JPH0654117A - Image sensor and driving method therefor - Google Patents

Abstract

PURPOSE:To make it possible to obtain an exact image signal without an after- image phenomena due to untransferred electric charge. CONSTITUTION:Optical electric charge generated in a photodiode 2 composing a photodiode group 6 is read by an IC 3 for driving for every photodiode group 6. After this optical electric charge is read, the output voltage of a gate pulse generation circuit 4 and a variable voltage circuit 5 temporarily change by synchronizing with each other, the photodiode 2 is made into a normal bias condition and the electric charge remaining in the photodiode 2 is swept after the optical electric charge is read.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor used for image scanners, facsimiles and the like for reading an image.

[0002]

2. Description of the Related Art Conventionally, as shown in, for example, Japanese Patent Application No. 2-265362, one of the image sensors of this type has a large number of photodiodes arranged linearly and each photodiode is -A thin film transistor (hereinafter, referred to as "TFT") is connected in series to each photo diode, and each photo diode is connected.
Charge generated in the photo diode through this TFT.
By temporarily accumulating in the capacitance between the wirings in a specific block unit of the code, the accumulated charges are sequentially read out in time series as an electric signal at a speed of several hundred KHz to several hundred MHz. There is a TFT drive type image sensor formed of the above, and it is already publicly known. Such TF
In the T drive type image sensor, in order to obtain a clear image signal without so-called afterimage, there is no residual charge in the photodiode after the charge generated in the photodiode is read by the TFT. Is desired.

[0003]

However, in reality, since there is a parasitic capacitance of the photodiode and an overlap capacitance between the drain and gate electrodes of the TFT, untransferred charges are stored in these capacitance portions. Remains, and the untransferred charges are added to the charges generated by reading a new image after reading the charges from the photodiode.
There is a problem that a so-called afterimage phenomenon occurs and accurate image reading cannot be performed.

The present invention has been made in view of the above circumstances, and an image sensor capable of obtaining an accurate image signal without an afterimage phenomenon due to untransferred charges and an image sensor capable of easily removing untransferred charges. To provide a method.

[0005]

In order to solve the above problems, an image sensor according to the present invention comprises a plurality of photodiodes, and these plurality of photodiodes are reverse-biased. In the image sensor in which the photocharge generated in the photodiode is read out as an image reading signal, the photocharge is read from the photodiode by being connected to the cathode electrode of the photodiode. In between, the photo diode ano
A first voltage higher than the potential on the side of the cathode electrode, a voltage smaller than the first voltage after the photocharges are read out, and then the first voltage are applied to the cathode electrode, respectively. The main bias means and the forward bias state of the photodiode are connected to the anode electrode of the photodiode via a capacitor in synchronization with the main bias means outputting a voltage smaller than the first voltage. And an auxiliary bias means for applying a potential change.

According to a second aspect of the present invention, there is provided an imager driving method comprising a plurality of photodiodes.
In an image sensor in which these photodiodes are reverse-biased and the photocharges generated in the photodiodes are read out as image reading signals, after reading the photocharges generated in the photodiodes,
Once the photodiode is forward biased, then
The photodiode is returned to the reverse bias state.

According to a third aspect of the present invention, there is provided a method for driving an image sensor, which comprises a plurality of photodiodes, and these photodiodes are reverse-biased to generate photocharges in the photodiodes. In the image sensor for reading out as an image reading signal, the anode of the photodiode is set to the ground potential via a capacitor, and the cathode of the photodiode is connected to the cathode of the photodiode. A positive potential pulse is applied to one end of the capacitor from a state in which the photodiode is reversely biased by applying a positive voltage higher than a voltage drop in the forward direction, and at the same time, the cathode of the photodiode is By setting the photodiode to the ground potential while the positive potential pulse is generated, the photodiode is kept in the forward bias state for a certain time. Is shall. The capacitance of the capacitor connected to the anode of the photodiode is such that the feedthrough voltage generated when a positive polarity pulse is applied to one end of this capacitor is the cathode of the photodiode. It is preferable that the voltage is set to a value higher than the feedthrough voltage generated when it is set to the ground potential. In addition, the capacitor is composed of two parallel plate electrodes,
It is also preferable that the opposing area of the two electrodes determines the feedthrough voltage generated when a positive polarity pulse is applied to one end of the capacitor. Alternatively, the feedthrough voltage generated when a positive polarity pulse is applied to one end of the capacitor is preferably adjusted by the magnitude of the positive polarity pulse.

