JPH07176711A - Photoelectric conversion device - Google Patents

Abstract

PURPOSE:To provide a photoelectric conversion device at low cost with a high S/N a small variation, and a high productive yield. CONSTITUTION:A photoelectric conversion unit Sn has a first electrode put opposite to an optical conductive layer and receiving a voltage from a power supply, a second electrode for generating a converted charge and a third electrode put on the optical conductive layer with an insulating layer in between. A photoelectric conversion device includes the photoelectric conversion unit Sn, a bias transistor Rn for applying a bias to the third electrode at the photoelectric conversion unit Sn, a bias capacitor Cgsn located between the second electrode and the third electrode, a storage capacitor Csn, for storing a charge converted photoelectrically, and a transfer transistor Tn for transferring the charge stored in the capacitor. In addition, the bias transistor Rn and the transfer transistor Tn are driven by the same signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device used in bar code readers, facsimiles, digital copying machines, and the like, and more particularly to a thin film transistor (thin film transistor formed by providing a semiconductor layer with a gate electrode via an insulating layer). Hereinafter, referred to as a TFT) type photoelectric conversion device.

[0002]

2. Description of the Related Art First, as a configuration example of a conventional TFT type optical sensor, a plan view is shown in FIG. 7 and a sectional view taken along line XX 'of FIG. 7 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, and 5 is a semiconductor layer 4 and source and drain electrodes. It is an n ⁺ layer for making ohmic contact with 6 and 7.

Since 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, the light quantity dependence characteristic of the photoelectric conversion output (hereinafter referred to as γ) is close to 1. Has good characteristics. Also,
Since a stable negative bias can be applied between the gate and source of the sensor, the recombination time of electrons and holes can be effectively shortened, and good characteristics in which the optical response falls quickly can be obtained. Further, it is characterized by good reproducibility and less variation within and between lots.

FIG. 9 shows a storage mode readout circuit using a TFT type sensor (see Japanese Patent Laid-Open No. 63-161666).

In the circuit of FIG. 9, in addition to the transfer switch sw2, a reset switch sw1-a for discharging the transfer residue of the storage capacitor C _s is provided. Further, the sensor electrode V _S is connected to the drain electrode D of the photoelectric conversion unit,
A gate bias switch sw1-b interlocking with the switch sw1-a is connected to the gate electrode.

Next, the operation of the photoelectric conversion device having the circuit configuration shown in FIG. 9 will be described with reference to FIG.

When the switches sw1-a and b are turned on,
Storage capacitor C_s Is discharged and V_C = 0 (V)
Becomes At the same time, the bias capacitor C_gsIs -V_B Devoted to
Be charged. When the switches sw1-a and b are turned off,
Photocurrent i generated in the photoelectric conversion unit _s Is the storage capacitor C_s 
Will be charged. Storage capacitor C_s Potential V_C Is charged
It rises as it goes. On the other hand, the bias capacitor C_gs
Switch SW1-b is in OFF state,
There is no flow path.

Therefore, the potential V _G of the gate electrode G operates so as to follow the change in the potential V _C of the storage capacitor C _s while keeping the gate-source voltage ΔV _GS = V _G -V _C constant. .

Storage time T_s Later, the transfer switch sw2 is O
The N state is set. At this time, the storage capacitor C_s Charging voltage
V_C Is the load capacity C_L Transferred to the load capacity C_L Charging power
Pressure V _L Is shown by a broken line in FIG.

In this configuration example, the potential ΔV _GS between the gate electrode G and the source electrode S is always kept at a constant value (-V _B ).
Therefore, the gate-source bias of the photoelectric conversion part can be set to an optimum value under any conditions such as the amount of incident light and the storage time, and a transient photocurrent flows due to the change of the potential ΔV _GS. In addition, good characteristics can be brought out with the influence of the interface of the insulating layer suppressed as much as possible.

[0011]

Although these characteristics show preferable results under the driving conditions of individual sensors (hereinafter also referred to as single-bit sensors), the operation of a multi-bit sensor such as that normally used for a line sensor, that is, a sensor. However, there is a problem when the blocks are arranged in an n × m array and used as a block drive. The problems will be described below.

