JPH02260968A - Image sensor - Google Patents

Abstract

PURPOSE:To improve sensitivity and response speed of a sensor by composing an image sensor of a first switching element, which applys a signal current from a sensor driving power source according to a photoelectric converting current value, and a second switching element, which switches off the first switching element at the time of discharging element capacity and grounds a light receiving element. CONSTITUTION:A counter bias is impressed to a photo diode 11 by a sensor driving power source Vb, a sensor current detected by the photo diode 11 is supplied to a signal current thin film transistor(TFT) 14, the gate of the signal current TFT 14 is opened by the sensor current, and the signal currents supplied from a drain electrode 14c according to the sensor current. The signal current is charged to element capacity 15 in an accumulating time, and the charge is read from a read electrode 15 by an external circuit. At the same timing as the read operation of the signal current, the gate of a reset switching TFT 13 is opened, a gate electrode 14a of a signal current TFT 14 is grounded, the gate of the signal current TFT 14 is closed, and only the charge accumulated in the element capacity 15 is applied to a read electrode 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an image sensor used in an image processing unit of an image processing device.

(Prior Art) In recent years, contact type image sensors have been actively developed in order to downsize image reading devices used in facsimiles and the like.

This image sensor is a device that reads image information by arranging photoelectric conversion elements (light receiving elements) in a negative row with the same width as the original and a desired resolution, and bringing the original close to the original.

In general, the basic configuration of an image sensor is to have a large number of photoelectric conversion elements arranged in a line for converting optical signals into electrical signals, and 1! It consists of a drive unit that reads the electricity/i number from the conversion element in time series. That is, the configuration of the image sensor is such that a plurality of individual electrodes as lower electrodes are arranged on a glass substrate, and a part of the individual electrodes is covered.
A photoelectric conversion layer such as 1. Kohasamu 1. It consists of common electrodes such as T, o, etc. Each individual electrode, this photoelectric conversion layer, and this common electrode constitute a photoelectric conversion element as a pixel.

As such an image sensor, there is a direct drive type image sensor as shown in FIG.

In the figure, the photoelectric conversion element 1 is a photodiode 1.
A and a capacitor 1b, the charge accumulated in the capacitor 1b is released according to the amount of light received, and the analog switch 3 for signal readout is sequentially closed by the shift register 2 at regular intervals to recharge the capacitor 1b. death,
The optical signal is detected by reading out the voltage or current from the signal reading terminal 4 at that time.

In recent years, the drive section of such an image sensor is often formed of a thin film in order to cope with the miniaturization of the device.

(Problem to be Solved by the Invention) However, in the conventional image sensor described above, since the sensor current of the photodiode 1a is minute, this sensor current is accumulated in the wiring capacitor 5, etc., and a component that can be extracted as a read signal is becomes smaller, decreases sensitivity, S
There was a problem that the /N ratio deteriorated, and there was also a problem that the signal response speed became slow.

The present invention was made in order to solve the above-mentioned conventional problems, and aims to provide an image sensor that can obtain good read signals and improve sensitivity and response speed. be.

[Structure of the Invention] (Means for Solving the Problems) The image sensor of the present invention has a plurality of photoelectric conversion units driven by a sensor drive power source arranged in a row on a high-low antibody substrate, and current from these photoelectric conversion units is In the image sensor configured to read by a reading electrode, the photoelectric conversion section is connected to a light receiving element connected to the sensor driving power source,
One end is connected to the sensor driving power source and the other end is connected to the reading electrode, and a switch is turned ON by the photoelectric conversion current from the light receiving element, and the signal current from the sensor driving power source is converted according to the photoelectric conversion current value. a first switching element that allows the signal current to flow, an element capacitor that is inserted between the output side of the first switching element and the reading electrode and that stores the signal current; A second switching element that turns off the first switching element and grounds the light receiving element when the element capacitance is discharged.
The device is characterized in that it is composed of a switching element.

Further, as the first switching element, poly
-81T F T analog switch is preferred, and the second switching element is preferably a TPT analog switch formed of a-3i.

(Function) According to the present invention, the photoelectric conversion current from the light receiving element is input to the gate of the first switching element. Then, a signal current from the sensor drive power source is caused to flow in accordance with the photoelectric conversion current value, and after this signal current is temporarily stored in the element capacitance, it is read out to an external circuit.

Furthermore, by turning on the second switch during reading, the gate of the first switching element is set to 0FFL, thereby resetting the light receiving element.

Here, the photoelectric conversion current from the light receiving element is about 10-'A, but the current flowing into the capacitor from the second switching element is as large as about 10'A, so the sensitivity decreases due to wiring capacitance, etc., and the S/N ratio can prevent deterioration. Also, since it is in accumulation mode, response speed is also improved.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view showing an equivalent circuit of a photoelectric conversion section of an example.