[0008]

Therefore, the image according to the invention of claim 1
In the di-sensor, after reading out the photocharges from the photodiodes, the photodiodes are positively biased once by the main bias means and the auxiliary bias means. It is possible to provide an image sensor capable of obtaining an accurate image signal without causing a so-called afterimage phenomenon, because the residual electric charge remaining after reading is swept by the forward current flowing in the photodiode. Becomes In the image sensor driving method according to the second aspect of the present invention, since the photo diode generated in the photo diode is read and then the photo diode is once put in the forward bias state, the photo charge is read. Even after that, the electric charge remaining in the parasitic capacitance of the photodiode is swept away by the forward bias, so that a so-called afterimage phenomenon does not occur and an accurate image signal can be obtained. Further, in the image sensor driving method according to the third aspect of the present invention, in particular, the feedthrough voltage generated when a positive polarity pulse is applied to one end of the capacitor as described in the fourth aspect is the photodiode. The potential on the anode side when the photodiode starts accumulating the photocharge is set to be higher than the feedthrough voltage generated when the cathode of the photodiode is set to the ground potential. As a result, the offset voltage of the output signal becomes smaller than in the conventional case, and the dynamic range in the circuit to which the image sensor is connected can be increased.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of an image sensor and an image sensor driving method according to the present invention will be described below with reference to FIGS.
Will be described with reference to. Here, FIG. 1 is an electrical equivalent circuit of the entire image sensor according to the present invention, FIG. 2 is an electrical equivalent circuit of one pixel of the image sensor according to the present invention, and FIG. 3 is an image sensor according to the present invention. FIG. 3 is a waveform diagram showing a signal waveform in the main part of FIG. First, as shown in the equivalent circuit of FIG. 1, the image sensor includes a photodiode array 1 linearly arranged with a length substantially the same as the lateral width of a document (not shown), and a photodiode. A plurality of additional capacitors Cad provided for each of the two and a plurality of thin film transistors TTi, j (i = 1 to N, j = 1) connected for each photodiode 2.
To n), the driving IC 3, and the gate pulse generating circuit 4
And the variable voltage circuit 5 as main constituent elements.

The photo diode array 1 is composed of N blocks of photo diode groups 6, and the photo diode group 6 is composed of n photo diodes 2. In each photodiode 2, the cathode electrode is connected to the variable voltage circuit 5 as the main bias means, while the anode electrode is connected to the thin film transistor TTi, j (i
= 1 to N, j = 1 to n), respectively. The thin film transistor TTi, j is
The charge transfer unit 7 is configured, and each thin film transistor TT
The i, j source electrodes are connected to a wiring group 8 of multi-layer wirings connected in a matrix. Further, the wiring group 8 is connected to the driving IC 3 via the common signal line 9. Here, the common signal lines 9 are provided in the same number as the number n of the photodiodes 2 constituting the photodiode group 6, and each common signal line 9 is connected to one of the photodiode groups 6 by one. The source electrodes of the two thin film transistors TTi, j are connected via the wiring group 8. Therefore, one common signal line 9 has N thin film transistors T.
The source electrode of Ti, j is connected through the wiring group 8.

Further, the gate of each thin film transistor TTi, j
The gate electrodes are collectively connected to the gate pulse generating circuit 4 as an auxiliary bias means for each photodiode group 6. On the other hand, the cathode electrode of the photodiode 2 is connected to the variable voltage circuit 5 for each block, and a voltage as described later is applied in block units.

The overall operation of the image sensor having such a structure will be briefly described. First, the gate pulse generating circuit 4 has the same number of blocks (same meaning as the number of photodiode groups 6). A gate pulse .phi.GT is sequentially transmitted for each block via the gate line GTi (i = 1 to N). By sending out the gate pulse .PHI.GT, for example, in the first block to which the first gate pulse .PHI.GT1 is input, the thin film transistors TT1,1 become conductive and are generated in the respective photodiodes 2 of the first block. The generated charges are transferred and accumulated in the wiring capacitance CL1. In the other blocks, charges are transferred and accumulated from each photodiode 2 to the wiring capacitance CLi in the same manner as described above.

