JPS6132833B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a semiconductor photosensitive device having a structure in which a photosensitive section and a charge transfer section are separated.

A semiconductor photosensitive device is already well known in which a photosensitive section that generates free carriers by irradiation with light and a charge transfer section that transfers the free carriers, that is, movable charges are set apart from each other by a certain distance on the surface of a semiconductor substrate. This type of semiconductor device has an electrode on a photosensitive section in addition to the charge transfer section, and has a gate electrode that controls charge transfer from the photosensitive section to the charge transfer section. Each of these electrodes is an insulated gate electrode separated from the substrate by an insulating film, but in order to differentiate each electrode, in this specification, the electrode on the photosensitive area is referred to as a light gate electrode and a photosensitive area. An electrode for opening and closing the charge transfer control electrode located between the charge transfer section,
The electrodes of the charge transfer section will be referred to as transfer electrodes. Such a semiconductor photosensitive device usually does not have a single photosensitive portion, but a large number of photosensitive regions arranged in a row, and one photogate electrode is provided for each region.
That is, each photosensitive area has one MOS
It is a type photosensitive element. The charge transfer section is provided to perform charge transfer in parallel to the array line of the photosensitive elements, and the opening/closing electrode extends between the photosensitive element array and the charge transfer channel in parallel to the channel.

The operation of the semiconductor photosensitive device described above is roughly as follows. First, a voltage with a polarity that creates a depletion layer in the substrate surface layer immediately below the photogate electrode is applied to the photogate electrode, and the opening/closing electrode is kept in a closed state. When light is irradiated through the photosensitive gate electrode in this state, free carriers are excited within the depletion layer by the energy of the incident light. After accumulating free carriers in the depletion layer for a certain time, the opening/closing electrode is opened to transfer the free carriers, that is, the movable charges in the depletion layer to the charge transfer section and transfer them to the output terminal.

When this type of semiconductor photosensitive device is used for imaging, a dark current is generated due to an undesired energy level generated in the depletion layer under the photogate electrode, and the value of the current is not large enough to reach a portion on the substrate. Therefore, the question of difference arose. In other words, even without light irradiation, an undesired energy level, especially a trap level, is generated within the depletion layer, and charges (free carriers) are captured at this level and released after a certain period of time. It becomes a mobile charge again, which is the main cause of dark current. The origin of such undesired energy levels is thought to be due to crystal defects in the substrate or undesired impurities, whose density is not uniform, resulting in non-uniform dark current. This non-uniformity of dark current not only causes local non-uniformity in the black level in the reproduced image when a semiconductor photosensitive device is used for imaging, but also causes non-uniformity in the saturation output level, degrading the reproduced image quality. There was an inconvenience.

The present invention has been made in view of the above-mentioned points, and is made by not transferring all the charges generated in the depletion layer under the photogate electrode to the charge transfer section, but by always allowing some charges to remain under the photogate electrode. It is an object of the present invention to provide a novel method for driving a semiconductor photosensitive device that can significantly reduce non-uniformity of dark current.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a cross-sectional view showing an example of the basic structure of a semiconductor photosensitive device, in which a substrate 1 is made of P-type silicon, and a P-type diffusion layer 2 with a high impurity concentration is used to define the width of the active region. is formed directly below the substrate surface, and the substrate surface is coated with silicon dioxide (SiO ₂ ). The optical gate electrode 4 and the transfer electrode 5 are embedded in the SiO ₂ film, while the opening/closing electrode 6 is deposited on the SiO ₂ film 3. The optical gate electrode 4 is a transparent electrode made of polycrystalline silicon, and incident light is transmitted through the optical gate electrode 4.
from the top of the substrate 1 through the electrode 4 to reach the surface of the substrate 1. Arrow 7 indicates incident light. Further, a positive DC voltage is applied to the optical gate electrode 4 from a power source (not shown), so that the optical gate electrode 4
A depletion layer 8 is formed in the surface layer of the substrate directly below. The dotted line shows the argument for the depletion layer 8. Further, a shaded area Q schematically shows the charges accumulated in the depletion layer 8. In this figure, it is assumed that no voltage is applied to the switching electrode 6 and the transfer electrode 5.

In order to transfer the accumulated charges (free carriers) Q to the bottom of the transfer electrode 5 in the semiconductor photosensitive device shown in FIG. 1, a voltage is applied to the switching electrode 6 to form a deep depletion layer in the substrate below. Just let it happen. In order to explain the mechanism of charge transfer in the conventional method, Fig. 2 is a simplified version of Fig. 1 showing only the shapes of the electrodes and depletion layer. A first pulse generator 10 is connected to the electrode 6.
However, a second pulse generator 11 is connected to each transfer electrode 5. The peak values of the pulses output from both of the pulse generators are selected so that a depletion layer of the shape shown by the dotted line in the figure is generated.