The anode electrode 11a of a light receiving element such as a Schottky photodiode 11 is connected to the negative electrode side of the sensor drive power supply vb via a common electrode 12, and its cathode electrode 11b is connected to a reset analog switching element such as a thin film transistor (hereinafter referred to as TPT). 13 source electrode 13H.

In addition, the photodiode 11 and the reset TPT1
A gate electrode 14a of a signal current analog switching element, for example, a TFT 14, is connected between the common electrode 1 and the source electrode 14b of the signal current TFT 14.
2, and the drain electrode 14c is connected to a signal reading electrode 16 having an element capacitance 15.

FIG. 2 is a sectional view showing the structure of a photoelectric conversion section of such an image sensor.
1 Drain electrode 14c and po I y-8i layer 1
8 are formed in sequence. This drain electrode becomes a signal reading electrode 16, and an element capacitor 15 is formed in a part thereof.

Then, on this po I y-31 layer 18, an insulating film 1 is formed.
9, a wiring pattern layer 20 made of a thin chromium film or the like;
a-3i layer 21.1 serving as a photoelectric conversion layer. T, o, membrane 22
are formed, and a Schottky photodiode 11 is formed by each of these layers. The cathode 11b of this shot diode 11 is connected to the signal current TFT 14.
This is the gate electrode 14a.

Furthermore, 1. T, 0. On the film 22, there is a TPT for resetting.
13 gate electrodes 13b are formed, and the cathode electrode 11b of the Schottky diode 11 is connected to this a-
The 8i layer 21 serves as a source electrode 13a of the reset TPT 13.

Furthermore, 1. T, 0. The film 22 and the source electrode 14b of the signal current TFT 14 are connected to the common electrode 12, which is a through hole.

The operation of the image sensor having such a configuration will be described below.

A reverse bias is applied to the photodiode 11 by the sensor drive power supply vb, and the sensor current detected by the photodiode 11 flows into the signal current TFT 14. This sensor current opens the gate of the signal current TFT 14, and the drain electrode A signal current according to the sensor current flows from 14c. This signal current charges the element capacitor 15 within the storage time, and then the charge charged in the element capacitor 15 is read from the read electrode 15 by an external circuit (not shown).

At the same timing as this signal current reading operation, the gate of the reset switch TPT13 opens and the signal current TPT13 opens.
The gate electrode 14a of the FT 14 is in a grounded state,
The gate of the signal current TFT 14 is closed and the read electrode 16
Only the charge charged in the element capacitor 15 flows through. At this time, the cathode electrode 11b of the photodiode 11 is also grounded and reset.

By repeating the above-mentioned operations in this manner, images are successively read.

According to such an image sensor, while the sensor current from the conventional photodiode is about 10-9A at most, the signal current from the signal current TPTI 4 of this embodiment is about 10'A, which is lower than the wiring capacitance. It is sufficiently large and there is no deterioration in sensitivity. Furthermore, high-speed response is possible due to the accumulation mode driving in which charge is accumulated in the element capacitor 15 and read out. Further, due to the action of the reset switch TPT 13, the gate of the signal current TFT 14 is closed during signal reading, thereby cutting off the common electrode 12 and the signal reading electrode.

Therefore, no deterioration of the S/N ratio of the signal occurs. Furthermore, since the photodiode 11 is reset at the same time, no afterimage components remain.

Note that the TPT 13 for the reset switch may be a TFT made of a-3i because the amount of flowing current is small, so it can be manufactured using the same film formation as the photodiode 11.
The manufacturing process will not become complicated.

[Effects of the Invention] As described above, according to the image sensor of the present invention, an image sensor with high sensitivity and high speed response without deterioration of the S/N ratio can be realized.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an equivalent circuit of a photoelectric conversion section in an embodiment according to the present invention, FIG. 2 is a sectional view showing the configuration of a photoelectric conversion section in an embodiment, and FIG. 3 is an equivalent circuit of a photoelectric conversion section in a conventional device. There is a diagram showing this. 11... Schottky photodiode 12... Common electrode 13... TPT 14 for reset switch
...... TPT15 for signal current...
...Element capacitance 16...Reading electrode vb...Sensor drive power supply 17...
...Glass substrate applicant Toshiba Corporation Representative Patent attorney Satoshi Suyama - Figure 1 Figure 2

Claims (1)

[Claims] An image sensor configured such that a plurality of photoelectric conversion units driven by a sensor drive power source are arranged in a row on a high-low antibody substrate, and current from these photoelectric conversion units is read by a reading electrode, The photoelectric conversion section is connected to a light receiving element connected to the sensor driving power source, one end is connected to the sensor driving power source and the other end is connected to the reading electrode, and the switch is turned on by the photoelectric conversion current from the light receiving element. a first switching element that causes a signal current from the sensor drive power source to flow in accordance with the photoelectric conversion current value; a second switching element connected to the current output side of the light-receiving element to switch off the first switching element and ground the light-receiving element when the element capacitance is discharged; An image sensor featuring: