JPH0628310B2 - Photoelectric conversion device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device used in a bar code reader, a facsimile, a digital copying machine and the like, and more particularly to a semiconductor layer having a gate electrode via an insulating layer. The present invention relates to a thin film transistor (hereinafter referred to as TFT) type photoelectric conversion device that is provided and configured.

(Prior Art) First, as a configuration example of a TFT type optical sensor, a plan view is shown in FIG. 1 and a sectional view taken along line XX ′ of FIG. 1 is shown in FIG.
In the figure, 1 is a substrate such as glass, 2 is a gate electrode, 3
Is an insulating layer, 4 is a photoconductive semiconductor layer, 6 and 7 are source and drain electrodes, respectively, and 5 is n for making ohmic contact with the semiconductor layer 4 and source and drain electrodes 6 and 7.
⁺ Layer.

The TFT type sensor can control the influence of the interface of the insulating layer and suppress the dark current by applying a bias voltage to the gate electrode, so that the photoelectric conversion output has a light quantity dependency characteristic (hereinafter referred to as γ) close to 1, which is a good characteristic. have. It also has the characteristics of good reproducibility and less variation within and between lots.

(Problems to be Solved by the Invention) These characteristics show favorable results under static (DC voltage) driving conditions, but dynamic operation such as that normally used in an image sensor, that is, charge accumulation. There was a problem when used in mode. The problems will be described below.

FIG. 3 shows a read-out circuit in the accumulation mode using a TFT type sensor. A sensor power supply V _S is connected to the drain electrode, and a bias power supply V _B is connected to the gate electrode.
A storage capacitor C is connected to the source electrode. The charge stored in the storage capacitor C is transferred to the transfer switch SW.
Is discharged to the load resistance R _L.

The operation waveforms in this circuit are shown in FIG. The transfer switch SW is repeatedly turned on / off in the accumulation time T _S cycle. That is, when the transfer switch SW is in the OFF state, the sensor photocurrent i _S is charged in the storage capacitor C, and when the transfer switch SW is in the ON state, the accumulated charge in the storage capacitor C is discharged to the load resistance R _L and is read as an output. Be done.

Here, paying attention to the voltage _{V C} across the storage capacitor C, and the conditions of _V S >> V _C by the integral value of the voltage _{V C} is _{i S, V C = ▲ ∫} t 0 ▼ i S dt = i S・ T, and the voltage V _C
Rises almost directly with respect to time t. The state of the voltage V _{C at} this time is shown by the broken line in FIG.

However, when actually driven by the circuit of FIG. 3, the voltage V _C has a distorted waveform as shown by the solid line in FIG. This is because when the transfer SW is turned on, the voltage V _C suddenly changes to the zero potential, the gate bias potential ΔV _gs becomes relatively shallow, and the transient current i _a between the source and the drain (indicated by the hatched line in FIG. 4). This is because the indicated current) flows. Due to the influence of this transient current, the light amount dependence of the output of the photoelectric conversion device by this circuit is γ = 0.4-
The value becomes 0.5, which is greatly deviated from γ = 1 calculated from the static characteristic measurement, and the S / N ratio decreases.

The present invention solves the above problems in dynamic operation,
It is an object of the present invention to provide a photoelectric conversion device in which the characteristics of the TFT type sensor are fully realized.

Further, another object of the present invention is to propose a driving circuit which can be easily formed on the same substrate as the sensor unit,
It is an object of the present invention to provide a photoelectric conversion device of low cost and high yield by utilizing the high S / N ratio and the reduction of variation distribution, which are features of the FT type sensor.

(Means for Solving Problems) A photoelectric conversion device of the present invention includes a semiconductor layer, first and second electrodes electrically connected to the semiconductor layer, and an insulating layer for the semiconductor layer. A third electrode provided through the third electrode, and a power conversion voltage is applied to the first electrode to obtain an output signal from the second electrode. The photoelectric conversion device is provided with means for making the second electrode and the third electrode have the same potential.