Then, the potential of each common signal line 15 is changed by the charges accumulated in each wiring capacitance CLi, and the driving IC 3
The same number of analog switches as the number of the photodiode groups 6 are sequentially turned on, so that the potential of the common signal line 9 is output to the output terminal 10 in time series. . Thereafter, the voltage ΦVB applied to the cathode electrode of the photo diode 2 is changed by the variable voltage circuit 5 and also on one electrode side of the additional capacitance Cad (photo diode 2).
The gate voltage generating circuit 4 changes the applied voltage .PHI.VR to the anode electrode (the electrode on the opposite side of the electrode connected to the anode electrode) by predetermined levels (details will be described later). Effectively in the forward bias state, each photodiode 2 has a parasitic capacitance Cp and an additional capacitance Cp.
The untransferred charges remaining in the overlap capacitance Cgd between ad and the drain-gate of the thin film transistor TTi, j are reset. By performing the above operation for each block, a pixel signal of one line in the main scanning direction of the document is obtained, and then the document (not shown) is moved by a known / known document feeding means such as a roller. Then, by repeating the above-mentioned operation again, the entire image of the document is obtained.

Next, an equivalent circuit for one pixel of the image sensor having the above structure will be described with reference to FIG. As shown in the equivalent circuit of one pixel of the image sensor, the photodiode 2 has a parasitic capacitance, which is represented by the symbol Cp in the figure. In addition, an overlap capacitance is formed between the gate electrode and the drain electrode of the thin film transistor TTi, j with the symbol Cgd, and the gate electrode and the source electrode of the thin film transistor TTi, j are connected. The capacitance formed between them is represented by the symbol Cgs. A wiring capacitance is formed between the wiring group 8 or the common signal line 9 and the ground, which is represented by the symbol CL in the figure.

Further, an additional capacitance Cad is connected between the anode electrode of the photodiode 2 and the gate pulse generating circuit 4. This additional capacitance Cad is equal to
As well known, it is formed of two electrodes and a dielectric material provided between the electrodes, unlike the case of electrically forming the burlap capacitance Cgd or the like. On the other hand, the source electrode of the thin film transistor TTi, j is connected to the amplifier 11 in the driving IC 3 and also to the drain electrode of the MOS transistor 12 for resetting the wiring capacitance CL. The source electrode of the MOS transistor 12 is grounded, while the gate electrode is connected to a drive pulse generator (not shown) provided in the drive IC 3 so that a pulse signal is input at the time of reset. It has become.

The operation of the above configuration will be described below. First, the operation of reading out the charges generated in the photodiode 2 will be described.
Is generated by receiving reflected light from an original (not shown).
, The additional capacitance Cad and the overlap capacitance Cgd, respectively. Looking at changes in the potentials of the drain electrode and the source electrode of the thin film transistor TTi, j during the accumulation of photocharge, as shown in FIGS. 3D and 3E, in the so-called bright state, the gate pulse is obtained. The drain electrode voltage rises relatively large before ΦGT is input (see FIG. 3D), but gradually rises in a so-called dark state (see FIG. 3D).

After a lapse of a certain time, the gate pulse .PHI.GT is input to the thin film transistor TTi, j and the thin film transistor TTi, j becomes conductive, whereby the parasitic capacitance C
The electric charge accumulated in p etc. is the thin film transistor TTi, j
Is transferred to and accumulated in the wiring capacitance CL via. Due to this wiring capacitance CL, the potential of the source electrode rises in comparison with the drain electrode of the thin film transistor TTi, j, and the transistor TTi, j becomes non-conductive, while the thin film transistor TTi, j due to the charge accumulated in the wiring capacitance CL. The increase in the voltage on the source electrode side of j is amplified by the amplifier 11 and is output from the driving IC 3 to the output terminal 10 of the photodiode 2
The photocharges generated in the above are output as an image reading signal.

After the image reading signal is output, the gate electrode of the MOS transistor 12 is supplied with a gate signal from a circuit portion (not shown) to be in a conductive state.
The previous wiring capacitance CL is reset. The potential of the input side of the amplifier 11 after the resetting of the wiring capacitance CL is
The reference potential (the voltage described as ΔVs in FIG. 3 (e)) when there is no image reading signal output from the image sensor is set. Gate pulse ΦG
The change in the potential on the source electrode side of the thin film transistor TTi, j from when T is input until the MOS transistor 12 becomes conductive and the wiring capacitance CL is reset is shown in FIG. 3 (e). Like That is, the potential on the source electrode side rapidly increases at the same time as the gate pulse .PHI.GT is input, by the amount of potential change due to the so-called feedthrough. Then, when the gate pulse ΦGT falls, this time it rapidly drops by the amount of the voltage change due to the feedthrough, and once settles to a certain potential. This potential is a potential generated by the charge being transferred and accumulated in the wiring capacitance CL. Thereafter, as described above, the MOS transistor 12 becomes conductive and the wiring capacitance CL is reset, so that the potential on the source electrode side becomes the ground potential,
Further, when the MOS transistor 12 becomes non-conductive, the potential increases by the voltage variation ΔVs due to the feedthrough, and this potential becomes the reference voltage in the non-output state of the driving IC 3.

