JPS5940347B2 - Driving method of solid-state image sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a solid-state image sensor that reads out optical information accumulated in a plurality of photodiodes formed on a semiconductor surface.

In the conventional example described below and the description of the present invention, a solid-state image sensor uses electrons as an information source, but by reversing the charge polarity and potential relationship, a solid-state image sensor using holes as an information source can be changed. It can be applied in exactly the same way.

FIG. 1 is an example of a conceptual diagram showing a part of the planar structure of a solid-state image sensor that uses a conventional charge transfer element as a signal readout device. The charge photoelectrically converted by the photodiode 1 is stored under the storage gate 2 and in the photodiode 1, and after a predetermined time (storage time), it is transferred to the charge transfer element 4 via the transfer gate 3, and is sequentially transferred to the charge transfer element 4. Read out to the output side.

FIG. 2 schematically shows an example of the conventional driving method and output waveform of the solid-state image sensor shown in FIG. 1.

In FIGS. 1 and 2, a DC voltage VpO is applied to the storage gate 2 to accumulate charges thereunder, and the transfer gate 3 is kept in a conductive state for a transfer time τ1 (as shown in 11).
Hereinafter, the state of the pulse that is in a conductive state or an accumulation state is called 0-ON, and the state of a pulse that is non-conductive or non-accumulative is called "OFF".
The charge transfer element is driven by two-phase pulses φ1 and φ2. As shown at 12 and 13 in the figure, one force is on during the transfer time τ1 of φ1 and φ2, and the other remains off, and then the on pulse (second
Signal electrons are received from below the photodiode 1 and the storage gate 2 (hereinafter this part will be referred to as a photosensor section) under the electrode to which φ1 in the figure is applied. When the transfer is completed, the signal electrons are sequentially sent to the output side by the charge transfer element 4. Photosensor section (photodiode 1 and storage gate 2
Since the transfer gate 3 is off, the electrons photoelectrically converted during this period are stored in the photosensor section (
Accumulation time τ8). In recent years, solid-state image sensors have come to be used for various purposes due to their advantages such as light weight and small size.

Along with this, the simple driving method shown in FIG. 2 is no longer applicable to all applications. For example, in applications where image signals are transmitted using telephone lines,
Because the signal band that can pass through telephone lines is low in the voice band, signals can only be sent at low data rates. However, due to the generation of dark current in semiconductor devices, false signals are likely to be mixed in when reading at a low data rate, and data rate conversion is required between reading from the semiconductor device and transmission. When the time required for this signal processing is longer than the sum of the accumulation time τ8 and the transfer time τ (one cycle in the case of the driving method shown in FIG. 2), the time required for this signal processing is as shown in FIG. A different drive method is required.

FIG. 3 is a diagram showing an example of a conventional driving method for a solid-state image sensor when the time required for signal processing as described above is shown.
After the transfer time τ, shown at 1, a signal processing time τ9 shown at 22 continues for a long time, including the signal readout time τRO by the charge transfer element.

During this signal processing time τ, the transfer gate pulse φ1 is off, so during this time signal charges are accumulated in the photosensor section in the same way as during the accumulation time τ8.

Since the charge accumulated during this period does not have a predetermined accumulation time, the information is different from the desired one. Therefore, as shown in FIG. 3, before entering the accumulation time 24, the transfer gate pulse φ1 is turned on as shown in 23, and the signal processing time τ, 2 is turned on.
(hereinafter, this is called a plinth scan; this output is not sent to the subsequent stage and is discarded). As shown in Figure 2, the accumulation time τ8 in one cycle
Even with a driving method using only the transfer time τ1, it is possible to achieve τ longer than τ8 by thinning out the readout signal by an appropriate number of cycles, but in this case, τ becomes τp′
Since it has to be an integral multiple of PS+τ1, it is inconvenient and cannot be applied to an actual system, and it is necessary to use the driving force method as shown in FIG. 3, which performs pre-scanning.

However, the driving method shown in FIG. 3 has the following serious drawbacks.

In other words, in order to take enough time for signal processing, τ, 〉τ8
shall be. Alternatively, when strong light hits the period τ, the amount of accumulated charge increases and cannot be completely accommodated in the charge transfer element, and the charge cannot be completely removed by pre-scanning and remains behind. The signal may be output superimposed on the subsequent original read signal 25. The magnitude of this varies depending on the strength of the optical signal irradiated during τ2, and like a kind of afterimage phenomenon, more charge remains on pixels that are particularly strongly hit by light. , the information of the previous image appears superimposed on the next main signal. In the example in Figure 3,
Even if the signal stored for a period of τ shown at 22 is taken out as shown at 26 by the pre-scanner p, a portion remains behind and is superimposed on the main output 25 to give an output like 27.

In order to eliminate this phenomenon, it is necessary to increase the number of pre-scans depending on the light intensity and signal processing time τ, which makes the peripheral circuitry complicated and increases the cost of configuring the system. This has become a serious defect in the practical use of solid-state image sensors.

