulated. Furthermore, by setting V _G to a negative voltage during the signal charge readout period, the diode formed by the amorphous semiconductor layer 3 is reliably brought into a reverse bias state, and the signal charge is reliably transferred to the CCD.

The operation of the solid-state imaging device shown in FIG. 1a will be explained using FIG. 2. 2b, c, d, and e show that V is applied to the transparent electrode 7 in the cell structure of FIG. 2a.
Semiconductor substrate 1 for explaining temporal changes in signal charge accumulation due to drive voltage waveform when _G is applied
It is a potential distribution diagram on the surface.

b shows the potential distribution when V _NO is applied to the transparent electrode 7 after the signal charge readout period. What is shown here is the potential of the second N ⁺ layer 5 portion after the signal charge readout operation, and its value is V _NO . Here, as described above, by applying _VNO to the transparent electrode 7, signal charge accumulation is not performed. Next, when a voltage V _I for accumulating signal charges is applied to the transparent electrode 7, the signal charges are accumulated. During this V _I holding period, as shown in ^FIG . lowers the potential of Then, in order to read out signal charges, a voltage is applied to the readout gate electrode 8 to transfer the signal charges to the first N ⁺ layer 2 of the CCD, and the potential distribution is shown in FIG. Here, the surface potential of the semiconductor substrate 1 under the readout gate electrode 8 is _VNO , and therefore, the signal charge 11 having a potential lower than the potential _VNO determined by the voltage applied to the readout gate electrode 8 is The second to the first N+ layer 2 of the CCD
The data is transferred as shown in Figure e.

As described above, the present invention allows automatic sensitivity adjustment to be easily performed by temporally changing the voltage applied to the transparent electrode.

FIG. 3 is a diagram for explaining an embodiment of the first invention that is further improved from that described in FIG. 1.
Since no voltage is applied to the amorphous semiconductor layer 3 during the _VNO holding period of the accumulation period in FIG. It is removed from the electrode 7. In this case, the signal charge is moved by diffusion and not by an electric field, as in the V _I holding period of the accumulation period, for example. Therefore, the period during which the signal charges stay in the amorphous semiconductor layer 3 is longer than the V _I holding period, and therefore the probability of being captured by the trap level existing in the amorphous semiconductor layer 3 increases.

The amorphous semiconductor layer 3 usually has many trap levels with different time constants within the layer. These signal charges captured in the trap level are released from the trap level with a certain time constant, so those with a long time constant become an afterimage.

The charges emitted with a longer time constant and reaching the metal electrode 4 cause a burn-in phenomenon that remains as an output on the reproduced image for a longer time. In particular, when the low voltage period T _I becomes short, that is, when the V _NO holding period becomes long, afterimages and burn-in phenomena increase, resulting in deterioration of image quality.

In view of this, as shown in FIG. 3, the voltage applied to the transparent electrode 7 is held at, for example, V _I , which is lower than the V _NO , for an arbitrary period following the signal readout period, and then held at the V _NO . Then, as in Figure 1, V
Automatic sensitivity adjustment is performed by holding V I at _I and changing the V _I holding period T _I according to the amount of incident light.

That is, according to this method, an electric field is generated in the amorphous semiconductor layer 3 during the first V _I holding period following the signal charge readout period, unlike the method shown in FIG. 1 described above. The signal charge generated in the amorphous semiconductor layer 3 during this period by this electric field quickly travels through the amorphous semiconductor layer 3, reaches the metal electrode 4, and is accumulated. Next, during the _VNO holding period, the signal charges accumulated in the metal electrode 4 are removed from the metal electrode 4 through the amorphous semiconductor layer and the transparent electrode 7. Then, in the second V _I holding period T _I , signal charges that become an output signal are generated in the amorphous semiconductor layer 3, travel to the metal electrode 4 side, and accumulate significantly. The signal charge generated during the first V _I retention period is an invalid charge with respect to the output signal, and the electric field shortens the residence time of this invalid charge in the amorphous semiconductor layer 3, which causes afterimage and burnout. The number of charges trapped at the trap level within the amorphous semiconductor layer 3, which causes the sticking phenomenon, is also reduced.