FIG. 11 shows the photoelectric conversion device shown in FIG.
An equivalent circuit of a line sensor type photoelectric conversion device configured by arranging xm arrays is shown.

S _{1 to} S _{n * m} are TFT type photoelectric conversion units, C
_{gs1 ~C gsn * m} the gate bias capacitor, R ₁ ~
R _{n * m} is a gate bias TFT, C _{s1 to} C _{sn * m} are storage capacitors, U _{1 to} U _{n * m} are reset TFTs, and T ₁ to
T _{n * m} is a transfer TFT.

The above-mentioned element group is divided into m blocks by n, and is matrix-connected to m + 1 gate drive lines and n signal lines. In the figure, 11 is a driver unit for sequentially applying voltages to the gate lines V _{G1 to} V _{Gm + 1} , and 12 is a signal processing unit for extracting the signal voltages of the signal lines L _{1 to} L _n . Further, V _S is the sensor bias and V _R is the reset voltage of the storage capacitor.

In this photoelectric conversion device, a reset TFT;
The gate electrode of U and the gate bias TFT; R is commonly connected to the gate electrode of the next block transfer TFT; T. By shifting the voltage pulse of the driver unit 11, the signal of the next block is transferred, and at the same time, the previous block is reset.

Here, paying attention to the wiring pattern for applying the voltage pulse of the driver section 11, V _G1 and V _G2 , ... V _Gm
And V _{Gm + 1} intersect (the place circled by the broken line in FIG. 11).

The intersections of these wirings will be described below with reference to manufacturing processes and pattern examples.

All the circuits shown in FIG. 11 can be formed on the same substrate. In particular, by using an a-Si: H film by the glow discharge method as the photoconductive semiconductor material, T
FT-type photoelectric conversion unit, storage and bias capacitor, transfer / reset and bias TFT, wiring part, etc., lower electrode, a-SiN: H insulating layer, a-Si: H layer, n ⁺ layer,
This can be realized by a simultaneous process due to the laminated structure of the upper electrode.
An example of a pattern of a line sensor type photoelectric conversion device by this type of process will be shown below.

FIG. 12 shows a one-bit configuration pattern diagram of the circuit of FIG. However, in order to avoid complication of the drawing, only the upper and lower wiring patterns and contact hole portions are shown. In the figure, 13 is a signal line matrix part, 14 is a photoelectric conversion part, 15 is a gate bias capacitor, 16 is a storage capacitor, 17 is a gate bias TFT, 18 is a transfer TFT, 19 is a reset TFT, and 20 is a gate drive. It is the wiring part of the line. Here, the gate drive lines G _j and G _{j + 1}
The overlapping part is surrounded by a broken line, but this overlapping part corresponds to the part surrounded by a broken line in FIG. The parasitic capacitance created by this overlapping portion is determined by the dielectric constant, the film thickness, and the overlapping area of the a-SiN: H insulating layer and the a-Si: H layer, but is 0 for one overlapping capacitance under general manufacturing conditions. 0.01 to 0.02 PF occurs.

Here, again, the load capacity C_L Voltage V across
_L Pay attention to the voltage V_L Is the voltage V _C To transfer C
_L ≫ C_s And under the condition of single-bit sensor drive,
Thus, in a certain time, the signal charge can be transferred almost completely.
It Voltage V at this time_L This is shown by the broken line in FIG.