An object of the present invention is to provide a driving force method that eliminates the above-mentioned problems.

To achieve the above object, the present invention makes the transfer gate conductive or pulses the storage gate for at least part of the time τ required for signal processing, τ
This function prevents the accumulation of charges that exceed the charge holding capacity of the charge transfer element during the period of time, and in conjunction with this, the pre-scanning prevents the mixing of false signals into the main signal even once, as in the past. Alternatively, we provide a driving force method that does not have the above-mentioned problems even without a pre-scan, and this makes it possible to drive the device with a simple peripheral circuit, simplifying the system configuration, reducing costs, and ultimately achieving high reliability. It becomes possible.

Example Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 4 is a timing chart showing an embodiment of the driving force method according to the present invention, and as is clear from the comparison with the conventional pre-scanker method shown in FIG.
1 applies a pulse that is off during the accumulation time τ8 (31, 32 in FIG. 4) and on at other times.

In this way, the signal charge is accumulated during the accumulation time τ8, but at other times it is not accumulated and the charge generated by the optical signal always flows into the charge transfer element and is removed, and the charge generated during the signal processing time However, it does not mix into the main signal. In this case, the time (transfer time τ1) for transferring the charge from the photosensor section to the charge transfer element is determined not by φ1 but by the time during which the drive pulses φ1 and φ2 of the charge transfer element are held on and off (Fig. 2, 12 and 13). automatically determined. According to the driving method shown in FIG. 4, an amount of charge corresponding to the intensity of light always flows into the charge transfer element except for the time τ, so this charge inflow also occurs in the main signal during the readout period τRO33, 34. . However, originally in principle τ5〉
τRO, and in practice it is easy to set τ8>τRO, so under this condition, the amount of charge flowing in is sufficiently small and can be ignored. However, if τ8〉τR
If it is not possible to set the condition of O, or if it is desired to strictly prevent the inflow of charges, the driving force method as shown in FIG. 5 may be adopted.

FIG. 5 is a timing chart showing another embodiment of the present invention. As can be seen from comparison with FIG. 4, the accumulation time τ,
The time τX45, 46, φ, which includes the readout time τRO41, 42 and the readout time τRO43, 44, is turned off to prevent charges caused by the optical signal from flowing into the main signal from other sources during the readout time. With the driving method shown in FIG. 5, the effects of the present invention can be obtained even when τ8 and τRO do not differ much. At this time, the time τ including reading of the main signal.

During 45 and 46, φ1 is off, so the charge due to the optical signal accumulated during this time is output as shown at 47 and 48, but this is because the signal processing section should not read anything other than this signal. Since this is easy, there is no change in the effect of the present invention.

FIG. 6 is a diagram showing another embodiment of the present invention.

In the embodiment shown in FIG. 6, instead of applying the DC voltage PG to the storage gate as shown in FIG. 2, a storage gate pulse φPG is applied. Since φPG is off during the period 51 other than the storage time τ8 and the transfer time τ1, no charge is accumulated, and no charge is mixed into this signal due to light irradiation during signal processing. In the embodiment shown in FIG. 6, a pulse φPG is applied to the storage gate, and τ, +τ
Except for distinguishing the signal processing time τ 1 and other signal processing times, the conventional simple driving force method in p shown in FIG. The peripheral circuitry of the device becomes very simple, and the effects of the present invention are significant.

In the solid-state imaging device driving method described in FIGS. 2 to 6 in the explanation of the present invention, the minimum requirement for the transfer pulse φ1 is that φ1 is off during the accumulation time τ8 and on during the transfer time τ1. This is what is happening.

Therefore, in the embodiment shown in FIG. 4, φ1 is τ. It does not need to be on for the entire period τ, as long as it is on for a period long enough to transfer all of the large amount of charge accumulated during the period τ to the charge transfer element during the period pτ. good.

For example, even in the conventional driving method shown in FIG.
, by turning on φ1 for an appropriate period of time, 2
It is possible to reduce the amount of charge taken out in 6 and avoid outputs mixed with false signals such as in 27, thereby achieving the effects of the present invention. Similarly, in the embodiment shown in FIG.
1 is off during the period indicated by τ8, τ, and on during the period τ1, and if τ, - τ is a sufficient time to remove the charge generated during τ, then τo≧τRO Under the condition of τ
The length of , is arbitrary.

In the drive system shown in FIG. 6, τ is as shown by the broken line at 52/ with respect to φPG.

Even if it is on during the period of , if it is possible to eliminate the charge accumulated for 52 periods during the remaining off period,
There is no change in the effects of the present invention. In the embodiments of the present invention described above in FIGS. 4 to 6, parts other than the schematic output line and φ1 or φPG are omitted, but the fourth
In Figures 4 and 5, a DC bias PG is applied to the storage gate, and in Figures 4 to 6, one of φ1 and φ2 is turned on only during the period marked as transfer time τ1, as in Figure 2. , the external force is held off to transfer the charge to the charge transfer element, and during other periods it is always driven at a predetermined frequency.