After the wiring capacitance CL is reset, the voltage ΦVB applied to the cathode electrode of the photodiode 2 is reset to a predetermined value by operating the variable voltage circuit 5 as shown in FIG. 3 (a). It is brought to a potential. In this embodiment, this reset potential is set to the ground potential. Further, at the same time as the potential on the cathode electrode side of the photodiode 2 changes, one electrode side of the additional capacitance Cad (the electrode opposite to the electrode connected to the anode electrode of the photodiode 2). As shown in FIG. 3B, the voltage applied to the gate voltage rises from the negative voltage by a predetermined voltage due to the operation of the gate pulse generating circuit 4. The output voltage .PHI.VR of the gate pulse generating circuit 4 is synchronized with the change in the cathode applied voltage .PHI.VB and the magnitude of the rising voltage is the same as that of the photodiode 2.
The voltage is set in the gate pulse generation circuit 4 so that the potential change is sufficient to bring the signal into the reverse bias state. In this embodiment, it is set to change from -5V to 0V.

In this way, the cathode applied voltage ΦVB of the photodiode 2 rises and the anode applied voltage ΦVR rises at the same time, so that the photodiode 2
Becomes a forward bias state, and each capacitance Cp,
The untransferred charges remaining in Cad and Cgd are swept away. When .PHI.VB rises and .PHI.VR falls and returns to their original potentials, the potential of the anode electrode of the photodiode 2 is a so-called feedthrough, as shown in FIG. 3 (d). The voltage suddenly drops, and the photodiode 2 is again in the reverse bias state and the photocharge is stored. Note that FIG. 3 shows the potential change of the main part in the dark state after the bright state.

Next, as described above in the present invention, in order to clarify the significance of changing the potentials at both ends of the photodiode 2 to the reverse bias state, the photodiode is used.
The operation of the photodiode 2 when only the potential of the cathode 2 on the cathode electrode side is changed will be described. First,
When the pulse .PHI.GT falls, the potential on the anode electrode side of the photodiode 2 rapidly drops by the so-called feedthrough, as shown in FIG. 3 (f). The potential change amount .DELTA.VFT (T) of the anode electrode of the photodiode 2 due to this feedthrough is (Cgd.times..DELTA.VG) / (Cp +
It is expressed as Cad + Cgd). Here, ΔVG is shown in FIG.
As shown in (b), it is the amount of change in the gate pulse ΦGT.

Then, after a lapse of a predetermined time after the application of the gate pulse ΦGT, only the voltage ΦVR on the cathode electrode side is lowered from the predetermined positive voltage to the reference voltage as shown in FIG. As shown in FIG. 3 (f), the potential on the anode electrode side of the diode 2 is shown in FIG. 3 (f). p), and the potential on the anode electrode side further decreases by this ΔVFT (p) (Fig. 3
(See (f)). Here, the voltage change amount ΔVFT (p) due to the feedthrough is (Cp × ΔVB) / (Cp + Cad + C
gd). Therefore, the potential on the anode electrode side of the photodiode 2 is affected by the changes in the gate pulse ΦGT and the cathode voltage ΦVB due to the feedthrough, and as a result, the cathode voltage of the photodiode 2 is increased. Is still at a negative potential even if the change is such that the photodiode can be reverse-biased, the photodiode 2 always remains in the reverse-biased state, and the residual charge is generated. The problem of afterimage occurs.

Therefore, in the present invention, as described above, one electrode side (photodiode 2) of the additional capacitance Cad is used.
Applied voltage ΦVR to the anode electrode connected to the anode electrode of the photo diode 2).
The photodiode 2 is effectively put into the forward bias state by raising the voltage by a predetermined level in synchronization with the change of VB. Then, even when ΦVR changes, a field surge occurs and the potential on the anode electrode side of the photodiode 2 changes. The amount of change ΔVFT (ad) is ΦVR, where ΔVR is (Cad x ΔVR) / (Cp + C
It is expressed as ad + Cgd). Therefore, in order to effectively put the photodiode 2 in the forward bias state, the change amount ΔVFT (ad) due to the feedthrough described here is calculated from the above ΦV.
Change in feedthrough due to change in B ΔVFT (p)
It is sufficient to adjust and set the magnitude of ΦVR so that the variation amount obtained by subtracting is larger than the threshold value of the photodiode 2 in the forward direction.

