JP2001320038A - Photoelectric conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion device in which an overlapping area of an electrode of a transistor and a gate electrode is reduced without reducing the channel length of the transistor. SOLUTION: In this photoelectric conversion device, a plurality of photoelectric conversion cells are formed in two-dimensional arrangement in the XY direction, switching elements of the cells are connected every line with driving lines in the X direction, the driving lines are driven sequentially, signal charge is transferred to a signal line in the Y direction, and a signal is led out sequentially. The switching elements are constituted of transistors. The length (channel length) between the source electrode of the transistor and a channel part is different from the channel length between the drain electrode and the channel part. The channel length of the electrode connected with the signal line is constituted to be shorter.

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, and more particularly, to a photoelectric conversion device in which photoelectric conversion cells are formed in a two-dimensional array in the XY directions on a substrate by using a large area process. The present invention relates to a photoelectric conversion device such as a facsimile, a facsimile, a digital copying machine, an X-ray imaging device, etc., which performs a 1: 1 reading.

[0002]

2. Description of the Related Art Conventionally, facsimile machines, digital copiers,
As an image reading system such as an X-ray imaging device, a reduction optical system or a reading system using a CCD sensor has been mainly used. In recent years, hydrogenated amorphous silicon (hereinafter referred to as “a-Si”) has been used. With the development of the photoelectric conversion semiconductor material represented by), a so-called contact type sensor is used in which a photoelectric conversion element cell and a signal processing unit are formed on a large-area substrate, and a medium to be read is read by an optical system for reading the same size. The development of the equipment was remarkable.

In particular, a-Si is used not only as a photoelectric conversion semiconductor material but also as a thin film field effect transistor (hereinafter, referred to as a thin film field effect transistor).
It is referred to as a “transistor”. ) Has the advantage that the photoelectric conversion semiconductor layer forming the photoelectric conversion element and the semiconductor layer of the transistor forming the switching element can be simultaneously formed on the substrate.

[0004]

However, the characteristic specifications for a large-area photoelectric conversion device have become stricter year by year. In a photoelectric conversion device provided on a substrate in a two-dimensional array of directions, X
It is required to reduce the number of lines as much as possible to obtain more accurate data.

In response to such a demand, it is conceivable to reduce the stray capacitance of the signal line in order to increase the signal output of the photoelectric conversion element even slightly. However, this stray capacitance is usually generated at an overlapping portion between an electrode of a transistor and a gate electrode and at an intersection of a signal line and a gate wiring. This also affects the S / N ratio of the device. This point will be specifically described with reference to FIGS. 5 to 9 regarding the photoelectric conversion device proposed earlier by the present inventors.

In FIG. 5 and FIG. 6, a photoelectric conversion cell comprises a photoelectric conversion element (hereinafter, referred to as a sensor) 8 on a glass substrate 7 and a field effect transistor (hereinafter, referred to as a TFT) 9 as a switching element. Have been. The photoelectric conversion cell is connected to the sensor 8 and the TFT 9.
Are simultaneously formed by an amorphous silicon thin film process. The sensor 8 is a MIS type sensor.

The sensor 8 and the TFT 9 are connected to the glass substrate 7
On top, a lower metal layer 10, an insulating layer 11, a semiconductor layer 12, and n
+ It is formed by sequentially forming a thin film 13, an upper metal layer 14, and a protective layer (not shown) such as an amorphous silicon nitride film or polyimide in a desired pattern.

FIG. 7 is an equivalent circuit of the photoelectric conversion cell of FIG. As shown in FIG. 7, the photoelectric conversion cell generates an electric charge according to the amount of light incident on the sensor 8 and transfers the electric charge to a reading device (not shown) by driving the TFT 9. FIG. 8 illustrates a circuit in which the photoelectric conversion cells illustrated in FIG. 5 are configured in, for example, a 3 × 3 array in the XY directions as illustrated. Here, S11 to S33
Corresponds to the sensor 8 described above, and the lower electrode side is denoted by G, and the upper electrode side is denoted by D. In addition, C11 to C33 are storage capacitors, which are shown on an equivalent circuit because the sensors of S11 to S33 function as capacitors by themselves.

In FIG. 8, T11 to T33 are transfer transistors, Vs is a read power supply, Vg is a refresh power supply, and all the sensors S11 are connected via switches SWs and SWg. ~ S
33 are connected to the D electrode. And the switch SW
s is connected via an inverter, and the switch SWg is directly connected to the refresh control circuit RF, and the switch SWg is controlled to be turned on during the refresh period.

The photoelectric conversion device proposed here divides a total of nine pixels (photoelectric conversion cells) into, for example, three blocks in the column direction (vertical direction) in the drawing and outputs three pixels per block. At the same time, the signal is transferred to the detection integrated circuit IC through the signal wiring SIG, and is sequentially converted into an output by the detection integrated circuit IC (Vout). Here, three pixels in one block are arranged in the horizontal direction, and three blocks are sequentially arranged in the vertical direction, so that each pixel is arranged in a two-dimensional array in the XY directions.

When this photoelectric conversion device is used in an X-ray imaging device, a phosphor CsI such as cesium iodide is formed above the pixel. Then, from above, X-rays (Xr
When ay) enters, the phosphor CsI emits light, and this light enters the sensor of the photoelectric conversion cell.

FIG. 9 is a timing chart showing the operation of FIG. Next, the operation of the above-described photoelectric conversion device proposed above will be described with reference to FIGS. 8 and 9. First, a voltage Hi is applied to the control lines g1 to g3 and s1 to s2 by the shift registers SR1 and SR2.