Here, the voltage applied to the transparent electrode 7 following the readout period is at a lower level than V _NO even if it is not V _I , and the accumulated signal charge is applied to the CCD.
Any voltage that does not leak into the first N ⁺ layer 2 is sufficient.

FIG. 4 is for explaining a second embodiment of the present invention, which is a further improvement of the embodiment shown in FIG. As shown in this figure, a low level voltage V is applied to the transparent electrode 7 during an arbitrary period following the signal readout period.
Apply a pulse voltage of _I , high level voltage _VNO ,
After that, the voltage V _I is held, and this V _I holding period T
Sensitivity is adjusted by changing _I according to the amount of incident light. Here, the low level voltage V _I
The period in which V is applied is a period in which signal charges can be accumulated, and the period in which high-level voltage V _NO is applied is a period in which signal charges are not accumulated; however, in Fig. 3, the first V _I holding period is constant. In this first V _I holding period _,
For example, if a strong light spot is amorphous semiconductor layer 3,
When incident on the amorphous semiconductor layer 3, the generated charge travels through the amorphous semiconductor layer 3 and reaches the metal electrode 4, causing the surface potential _VN of the second N ⁺ layer 5 to change with time, and causing the amorphous semiconductor layer to become free of electric field. It brings you closer to the V _I that you want to make. That is, when an object with high light intensity is imaged, afterimages and burn-in phenomena are likely to occur even if the first _VI holding period is provided as described above. On the other hand, by applying a pulse voltage as shown in Fig. 4, signal charges can be accumulated once, that is, by applying the V _I voltage and then changing to V _NO , the signal charges are accumulated for this period. Signal charges are removed from the transparent electrode 7. By repeating this process, the state in which the electric field always exists in the amorphous semiconductor layer 3 can be made longer than in the case of FIG. 3, and the afterimage and burn-in phenomena can be further reduced. At this time, the number of pulse voltages may be arbitrary, and the waveform thereof is not limited to a rectangular shape, and may be a sine wave, for example.

FIG. 5 is for explaining still another driving method for more reliable sensitivity adjustment. In the explanation of FIGS. 1, 2, 3, and 4, following the signal readout period, the readout gate electrode 8 in FIG.
In this method, the voltage applied to the transparent electrode 7 is maintained at the surface potential V _NO of the second N ⁺ layer 5 immediately after the signal charge is read out by applying a voltage to the transparent electrode 7 . During the period in which V _NO is applied to the transparent electrode 7, there is no accumulation of signal charge, which is ideal for sensitivity adjustment, but in reality, this V _NO is detected _and It is difficult to accurately set the voltage applied to the transparent electrode 7. For example, when the voltage applied to the transparent electrode 7 becomes a higher level voltage than V _NO , the second N ⁺
Electrons move from the layer 5 to the transparent electrode 7, pushing the potential of the second N ⁺ layer 5 above V _NO . Then, when V _I is applied to the transparent electrode 7 and signal charge accumulation is started, the second
The signal charge accumulated during the period until the surface potential of the N ⁺ layer 5 reaches _VNO is used for the next signal charge readout.
It is not transferred to the first N ⁺ layer 2 of the CCD and cannot be read out as a signal. For this reason, it becomes impossible to obtain the contrast of dark parts of the subject, and the quality of the reproduced image deteriorates. In order to prevent such image quality deterioration, as shown in FIG. 5a, the voltage V _N1 is maintained at a lower level than the V _NO for an arbitrary period following the signal readout period. This makes it possible to solve the above-mentioned problem that the signal charge cannot be taken out as a signal. Here, signal charges are accumulated during the V _N1 holding period, and by bringing V _N1 closer to V _NO , the amount of accumulated signal accumulation can be reduced.