In this embodiment, the photodiode 2 is used.
In order to effectively put the capacitor in the forward bias state, the voltage ΦVB applied to the cathode electrode is temporarily changed from + 5v to 0v, and at the same time, the additional capacitance Cad whose one end is connected to the anode electrode of the photodiode 2 is added. Applied voltage ΦVR at the other end of −
Although the photodiode 2 was temporarily set in the reverse bias state by changing from 5v to 0v, it goes without saying that the voltage values of ΦVB and ΦVR are not limited to the values of the embodiment. That is, the polarities of ΦVB and ΦVR do not matter, but the difference between the potential on the cathode electrode side and the potential on the anode electrode side of the photodiode 2 causes the photodiode 2 to be forward-biased and reverse-biased. It suffices if there is a sufficient relative relationship to establish the state.

As described above, in this embodiment, after the photocharges are read out from the photodiode 2, the photodiode 2 is effectively forward-biased to the anode electrode. The applied voltage ΦVB is temporarily set to the ground potential, and at the same time, one electrode of the additional capacitance Cad connected to the anode electrode of the photodiode 2 (opposite to the electrode on the side connected to the photodiode 2). Since the potential of the anode 2) is set to the ground potential, the photodiode 2 is substantially forward-biased while considering the potential change in the anode electrode due to the feedthrough. The diode 2 is surely put in a forward bias state, and the residual charges accumulated in the parasitic capacitance Cp, the additional capacitance Cad, and the overlap capacitance Cgd can be swept away, which causes an afterimage phenomenon. Therefore, it is possible to obtain an accurate image signal without any trouble.

Further, in the present embodiment, the so-called bias voltage of the photodiode can be changed, so that the photodiode itself and the semiconductor element portion in the periphery thereof have the same structure as the conventional one, and therefore the manufacturing process. Does not change at all, and in that respect it is possible to easily obtain an image sensor in which a so-called afterimage phenomenon does not occur.

By the way, in the above embodiment, the image signal output from the driving IC 3 to the outside ideally has the wiring capacitance CL in the bright state in the absence of the offset voltage due to the dark current in the dark state. Reset (Fig. 4
The voltage signal Vp read into the driving IC 3 immediately before (see (g)) becomes an image signal. However, in reality, due to the offset voltage due to the dark current or the like, such an ideal output is not obtained but an output as described below is obtained. That is, the image signal is represented by the difference voltage V between the read voltage Va in the bright state (state of reading the original) and the read voltage Vb in the dark state. Here, the read voltage Va in the bright state is the reset voltage of the wiring capacitance CL (FIG. 4 (g)).
The voltage Vp read into the driving IC 3 immediately before,
Voltage ΔVs generated on the input side of the driving IC 3 after reset
Further, the voltage Vb in the dark state is the difference between the voltage VL read immediately before the resetting of the wiring capacitance CL (see FIG. 4 (g)) and the voltage ΔVs at the input side of the drive circuit IC3 after the resetting. Is.

As described above, since the voltage Vb, that is, the offset voltage is actually generated even in the dark state, the image processing circuit (amplifies the output signal of the image sensor in the above-described embodiment to perform various image processes ( (Not shown), the dynamic range is the offset voltage V
Since it becomes smaller by the amount of b, the so-called noise margin becomes smaller accordingly, which causes a disadvantage that the image quality is deteriorated.