However, it is actually driven by the circuit of FIG.
Voltage V_L Is the distortion as shown by the solid line in FIG.
It became a waveform. This is due to the transfer sw2 in FIG.
It depends on the overlap capacity of the gate drive that is triggered when it is turned on.
Crosstalk. In detail, transfer sw2 turned on.
Sometimes this crosstalk causes a switch for setting sw1-
Raise the gate potential of a, causing leakage of signal charge.
Rub That is, the storage capacitor C_s Signal accumulated in
Part of the electric charge (the amount of electric charge shown by the diagonal lines in FIG. 10) is the load capacity.
Quantity C_L Since it cannot be transferred to the
Load capacity C _L Potential V_L Smaller potential V than_L Shi
You will not be able to get it. The confidence of this crosstalk
While the transfer sw2 is ON, the leak current of the signal charge is
There is almost no effect on a steady basis. But the transfer sw2
When it is turned on, the switch sw1-a for reset is suddenly turned on.
Gate electrode voltage rises, causing leakage as inrush current
It As a result, the load capacitance C which is the signal output potential_L Potential of
V_L Deteriorates and the S / N ratio deteriorates.

[Object of the Invention] The present invention proposes a driving circuit which solves the above-mentioned problems and can be easily formed on the same substrate as a sensor section, and has a high S / N characteristic of a TFT type sensor. It is an object of the present invention to provide a photoelectric conversion device with low cost and high yield by taking advantage of the characteristic that the ratio and the variation distribution are small.

[0023]

A photoelectric conversion device according to the present invention comprises a photoconductive layer, a first electrode provided opposite to the photoconductive layer, to which a power source voltage is applied, and a photoelectric conversion device for obtaining photoelectrically converted charges. A second electrode and a third electrode provided on the photoconductive layer via an insulating layer, and a bias transistor for applying a bias to the third electrode of the photoelectric conversion unit, A photoelectric converter including a bias capacitor provided between the second electrode and the third electrode, a storage capacitor that stores the photoelectrically converted charge, and a transfer transistor that transfers the charge stored in the storage capacitor. In the conversion device, the bias transistor and the transfer transistor are driven by the same signal, which is a photoelectric conversion device.

[0024]

According to the photoelectric conversion device of the present invention, the transfer operation of the signal charge accumulated in the storage capacitor and the bias operation of the storage capacitor and the third electrode of the photoelectric conversion portion can be performed at the same time. The leak current of the signal charge can be prevented from flowing, and a photoelectric conversion device having a high S / N ratio and excellent reproducibility can be obtained.

Further, since it is not necessary to provide the reset switch (reset TFT), it is possible to provide a photoelectric conversion device of low cost and high yield. Also,
Further, since it is not necessary to drive the reset switch, it is possible to prolong the time for charging the photocurrent i _s to the storage capacitor C _s . As a result, the signal charge is increased and the S / N ratio is excellent. It is possible to obtain an improved photoelectric conversion device.

[0026]

FIG. 1 shows an equivalent circuit diagram of an embodiment of the present invention.

In the circuit of FIG. 1, similar to the conventional example shown in FIG. 9, the transfer switch sw-a can almost completely transfer the signal charge of the storage capacitor C _s to the load capacitance C _L (C _L >> C _s ). . In other words, the transfer causes the potential V _C of the storage capacitor C _s to be almost completely reset to the potential V _L of the load capacitance C _L. Further, a gate bias switch sw-b interlocking with the transfer switch sw-a is connected to the gate electrode of the photoelectric conversion unit.

Next, FIG. 2 shows the operation timing.

When the switches sw-a and b are turned on,
The charge on the storage capacitor C _s is almost completely transferred to the load capacitor C _L and V _C ≈V _L. At the same time a bias capacitor C _gs is charged to -V _B. Switch sw-
When a and b are turned off, the photocurrent i generated in the photoelectric conversion unit
_s is charged in the storage capacitor C _s. The potential V _C of the storage capacitor C _s rises as it is charged. on the other hand,
The switch sw-b of the bias capacitor C _gs is OFF.
Since there is a state, there is no path for current to flow.

Therefore, similarly to the conventional example, the potential V _G of the gate electrode G changes in the potential V _C of the storage capacitor C _s while keeping the gate-source voltage ΔV _GS = V _G -V _C constant. Operates to follow.

[0031] The storage time T _s again after transfer switch sw-
The a and b are turned on, and the above operation is repeated.

