JPH05129577A - Solid-state image sensing element - Google Patents

Abstract

PURPOSE:To reduce spike noise of a solid-state image sensing element. CONSTITUTION:Photodiodes PD and switch diodes SG are sequentially connected in series, and driving pulses inverted to each other are applied to opposite terminals. Accordingly, both the diodes are reversely biased during a storage period, and both the diodes are forwardly biased during a reading period. Therefore, signal charge is stored in a capacitor during the storage period, and a drive pulse flows through both the diodes during the reading period to be canceled, and hence a signal having no noise can be outputted. If both the diodes are equivalent, a dark current can be canceled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state image sensor,
For example, it is used for a one-dimensional or two-dimensional image sensor.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field, for example, the Institute of Electronics and Communication Engineers, 1986, Vol. J69
-C, No. 11, PP. 1438 to 1442 and Technical Report Vol. 12, No. 50, P
P. The thing disclosed in 55-60 is known. Figure 4
Shows the main part of the circuit and the waveform of the drive pulse.

As shown, each pixel has a photodiode PD and a blocking diode BD. Here, the photodiode PD and the blocking diode BD are connected in anti-series with each other. Then, by applying the drive pulse Φ _y shown in FIG. 7B to the blocking diode BD, it is turned on during the reading period,
The signal charge of the photodiode PD is read out. The read signal charge is I /
V converted and output.

By the way, in such a device, spike-like noise is generated when the signal charge is read out. That is, since a large reset pulse (driving pulse) is applied to the blocking diode BD during signal reading, the reset pulse is mixed into the output line (reading line) through the photodiode PD, which causes spike noise. And
If this noise exceeds the signal current level, the amplifier 1
Is saturated, the amplification factor of the first stage cannot be increased to avoid this. Therefore, in the former known document, the influence of the spike noise is avoided by providing an external noise canceling circuit.

[0005]

However, when the noise canceling circuit is externally attached, there is a drawback that the noise of the externally attaching circuit itself is mixed into the signal. Further, in this conventional technique, it is necessary to raise the signal level to such an extent that noise cancellation can be performed. However, in this case, the saturation of the amplifier easily occurs as described above, and therefore the first stage gain cannot be sufficiently increased. Suitable noise processing is difficult under constraint conditions. The present invention solves such a problem.

[0006]

According to the present invention, a plurality of photodiodes are provided for each of a plurality of pixels, and by applying a driving pulse to the plurality of photodiodes, signal charges are accumulated in an accumulation period and read out. In a solid-state imaging device including a drive control unit that reads out a signal charge accumulated during a period through a read line, each of the plurality of pixels has a switching diode connected in series to the photodiode, The common connection point of the switch diode is connected to the read line via the capacitor, and the drive control means is configured to give an inverted signal of the drive pulse to the terminal of the switch diode on the opposite side of the common connection point. It is characterized by being here,
The photodiode and the switch diode may have the same current / voltage characteristics and capacitance characteristics.

[0007]

According to the structure of the present invention, the photodiode PD
And the switching diode SD are connected in series in series with each other, and drive pulses inverted from each other are applied to the opposite terminals, so that both diodes are reverse biased during the accumulation period and both diodes are forwarded during the reading period. It becomes a bias. Therefore, since the signal charge is stored in the capacitor during the storage period and the drive pulse flows through both diodes and is canceled during the read period, noise-free signal output is possible. If both diodes are made equal, dark current can be canceled.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. The same reference numerals indicate the same elements.

FIG. 1 shows the configuration of the first embodiment. FIG. 1A is a circuit diagram and FIG. 1B is a waveform diagram of drive pulses Φ ₁ and Φ ₂ and a potential Vj at a common connection point. As shown in the figure, the photodiode PD and the switching diode SD are connected in the same direction (sequentially connected in series), and the common connection point is connected to the readout line 2 for signal charge output via the charge storage capacitor C _i. Has been done. The anode of the photodiode PD is connected to the shift register 3, and the cathode of the switching diode SD is connected to the shift register 3 via the inverter 4. Therefore, inverted drive pulses Φ ₁ and Φ _{2 are} supplied to these diodes PD and SD. Is entered. The read line 2 is connected to the multiplexer 5 via the I / V conversion amplifier 1.

Next, the operation of the above circuit will be described. The drive pulses Φ ₁ and Φ ₂ are as shown in FIG. 1B, and therefore the photodiode PD and the switching diode SD are both reverse biased during the signal charge accumulation period. Therefore, the charge amount of the charge storage capacitor C _i changes according to the signal charge amount corresponding to the amount of light incident on the photodiode PD, and the potential Vj at the common connection point also changes. At this time, the dark current component of the output of the photodiode PD is canceled with the dark current of the switching diode SD, so that the characteristic does not change due to the temperature change. In particular, this effect is remarkable when the photodiode PD and the switching diode SD have the same current / voltage characteristics and capacitance characteristics. During the storage period, the output current does not appear on the read line 2 due to the DC component blocking function of the charge storage capacitor C _i .

From the accumulation period to the signal reading period, the drive pulses Φ ₁ and Φ ₂ are inverted in synchronism, and the photodiode PD and the switching diode SD are both forward biased. As a result, the potential Vj at the common connection point is reset, and accordingly, the read line 2 of the charge storage capacitor C _i is reset.
A current corresponding to the signal charge amount appears on the side. This output is I / V converted by the amplifier 1 and output from the multiplexer 5 to the outside.