A second embodiment as a technique for suppressing the above-mentioned offset voltage Vb will be described below with reference to FIG. 4A to 4C are the same as those in FIG. 3, showing the gate pulse ΦGT, the cathode voltage ΦVB, and the applied voltage ΦVR to the additional capacitance Cad, respectively.
(D) is the anode potential of the photodiode in the second embodiment, (e) is the source potential of the thin film transistor in the second embodiment, and (f) and (g) are respectively FIG. (D) and (e), and (f) is the anodic potential of the photodiode in the first embodiment, (g)
Represents the source potential of the thin film transistor in the first embodiment. The circuit configuration of the second embodiment is basically the same as the circuit configuration shown in FIGS. 1 and 2, but its driving method is the same as that of the first embodiment as described below. Is different. In the second embodiment, the applied voltage .PHI.VB to the cathode electrode of the photodiode 2 is lowered to the reset potential in order to wipe out the residual charge of the photodiode 2, and at the same time, one of the additional capacitors Cad This is the same as the first embodiment in that the voltage ΦVR applied to the electrodes of the above is raised to a predetermined voltage, but at this time,
The feedthrough amount ΔVFT (ad) generated by the change in the voltage ΦVR applied to one electrode of the additional capacitor Cad is the feedthrough generated by the change in the voltage ΦVB applied to the cathode electrode.
It is different from the first embodiment in that it is set larger than the amount ΔVFT (p). In this way, by keeping the feedthrough amount constant, the data reading on the anode side of the photodiode 2 (the charge accumulated in the additional capacitance CL is input to the driving IC 3). Operation) At the start of the operation, the potential on the anode side of the photodiode 2 (see the potential indicated by the symbol a in FIG. 4 (d)) is the case of the first embodiment (FIG. 4 (f). ), The voltage VL (see FIG. 4 (g)), which has been conventionally generated in the dark state, can be made zero (see FIG. 4).
(Refer to the dark state in (e)). Therefore, the dynamic range in the circuit (not shown) for processing the output signal of the image sensor becomes larger than the conventional one.

Here, the setting of the feedthrough amount ΔVFT (ad) larger than the feedthrough amount ΔVFT (p) will be specifically described. First, the feedthrough amount ΔV.
FT (ad) is as described above in the first embodiment,
If the amount of change in the voltage ΦVR applied to one electrode of the additional capacitance Cad is ΔVR, then ΔVFT (ad) = (Cad × ΔVR) /
It is represented by (Cp + Cad + Cgd). Therefore, ΔVFT
To increase C, increase Cad or ΔVR, or increase C
Although it is theoretically possible to reduce either p or Cgd, as a practically suitable method, firstly, the additional capacitance Cad is increased, and secondly, ΔVR is increased. Conceivable. For example, when the additional capacitance Cad is composed of a so-called plate capacitor, the electrode area is made as large as possible, the electrode interval is made as small as possible, and the dielectric material interposed between the electrodes has as large a dielectric constant as possible. Is used, the value of the additional capacitance Cad can be increased.

In this second embodiment, the additional capacitance C
The feedthrough amount ΔVFT (ad) generated by the change of the applied voltage ΦVR to ad is larger than the feedthrough amount ΔVFT (p) generated by the change of the applied voltage ΦVB to the cathode electrode of the photodiode 2. By so doing, the so-called offset voltage VL can be suppressed, so that this image sensor is connected and the image
It is possible to increase the dynamic range in the external circuit that amplifies the output signal of the sensor and performs necessary signal processing. When the feedthrough amount .DELTA.VFT (ad) is increased by adjusting the additional capacitance Cad, the gate voltage applied to the gate electrode of the thin film transistor TTi, J and the cathode of the photodiode 2 are used. -The same power supply is used because either the voltage ΦVB applied to the negative electrode or the voltage ΦVR applied to one electrode of the additional capacitance Cad does not take an extremely different value compared to the other voltage. Therefore, the cost can be reduced.

When the feedthrough amount ΔVFT (ad) is increased by adjusting the amount of change in the voltage ΦVR applied to one electrode of the additional capacitance Cad, the additional capacitance Cad
Size and shape, and further, thin film transistor TTi, J
Also, since it is not necessary to consider the size and shape of the photodiode 2, the degree of freedom in design is increased in these designs.

[0033]

As described above, according to the first aspect of the invention, the main bias means for applying a voltage to the cathode electrode of the photodiode and the capacitor for the anode electrode of the photodiode are provided. By providing an auxiliary bias means for applying a voltage via the photo diode, the photo diode from which the photo charge is read is configured to be temporarily in the forward bias state. It is possible to provide an image sensor which can eliminate the so-called afterimage phenomenon and can obtain an accurate image signal because the electric charge remaining in the image can be wiped out. In addition, claim 2
According to the above-mentioned invention, the photodiode is temporarily set to the forward bias state after the photocharge is read out from the photodiode, so that the photodiode remains after the photocharge is read out. Since it is possible to clean up the electric charges, it is possible to eliminate an afterimage phenomenon and obtain an accurate image signal.