FIG. 5b shows the photoelectric conversion characteristics of this driving method. In this figure, the vertical axis is the output, the horizontal axis is the amount of incident light, the solid line shows the photoelectric conversion characteristics obtained by the drive method shown in Figure 1c, and the dashed-dotted line shows the fifth
This is the photoelectric conversion characteristic during the V _N1 holding period shown in Figure a. Since the amount of signal charge that can be accumulated during this V _N1 holding period is small, it is saturated at a low amount of incident light.
Therefore, the obtained overall photoelectric conversion characteristic is the sum of the solid line characteristic and the one-dot chain line characteristic, as shown by the dotted line. As described above, according to this method, even when the amount of incident light is small, the problem that the signal charge is not read out does not occur. In addition, the photoelectric conversion characteristics shown by the dotted line in FIG. 5b show that the gain is high for dark subjects with a small amount of incident light, and the image quality can be further improved.

Although the embodiments have been described using a photoconductive amorphous semiconductor, it goes without saying that various other photosensitive materials can be used. Therefore, although the amorphous semiconductor layer 3 in the embodiment has been described as a single layer, it may be a plurality of amorphous semiconductor layers or a plurality of various photosensitive layers.

Further, although the present invention has been explained using a CCD as a signal charge readout section, for example, a BBD (Bucket
Brigade Device). That is, in the present invention, the signal accumulation and readout constitute one period.
It can be applied to "story building sensors" and is not limited to CCDs.

It goes without saying that the present invention can be applied to both one-dimensional and secondary sensors. Furthermore, as an example, the amorphous semiconductor layer 3 is sandwiched between a transparent electrode 7 and a metal electrode 4.
Although the description has been given of a device that utilizes the characteristics in the thickness direction, it goes without saying that the present invention can also be applied to a device that utilizes the characteristics in the lateral direction. Furthermore, although a single transparent electrode 7 has been described, it is also possible to control a plurality of transparent electrodes. In the embodiment, the voltage applied to the transparent electrode 7 changes with time during the V _I holding period of signal charge accumulation, and the V _I
It may be something that reaches .

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams for explaining the basic configuration of the present invention, Figure 3 is a diagram for explaining an embodiment of the first invention, and Figure 4 is a diagram for explaining the second invention. and FIG. 5 are diagrams showing other examples of drive waveforms and optical conversion characteristics in the sensitivity adjustment method of the present invention. 1: p-type semiconductor substrate, 2: first N ⁺ layer, 3:
amorphous semiconductor layer, 4: metal electrode, 5: second
N ⁺ layer, 6-1, 6-2: P ⁺ layer, 7: transparent electrode, 8: readout gate electrode, 9: CCD transfer electrode, 10-1, 10-2: insulating film.

Claims (1)

[Claims] 1. A layer of an opposite conductivity type is provided on a semiconductor substrate of one conductivity type, and a photoelectric conversion layer is formed directly or electrically connected to the layer of the opposite conductivity type, and a conductor electrode is formed on the photoelectric conversion layer. In the solid-state imaging device, the signal charge generated by light incidence on the photoelectric conversion layer is read out by a readout section provided on the semiconductor substrate adjacent to the opposite conductivity type layer. One cycle is composed of a period for accumulating charges and a period for reading signal charges from the reading section, and in the middle of the accumulation period, an operation is performed to release the accumulated signal charges to the electrodes, and the subsequent accumulation A method for adjusting sensitivity of a solid-state imaging device, characterized in that sensitivity is adjusted by controlling a period length according to an amount of incident light. 2. A semiconductor substrate of one conductivity type is provided with an opposite conductivity type layer, and a photoelectric conversion layer is formed on this opposite conductivity type layer directly or electrically connected, a conductive electrode is provided on the photoelectric conversion layer, and the opposite A period for accumulating signal charges in a solid-state imaging device in which signal charges generated by light incident on the photoelectric conversion layer are read out by a readout section provided on the semiconductor substrate adjacent to the conductivity type layer; One period is constituted by a period of reading out the signal charge from the reading section,
A solid-state imaging device characterized in that sensitivity is adjusted by performing an operation of discharging the signal charge accumulated during the accumulation period to the electrode a plurality of times, and controlling the length of the subsequent accumulation period according to the amount of incident light. How to adjust the sensitivity.