That is, in the embodiment of the present invention, the transfer time τ1 is equal to φ1, φ
2 refers to the time during which the charge transfer element remains on and off for a time longer than the driving cycle, and the accumulation time τ8 is equal to τ1
Immediately before, in Figures 4 and 5, φ1 is turned off,
FIG. 6 indicates the time when φPG is turned on.

Further, the readout time τRO is determined by the number of picture elements of the element and the data rate. In a broad sense, signal processing in an external circuit can be performed from the time a signal begins to be read until the next signal begins to be read out again, that is, throughout one cycle of the device's operation. In the description of the present invention, for convenience, the time other than τ8+τ is referred to as signal processing time τ. The embodiments of the present invention described above are applied to the solid-state imaging device whose configuration is conceptually shown in FIG. 1, but this applies regardless of whether the device is one-dimensional or two-dimensional. Of course, it can be applied to any solid-state imaging device as long as the structure is such that a photodiode, a storage gate transfer gate, and a charge transfer element are connected in sequence. It is also possible to have a structure in which storage gates, transfer gates, and charge transfer elements are arranged and photodiodes are alternately connected to these.

In the structure of the solid-state image sensor shown in FIG. 1, a storage gate 2 is provided adjacent to a photodiode 1. However, it is recommended that the storage gate also have a photoelectric conversion function, for example by using a transparent electrode, or that the photodiode There are also devices with a structure in which one electrode serves as the storage gate. It goes without saying that the present invention can be applied to such devices, and the embodiments shown in FIGS. 4 and 5 have a structure consisting of a photodiode, a transfer gate, and a charge transfer element without an accumulation gate. It is clear that the present invention can also be applied to elements. In the description of the present invention, the drive of the charge transfer element is φ1, φ
However, the effect of the present invention remains the same no matter how many phases there are, and it depends on which phase of the drive pulse is turned on or off to receive the charge during the transfer time τ. Of course, this will vary depending on each case.

In the embodiment of the present invention, an example in which the present invention is applied to FIG. 4, φ in FIG. 5, and φPO in FIG. 6 is shown. Simultaneously with the driving method shown, φPG may be applied to the storage gate at the timing shown in FIG.

FIG. 7 shows an example of a readout control circuit specifically implementing FIG. 6, and FIG. 8 shows its timing chart.

A predetermined accumulation time signal INT generated in an external circuit is sent to a one-shot multipipulator (0N) 73 and an 0R circuit 7.
Join 4. 0M73 is triggered by the rear end of the INT signal and becomes Q.

Generates an M signal. The QOM signal is applied to 0R74.
The output of the 0R circuit 74 is applied to the storage gate 2.

The high level value of the 0R circuit 74 is a predetermined voltage value for placing the solid-state image sensor 75 in the accumulation mode.

Therefore, while the NT signal and QOM signal are being output, the solid-state image sensor 75 is in the accumulation mode. The QOM signal is also applied to a D-type flip-flop 72 to generate the φ1 signal in synchronization with the clock pulse CP. Therefore, the signal accumulated during the INT signal period is transferred to the charge transfer element 4 through the transfer gate 3 and to the output via the output gate 76 and the output amplifier JMOV. In addition, 71 is a phase shift pulse φ1, φ2
0G is the output gate pulse. As explained above, according to the present invention, when driving a solid-state image sensor,
Either the transfer gate is conductive during the time required to process the output signal, or the storage gate is pulsed to be non-conducting during the time required to process the output signal, eliminating the charge generated during the signal processing time to achieve the desired accumulation. It is possible to easily prevent false signals from being mixed into the main signal that is stored over time. This makes it possible to obtain clear signals even with simple peripheral circuits, improve the image quality of solid-state image sensors, and improve system configuration. Significant improvements can be made in terms of simplification and cost reduction.

[Brief explanation of drawings]

Figure 1 is a diagram schematically showing a part of the planar structure of a conventional solid-state image sensor, and Figures 2 and 3 are diagrams showing the structure of a conventional solid-state image sensor.
The drawings showing the driving force method and FIGS. 4 to 8 are drawings showing examples of the driving method of the present invention.

Claims (1)

[Claims] 1 A plurality of photodiodes, an accumulation gate that accumulates signal charges generated in the photodiodes by applying a signal voltage, a transfer gate that transfers the accumulated charges of the accumulation gates, and a transfer gate that is driven by a transfer pulse and is transferred from the transfer gate. In a solid-state imaging device including a charge transfer element that sequentially reads out the accumulated charges to the output side, one reading cycle of the solid-state imaging device is a time period during which the photodiode photoelectrically converts an optical signal and accumulates it as a signal charge. , consisting of a time for transferring the signal charge to the charge transfer element, a time for reading it with the charge transfer element, and a time for processing the read signal in an external circuit, and at least a part of the processing time is spent on the accumulation. A method for driving a solid-state image sensing device, characterized in that the gate is non-accumulating or the transfer gate is conductive so that no charge is accumulated during a part of the processing time.