In this way, the shape of the depletion layer formed under each electrode will be as shown by the dotted line in Figure 2, and the depth of the depletion layer formed under the optical gate electrode, the opening/closing electrode, and the transfer electrode will be d. ₁ , d ₂ , d ₃ then d ₁ < d ₂
<d ₃ . Therefore, the movable charges accumulated under the optical gate electrode pass under the opening/closing electrode 6 as indicated by arrow 12 and are accumulated under the transfer electrode 5. This is a conventionally known driving method.

However, when applying the driving method according to the present invention, the peak value of the pulse voltage applied to the switching electrode 6 is slightly lowered, so that the depth of the depletion layer generated under the switching electrode 6 can be adjusted to the depth of the depletion layer formed under the switching electrode 6. It is made slightly shallower than that under 4 so that some charge remains under the photogate electrode. FIG. 3 shows this state, and in this figure, if the depth of the depletion layer under the switching electrode 6 is d ₄ , then d ₄ >d ₁ . Therefore, charges like 13 remain in the depletion layer 8 under the photogate electrode. Since this residual charge 13 is almost buried in unnecessary energy levels such as trap levels, the energy levels become apparently inactive, and therefore, as stated at the beginning of this specification, Deterioration of reproduced image quality due to non-uniformity of dark current is significantly reduced.

According to an experimental example by the present inventors, for a semiconductor photosensitive device in which a local difference of ±30% appeared in the saturated output signal when using the conventional driving method, the driving method according to the present invention sometimes caused a local difference of ±30%. , the local difference above is ±
This was an improvement of 7%, or less than 1/4 of that achieved using the conventional method.

The effects described above are obtained by making the depth of the depletion layer formed under the switching electrode shallower than that under the optical gate electrode. By the way, if the impurity concentrations in the surface layer of the substrate under both electrodes are made different, the depths of the depletion layers formed under each electrode will be different even when the same voltage is applied to both electrodes. For a substrate that initially had a uniform resistivity, for example, if the surface layer of the substrate directly under the switching electrode is doped with an impurity of a type that gives the same conductivity type as the substrate, the switching Even if the potentials of the electrode and the optical gate electrode are the same, the depth of the depletion layer under the switching electrode is shallower than that under the optical gate electrode, so the same effect as in the embodiment can be obtained. In addition, if the substrate surface is doped with an impurity that gives a conductivity type opposite to that of the substrate, the bottom of the depletion layer formed under the doped layer will become deeper, contrary to the above, so dope the above impurity to the part of the substrate surface under the optical gate electrode. You can also get similar results.

According to the driving method of the present invention, fluctuations in the output signal level of the semiconductor photosensitive device due to unnecessary energy levels generated in the depletion layer under the optical gate electrode can be significantly reduced, resulting in significantly better reproduced image quality. There are some advantages. Therefore, this device is particularly advantageous when performing analog imaging, and is also advantageous in cases where halftones are not required, such as when imaging documents by facsimile, in that unexpected signal level fluctuations can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing an example structure of a semiconductor photosensitive device, FIG. 2 is a diagram for explaining the mechanism of charge transfer using a conventional driving method, and FIG. 3 is a diagram showing a case using the driving method of the present invention. FIG. 3 is a diagram for explaining the shape of a depletion layer and the mode of charge transfer. 1: Silicon substrate, 2: P-type diffusion layer, 3:
SiO ₂ film, 4: optical gate electrode, 5: transfer electrode,
6: Opening/closing electrode, 8: Depletion layer under the optical gate electrode,
Q: Accumulated charge, 9: DC power supply, 10: First
Pulse generator, 11: Second pulse generator, 13:
residual charge.

Claims (1)

[Claims] 1 A semiconductor substrate is provided with a plurality of photosensitive element regions that generate movable charges upon irradiation with light, and a photogate electrode provided in an insulating relationship with the substrate directly above each photosensitive element region. A photoelectric conversion section is constituted by an array of elements, and a charge transfer section is provided on the side of the photoelectric conversion section for inputting movable charges from each photosensitive element and transferring them in parallel to the arrangement direction. In a semiconductor substrate photosensitive device equipped with an opening/closing electrode that opens and closes a charge path between a photoelectric conversion section and a photoelectric conversion section, when moving charges generated in each of the photosensitive element regions in response to the light irradiation is transferred to a charge transfer section,
The voltage levels applied to the photogate electrode and the switching electrode are adjusted so that when the charge path is opened, the bottom of the depletion layer formed in the photosensitive element region under each photogate electrode is a depletion formed in the charge path under the switching electrode. The potential is selected to be slightly deeper than the bottom of the layer, and only the charge in the portion exceeding the depth level of the depletion layer under the opening/closing electrode is transferred to the charge transfer section, and the remaining charge remains in the photosensitive element area. A method for driving a semiconductor photosensitive device, characterized in that at least the influence due to the difference in dark current between each photosensitive element region is reduced.