Further, according to the invention of claim 4, the capacitance of the capacitor connected to the anode of the photodiode is adjusted to set the feedthrough amount to a desired amount. With this configuration, the potential on the anode side when the photodiode starts accumulating photocharges drops compared to the conventional one, so the offset voltage of the output signal becomes smaller by that amount, and the image sensor is connected. It is possible to secure a large dynamic range in the circuit. The gate voltage applied to the gate electrode of the thin film transistor and the voltage .PHI.V applied to the cathode electrode of the photodiode.
B, and the applied voltage ΦV to one electrode of the additional capacitance Cad
Since any one of R does not take an extremely different value as compared with the other voltages, the same power supply can be used, and the cost can be reduced.

Further, according to the invention of claim 5, the feedthrough amount is adjusted by adjusting the voltage applied to the capacitor connected to the anode of the photodiode. Therefore, it is not necessary to finely adjust the capacitance of the capacitor connected to the photodiode node, which increases the degree of freedom in designing the shape and size of the capacitor and facilitates the design. Is played. In addition, by adjusting the voltage applied to the capacitor connected to the anode of the photodiode, by adjusting the feedthrough amount,
Also in the photodiode and the thin film transistor, the shape and the size thereof can be freely set, and the degree of freedom in design is increased and the design is facilitated.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an electrical equal circuit in one embodiment of an image sensor according to the present invention.

FIG. 2 is a circuit diagram showing an electrically equivalent circuit per pixel in the image sensor according to the present invention.

FIG. 3 is a waveform chart of the main part for explaining the operation of the main part of the equivalent circuit shown in FIG.

FIG. 4 is a waveform chart of a main part for explaining a second embodiment of a driving method.

[Explanation of symbols]

2 ... Photodiode, 3 ... Driving IC, 4 ... Gate
Pulse generation circuit, 5 ... Variable voltage circuit, Cad ... Additional capacitance, Cgd ... Overlap capacitance, CL ... Wiring capacitance,
Cp ... parasitic capacitance, TTi, j ... thin film transistor

Claims (6)

[Claims] 1. An image sensor comprising a plurality of photodiodes, wherein the plurality of photodiodes are reverse-biased and the photocharges generated in the photodiodes are read out as an image reading signal. , While the photoelectric charge is read from the photodiode by being connected to the cathode electrode of the photodiode, a first voltage higher than the potential on the anode electrode side of the photodiode is applied, After the photocharge is read out, a voltage smaller than the first voltage and then the first voltage are applied to the cathode electrode, respectively. In synchronization with the output of a voltage smaller than the voltage, a potential change is applied to the anode electrode of the photodiode through a capacitor to put the photodiode in a forward bias state. And an auxiliary biasing means for providing the image sensor. 2. An image sensor comprising a plurality of photodiodes, wherein the photodiodes are reverse-biased and the photocharges generated in the photodiodes are read out as image reading signals. After reading out the photocharges generated in the photodiode, the photodiode is once put in a forward bias state, and then the photodiode is returned to a reverse bias state. Driving method of the di-sensor. 3. An image sensor comprising a plurality of photodiodes, wherein the photodiodes are reverse-biased and the photocharges generated in the photodiodes are read out as image reading signals. By setting the anode of the photodiode to the ground potential via a capacitor and applying a positive voltage equal to or higher than the voltage drop in the forward direction of the photodiode to the cathode of the photodiode. Applying a positive polarity pulse to one end of the capacitor from the state in which the photodiode is reverse-biased, and at the same time, setting the cathode of the photodiode to the ground potential while the positive polarity pulse is generated. A method for driving an image sensor, characterized in that the photodiode is forward-biased for a certain period of time. 4. The capacitance of the capacitor connected to the anode of the photodiode is such that the feedthrough voltage generated when a positive pulse is applied to one end of the capacitor is the cathode of the photodiode. 4. The method of driving an image sensor according to claim 3, wherein the voltage is set to a value higher than the feedthrough voltage generated when the ground potential is set to ground. 5. The capacitor comprises two parallel plate electrodes, and the facing area of the two electrodes determines the feedthrough voltage generated when a positive polarity pulse is applied to one end of the capacitor. The image sensor driving method according to claim 4. 6. The feedthrough voltage generated when a positive polarity pulse is applied to one end of a capacitor is adjusted by the magnitude of the positive polarity pulse.
A method for driving the image sensor described.