It is preferable that the means is a contact hole that short-circuits the second electrode and the third electrode.

Further, it is desirable that the second electrode is provided with a storage unit for storing the output signal and a transfer unit for transferring the output signal.

In the photoelectric conversion device of the present invention, since the aforementioned transient current does not flow, γ is close to 1, the S / N ratio is high, and the reproducibility is excellent. In addition, since the drive circuit can be formed simultaneously with the photoelectric conversion unit on the same substrate, a low-cost and high-performance photoelectric conversion device can be realized.

Examples of the present invention will be described below.

[Embodiment 1] FIG. 6 shows an equivalent circuit of an embodiment of the photoelectric conversion device according to the present invention.

Sensor supply V _S is connected to the drain electrode D, the gate electrode G is connected in common with the source electrode S. A storage capacitor C is connected to the source electrode S,
Further, the load resistance R _L is connected via the transfer switch SW.

Next, the operation of the photoelectric conversion device having the equivalent circuit of FIG. 6 will be described.

When the transfer switch SW is switched from ON to OFF, the photoelectric current i _S flows into the storage capacitor C and charging is started. As the storage capacitor C charges, the voltage V _C increases.

On the other hand, since the gate electrode G and the source electrode S are short-circuited and connected as a means for making the gate electrode G and the source electrode S have the same potential, the gate-source voltage ΔV _GS is always zero potential. Therefore, the transient photocurrent does not flow when the transfer switch SW is ON.

[Embodiment 2] FIG. 7 shows an equivalent circuit of a line sensor type photoelectric conversion device configured by arranging the photoelectric conversion devices shown in FIG. 6 in an nxm array.

In the figure, S _{1 to} S _nxm are TFT-type photoelectric conversion units, and Cs ₁ to
C _{s nxm} is a storage capacitor, U _{1 to} U _nxm are reset TFTs, and T _{1 to} T _nxm are transfer TFTs.

The above element group is divided into n blocks each having n elements, and m + 1
The gate lines and the n signal lines are connected in a matrix. Reference numeral 11 is a driver unit for sequentially applying voltages to the gate lines V _{G1 to} V _{G m + 1} , and 12 is a signal processing unit for extracting the signal voltages of the signal lines S _{1 to} S _n . Also, V
_S is a sensor bias, V _R is a reset voltage of the storage capacitor, and C _{L1 to} C _Ln are load capacitors.

In this circuit, a reset TFT U is provided so that after the charge of the storage capacitor C _S is transferred, the remaining charge can be completely reset. In addition, the reset TFTU
Is commonly connected to the gate electrode of the transfer TFT T of the next block. By shifting the voltage pulse of the driver unit 11, the signal of the next block can be transferred and the previous block can be reset at the same time.

The circuits described above can all be formed on the same substrate. In particular, by using an a-Si: H film by glow discharge as the photoconductive semiconductor material, the TFT type photoelectric conversion part, the storage capacitor, the transfer and reset TFT, the wiring part, etc. are formed into the lower electrode, the SiNH insulating layer, the a-Si. : H layer, n
^It can be realized in the same process by the laminated structure of ⁺ layer and upper electrode. The photoelectric conversion device of the present invention can be suitably applied to such a line sensor type photoelectric conversion device by the same substrate and simultaneous process. Hereinafter, a pattern example of the photoelectric conversion device by this type of process will be described.

FIG. 8 shows a configuration pattern diagram of the circuit of FIG. 7 for one bit. However, in order to avoid making the figure complicated, only the upper and lower wiring patterns and contact hole portions are shown.