As described above, in this embodiment, the signal charge of the storage capacitor C _s is almost completely transferred by the transfer switch sw-a. Therefore, the residual charge of the storage capacitor C _s is almost no, there is no need to provide a reset switch to discharge the storage capacitor C _s. Further, since it is not necessary to provide the reset switch, the gate device switch sw-b for charging the bias capacitor C _gs to −V _B can be driven simultaneously with the transfer switch sw-a.

As described above, since it is not necessary to provide the reset switch in this embodiment, it is possible to provide a photoelectric conversion device of low cost and high yield. Further, since the time for driving the reset switch is not necessary, the time for charging the photocurrent i _s to the storage capacitor C _s can be lengthened, and as a result, the signal charge is increased and the S / N ratio is increased. A highly excellent photoelectric conversion device can be obtained.

In this embodiment, the potential ΔV _GS between the gate electrode G and the source electrode S is always kept at a constant value (-V _B ).
Therefore, the gate-source bias of the photoelectric conversion unit can be set to an optimum value under any conditions such as the amount of incident light and the accumulation time, and a transient photocurrent flows due to the change in the potential ΔV _GS. In addition, good characteristics can be brought out with the influence of the insulating layer interface being suppressed as much as possible, and such effects are similar to those of the conventional example.

FIG. 3 shows the photoelectric conversion device shown in FIG.
An equivalent circuit of a line sensor type photoelectric conversion device configured by arranging m pieces in an array is shown.

In FIG. 3, the same reference numerals as in the conventional example of FIG. 11 indicate the same or corresponding portions. Further, with respect to the names of the respective elements and the description of the configuration of the photoelectric conversion device, the same parts as those described in the related art in FIG. 11 will be omitted.

FIG. 3 is different from FIG. 11 of the conventional example.
Is the storage capacitor C_s ~ C _{sn * m}Releasing the residual charge of
Reset TFT to turn on; U₁ ~ U_{n * m} Has been deleted
Second, the gate bias TFT; R₁ ~ R
_{n * m} The number of gate drive lines for driving
Third, each gate drive line must have the same bit.
Transfer TFT; T₁ ~ T_{n * m} To the same gate drive line as
The point is that they are connected. This first and third difference
Therefore, the gate drive lines do not overlap in the equivalent circuit.
Will not be lost.

By eliminating the overlap between the gate drive lines, that is, eliminating the overlap capacitance, crosstalk, which is the above-mentioned problem, is eliminated. In detail, transfer sw
When is turned on, the gate potential of the reset switch sw is raised, and the leakage of signal charges is not caused. That is, since a part of the signal charge accumulated in the storage capacitor C _s (the charge amount shown by the slanted line in FIG. 10) can be almost completely transferred to the load capacitance C _L , the load capacitance obtained when the single-bit sensor is driven. it is possible to obtain an accurate potential V _L of the same as the potential V _L of C _L.

Since the leak current of the signal charge due to the crosstalk due to the overlapping capacitance of the gate drive lines is eliminated in this way, the potential V _L of the load capacitance C _L which is the signal output potential shows a normal value and the S / N ratio is increased. Improve.

The photoelectric conversion device of the present invention can be suitably applied to a line sensor type photoelectric conversion device by the same substrate and simultaneous process as in the conventional example. An example of a pattern of this type of line sensor type photoelectric conversion device will be shown below.

FIG. 4 shows a one-bit configuration pattern diagram of the circuit of FIG. Also in FIG. 4, as in FIG.
The same reference numerals as those of the conventional example 2 denote the same or corresponding portions, and the same portions as those described in the conventional technique are omitted.

4 is different from FIG. 12 in that
For resetting to discharge the residual charge of the storage capacitor 16
Reset voltage wiring V of the TFT 19 and the storage capacitor_R 
The second point is that the gate bias T has been deleted.
The gate drive line that drives the FT17 is G_{j + 1} Not the same
The same gate drive line as the transfer TFT 18 of the same bit is G _j 
It is a point connected with. Therefore, on the actual pattern
However, the gate drive lines will not overlap each other.