When the drive pulses Φ ₁ and Φ ₂ are inverted,
The drive pulses Φ ₁ and Φ ₂ themselves flow through the photodiode PD and the switching diode SD, but since the diodes PD and SD are forward biased, they are not canceled by each other to generate spike noise. Therefore, the noise component of the output current in the read line 2 is reduced, and the gain of the first stage amplifier can be increased.

2A and 2B show the structures of the photodiode PD and the switching diode SD in the above embodiment, FIG. 2A being a sectional view and FIG. 2B being a top view. The photodiode PD and the switching diode SD are formed by depositing a-Si: H11 on the glass substrate 10. The first layer metal electrode 12 of Cr or the like which becomes the cathode of the photodiode PD is connected to the second layer metal electrode 13 which becomes the anode of the switching diode SD. An insulating film 14 of a-SiN or the like is formed on the second layer metal electrode 13, and a third layer metal electrode 15 is formed on the insulating film 14 to form a charge storage capacitor C _i . The third-layer metal electrode 15 is integrally formed with the read line 2 so as to be connected to the first stage amplifier.

While the transparent electrode 16 such as ITO is provided on the light receiving surface of the photodiode PD,
Light is not incident on the switching diode SD. The a-Si: H layer 11 is formed in the same process with substantially the same characteristics. Therefore, it is possible to cancel the dark current during the accumulation period and prevent the spike noise from entering during the reading period at the same time.

3A and 3B show a solid-state image pickup device according to the second embodiment. FIG. 3A is a circuit diagram and FIG. 3B is a signal waveform diagram. In this embodiment, the drive pulse Φ ₁ is input to the anode of the switching diode SD and the inversion drive pulse Φ ₂ is input.
Is input to the cathode of the photodiode PD. In addition, the set pulse Φ _S passes through two P-channel and N-channel MOS transistors 6 connected in parallel,
This is a signal for setting the level of the read line 2 to an initial value. The reset pulse Φ _R is a signal that resets the integrating amplifier 9 via the MOS transistor 7. further,
The read pulses Φ _{X1 to} Φ _Xn are signals for reading the output of the read line 2 to the outside via the MOS transistor 8.

The operation of the second embodiment will be described with reference to FIG. First, immediately before the reading period, the reset pulse Φ _R turns off the transistor 7, and the integrating amplifier 9 can integrate the signal. In the read period, both the photodiode PD and the switching diode SD are forward biased, and the read line 2 of the charge storage capacitor C _i is read.
A potential change occurs on the side. At this time, since the set pulse Φ _S is at the high level, the transistor ST6 is ON, and therefore the potential change of the read line 2 is transmitted to the integrating amplifier 9.

In the accumulation period after the read period, the set pulse Φ _S is inverted to turn off the transistor 6, and the integrating amplifier 9 is disconnected from the read line 2. Therefore, the signal charge can be accumulated in the charge accumulating capacitor C _i and the signal output from the integrating amplifier 9 can be simultaneously executed. The signal output from the integrating amplifier 9 is performed by sequentially setting the read pulses Φ _{X1 to} Φ _Xn to the high level, and when all the reading is completed, both the pulses Φ _S and Φ _R are set to the high level.
As a result, both of the trenchers 6 and 7 are turned on, and the next reading period starts.

[0018]

As described above, according to the present invention, the photodiode PD and the switching diode SD are connected in series with each other, and the drive pulses inverted from each other are applied to the opposite terminals. Both diodes are reverse biased, and both diodes are forward biased during the read period. Therefore, since the signal charge is stored in the capacitor during the storage period and the drive pulse flows through both diodes and is canceled during the read period, noise-free signal output is possible. If both diodes are made equal, dark current can be canceled. If a scintillator is attached to the element of the present invention, it can be used as a radiation detection element.

[Brief description of drawings]

FIG. 1 is a diagram showing a solid-state image sensor according to a first embodiment.

FIG. 2 is a diagram showing specific examples of a photodiode PD and a switching diode SD.

FIG. 3 is a diagram showing a solid-state image sensor according to a second embodiment.

FIG. 4 is a diagram showing a conventional solid-state image sensor.

[Explanation of symbols]

1 ... Amplifier, 2 ... Read-out line, C _i ... Charge storage capacitor, PD ... Photo diode, SD ... Switch diode

Claims (2)

[Claims] 1. A plurality of photodiodes provided for each of a plurality of pixels, and by applying a drive pulse to the plurality of photodiodes, signal charges are accumulated in an accumulation period and at the same time, the accumulated signal charges are accumulated in a reading period. And a drive control means for reading the light via a read line, wherein each of the plurality of pixels has a switch diode connected in series to the photodiode, and the photodiode and the switch A common connection point of the diodes is connected to the read line via a capacitor, and the drive control means applies an inverted signal of the drive pulse to a terminal of the switching diode on the opposite side of the common connection point. A solid-state imaging device characterized by being configured. 2. The solid-state image sensor according to claim 1, wherein the photodiode and the switching diode have the same current / voltage characteristics and capacitance characteristics.