In the figure, 13 is a signal line matrix section, 14 is a photoelectric conversion section,
Reference numeral 15 is a contact hole for connecting the gate and source as means for making the potentials the same, 16 is a storage capacitor, 17 is a transfer TFT, 18 is a reset TFT, and 19 is a wiring portion of a gate drive line. In this example, a so-called lensless configuration is adopted in which the original is read by directly contacting the sensor portion without using the imaging lens. Therefore, the window 20 for illuminating the original is provided, and the lower gate electrode of the sensor portion is formed of an opaque material and also serves as a light shielding film.
The transfer and reset TFTs 17 and 18 are arranged at two mirror symmetrical positions. This is to prevent the gate-source capacitance of the TFT from being compensated by the TFT of this pair and not changing when the alignment accuracy of the upper electrode pattern of the lower electrode pattern changes in the longitudinal direction of the substrate. This change in the gate-source capacitance in the longitudinal direction appears as an offset component of the signal output. This offset component can be removed by using the above pattern. The load capacitor C _Li (i = 1 to n) is not shown in FIG. 8, but its capacitance is 10 to several hundreds with respect to the stray capacitance between the signal lines S _{1 to} S _n generated in the signal matrix section 13.
Doubled. Of course, the load resistance may be used to directly read in the form of current without using the load capacitance as in the above embodiment.

9 is a sectional view taken along line XX 'in FIG. 8, and FIG.
-Y 'sectional drawing is shown.

In the figure, 1 is a substrate such as glass, 21 is a lower electrode, which is the gate electrode of the sensor section in FIG. 9 and the gate electrode of the TFT in FIG.

An insulating layer 3 is made of SiNxH, SiO _2, or the like.

A photoconductive semiconductor layer 4 is formed of α-Si: H or the like.

5 is an n ⁺ layer for establishing ohmic contact with the upper electrode, 2
Reference numerals 2 and 23 are upper electrodes, which are the source electrodes of the sensor portion in FIG. 9 and the source / drain electrodes of the TFT in FIG.

(Effects of the Invention) According to the present invention as described above, since the gate and source electrodes of the TFT type photoelectric conversion device are connected in common, (1) a transient photocurrent flows in the current accumulation operation. Therefore, γ =, and the same high S / N ratio as static operation can be obtained.

(2) TFT type sensor unit, TF for transfer and reset
Since the T, the storage capacitor, and the like can all be manufactured on the same substrate in the same process, a line sensor type photoelectric conversion device having a high S / N ratio and low cost can be provided.

And so on.

[Brief description of drawings]

FIG. 1 is an enlarged plan view of a sensor section in a conventional photoelectric conversion device, FIG. 2 is a sectional view taken along line XX ′ in FIG. 1,
FIG. 3 is an equivalent circuit of a conventional photoelectric conversion device, FIG. 4 is the same operation timing diagram, FIG. 5 is the same characteristic diagram, FIG. 6 is an equivalent circuit diagram of an embodiment of the present invention, and FIG. 7 is the present invention. 8 is an equivalent circuit diagram of another embodiment of the present invention, FIG. 8 is a partially enlarged pattern diagram of the same, FIG. 9 is a sectional view taken along line XX 'in FIG.
FIG. 0 is a sectional view taken along the line YY 'in FIG. 1 ... Substrate, 2 ... Gate electrode 3 ... Insulating layer, 4 ... Semiconductor layer 5 ... N ⁺ layer 6, 7, 21, 22, 23 ... Electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location 9056-4M H01L 29/78 311 J (56) Reference JP-A-59-185474 (JP, A) JP 61-288474 (JP, A) JP 59-25280 (JP, A) JP 60-239072 (JP, A) JP 59-8073 (JP, B2) West German patent 2440325 (DE , A)

Claims (3)

[Claims] 1. A semiconductor layer, first and second electrodes electrically connected to the semiconductor layer, and a third electrode provided on the semiconductor layer via an insulating layer. And the first
In a photoelectric conversion device comprising a photoelectric conversion unit configured to obtain a output signal from the second electrode by applying a power supply voltage to the electrode, the photoelectric conversion device includes the second electrode and the third electrode. A photoelectric conversion device, characterized in that it is provided with means for making the same potential. 2. The photoelectric conversion device according to claim 1, wherein the means is a contact hole that short-circuits the second electrode and the third electrode. 3. The second electrode is provided with storage means for storing an output signal and transfer means for transferring the output signal, according to claim 1. Photoelectric conversion device.