The load capacitor C in the equivalent circuit of FIG.
_{Although Li} (i = 1 to n) is not shown in FIG. 4, its capacitance is equal to that of the signal lines L _{1 to} L generated in the signal line matrix unit 13.
The stray capacitance between _{n is} usually set to 10 to several hundred times. Needless to say, the data may be directly read out in the form of current without using the load capacitance C _Li .

In this example, a so-called lensless structure is adopted without directly using the image forming lens to read the original by directly adhering it to the sensor section. Therefore, a window 21 for illuminating the original is provided, and the lower gate electrode of the photoelectric conversion portion is formed of an opaque material and also serves as a light shielding film.

FIG. 5 is a sectional view taken along line XX 'in FIG. 4, and FIG.
YY 'sectional drawing of is shown. In the figure, 1 is a glass substrate, 2 is a lower electrode, a lower electrode of a capacitor in FIG. 5, and a TFT in FIG.
Gate electrode. An insulating layer 3 is formed of a SiN: H film / SiO ₂ film or the like. 4 is a photoconductive semiconductor layer,
Reference numeral 5 denotes an n ⁺ layer formed of a-Si: H or the like to form an ohmic contact with the upper electrode. 6 and 7 are upper electrodes, which are the upper electrodes of the capacitor in FIG. 5 and the source / drain electrodes of the TFT in FIG.

[0046]

As described above, according to the present invention, the transfer TFT for transferring the signal charge of the TFT type photoelectric conversion part in which the capacitor is provided between the gate and the source is used for the reset TFT for resetting the source potential. Since it also has a function, the transfer TFT and the gate bias TFT can be driven by the same signal.

As a result, the overlapping capacitance of the gate bias drive lines is eliminated and crosstalk is not induced, so that the leak current of signal charges is eliminated and the same S / N ratio as that of the single bit operation can be obtained. Since it is not necessary to provide the TFT for resetting the source potential, the photoelectric conversion device can be miniaturized in a pattern, and a photoelectric conversion device with low cost and high yield and good productivity can be obtained. Since the time for driving the reset TFT is not required, it is possible to lengthen the time for charging the photocurrent.
A high-performance photoelectric conversion device having a high ratio can be realized. There are various effects such as.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of an embodiment of the present invention.

FIG. 2 is an operation timing chart of the embodiment of the invention.

FIG. 3 is a partial equivalent circuit diagram of a line sensor type photoelectric conversion device configured according to the present invention.

FIG. 4 is a partial pattern diagram of a line sensor type photoelectric conversion device configured according to the present invention.

5 is a cross-sectional view taken along line X-X ′ in FIG.

6 is a cross-sectional view taken along line Y-Y ′ in FIG.

FIG. 7 is a pattern diagram of a conventional photoelectric conversion device.

8 is a sectional view taken along line X-X ′ in FIG. 7.

FIG. 9 is an equivalent circuit diagram of a conventional photoelectric conversion device.

FIG. 10 is an operation timing chart of the conventional photoelectric conversion device.

FIG. 11 is a partial equivalent circuit diagram of a line sensor type photoelectric conversion device configured according to a conventional invention.

FIG. 12 is a partial pattern diagram of a line sensor type photoelectric conversion device constructed according to a conventional invention.

[Explanation of symbols]

 1 substrate 2 gate electrode 3 insulating layer 4 photoconductive layer 6.7 source / drain electrode 14 photoelectric conversion unit C capacitor R resistance sw switch

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hidemasa Mizutani 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (1)

[Claims] 1. A first electrode provided opposite to the photoconductive layer to which a power supply voltage is applied, a second electrode for obtaining photoelectrically converted charges, and provided on the photoconductive layer via an insulating layer. And a bias transistor for applying a bias to the third electrode of the photoelectric conversion unit, and a photoelectric conversion unit including the third electrode formed between the second electrode and the third electrode. A photoelectric conversion device comprising a bias capacitor, a storage capacitor that stores the photoelectrically converted charges, and a transfer transistor that transfers the charges stored in the storage capacitor, wherein the bias transistor and the transfer transistor Are driven by the same signal.