Then, the transfer transistors T11 to T3
3 and the switches M1 to M3 are turned on to be in a conductive state. The G electrodes of all the sensors S11 to S33 are GN
D potential (the input terminal of the integration detector Amp is GND
Designed to potential). At the same time, the refresh control circuit RF outputs the voltage Hi, turns on the switch SWg,
As a result, the D electrodes of all the sensors S11 to S33 become positive potential by the refresh power supply Vg.

Thus, all the sensors S11 to S33 are
It enters the refresh mode and is refreshed. Next, the refresh control circuit RF outputs the signal Lo, the switches SWs are turned on, and the D electrodes of all the sensors S11 to S33 are set to a higher positive potential by the reading power supply Vs.

Then, all the sensors S11 to S33 enter the photoelectric conversion mode, and at the same time, the capacitors C11 to C33 are initialized. In this state, the signal Lo is applied to the control lines g1 to g3 and s1 to s2 by the shift registers SR1 and SR2. Then, the transfer transistor T1
The switches M1 to M3 of 1 to T33 are turned off, and all the sensors S
The G electrodes 11 to S33 are DC open, but their potential is held by the capacitors C11 to C33.

However, at this time, since no X-rays are incident, no light is incident on all the sensors S11 to S33, and no current flows. In this state, X-rays are emitted in a pulsed manner, pass through a subject such as a human body, and emit phosphors C.
When the light enters sI, the phosphor emits light, and this light enters each of the sensors S11 to S33.

This light contains information on the internal structure of the human body and the like. The photocurrent flowing by this light is accumulated as a charge in each of the capacitors C11 to C33,
It is retained even after the end of X-ray incidence.

Next, a control pulse of a voltage Hi is applied to the control line g1 by the shift register SR1. Then, by applying a control pulse to the control lines s1 to s3 of the shift register SR2, the transfer transistors T11 to T11 are transferred.
V1 to v3 are sequentially output through the switches M1 to M3 of T33. Similarly, shift registers SR1, SR
Under the control of 2, other optical signals are sequentially output.

Thus, two-dimensional information of the internal structure such as the human body is obtained as v1 to v9. The operation up to this point is performed to obtain a still image, but the operation up to this point is repeated to obtain a moving image.

In this photoelectric conversion device, the D electrode of the sensor is commonly connected, and this common wiring is controlled to the potential of the refresh power supply Vg and the read power supply Vs via the switches SWg and SWs. Therefore, all the sensors can be simultaneously switched between the refresh mode and the photoelectric conversion mode. Therefore, an optical output can be obtained with one transistor per pixel without complicated control.

In the photoelectric conversion device, nine pixels are two-dimensionally arranged in 3 × 3, and three pixels are simultaneously transferred and output three times at a time. However, the present invention is not limited to this. If 5 × 5 pixels per 1 mm (vertical and horizontal) are set as 2000 × 2000 pixels, 40 cm × 40 cm if arranged two-dimensionally
The photoelectric conversion device as the X-ray detector of (1) is obtained.

Here, in order to further improve the SN ratio of this photoelectric conversion device, the following proposals have been made. That is, in FIG. 8, when the signal charge Q accumulated in the photoelectric conversion element S11 is transferred to the SIG1 via the transistor 11 and is read by the Amp via the switch M1, Vout is substantially equal to or less than the following. It is expressed by an equation.

[0023] Vout = Q / (ΣCgs + ΣC cross +
CAMP) As also shown in FIG. 8, Cgs is the overlap capacitance of the gates and signal lines of the transistors, C _cross is the overlap capacitance of Vg line and the signal line, cAMP is a read capacity of Amp. Actually, CAmp is {Cgs and {
It is considered that the negligibly small with respect to C _cross, reduce the ΣCgs and .SIGMA.C _cross, to improve signal output, thus improving the SN ratio.

Here, 5 × 5 pixels per 1 mm in height and width as described above are replaced by a much larger number of 2000 × 200 pixels.
40 pixels x 40 pixels, two-dimensionally arranged as 0 pixels
When an X-ray detector having a width of 10 cm is manufactured, the transistor of the pixel shown in FIG.
μm, channel length is 150 μm, signal line width, and V
g linewidth becomes higher 10 [mu] m, when prepared using a general amorphous silicon lamination process, the ΣCgs = 300pF ΣC _cross = 20pF.

The present invention has been made based on the above circumstances, and it is possible to reduce an overlapping area between an electrode of a transistor and a gate electrode without reducing a channel length of the transistor.
It is an object to provide a small-sized photoelectric conversion device.

Still another object of the present invention is to provide a photoelectric conversion device which can reduce on-resistance when transferring signal charges to a signal line in the transistor.

[0027]

In order to solve the above-mentioned problems, the present invention provides a photoelectric conversion cell in which a photoelectric conversion element and a switch element are connected on a substrate in a two-dimensional array in the XY directions. , A plurality of the switching elements are connected to the driving lines in the X direction for each line, the driving lines are sequentially driven, the signal charges are transferred to the signal lines in the Y direction, and the signals are sequentially extracted. In the photoelectric conversion device, the switch element is formed of a transistor, and each of a source electrode and a drain electrode of the transistor has a different length (channel length) in contact with a channel portion, and is connected to the signal line. The channel length of the electrode is shorter.

In this case, as an embodiment of the present invention, in the transistor, when transferring the signal charge to the signal line, the potential of the electrode on the signal line side is higher than the potential of the electrode on the photoelectric conversion element side. It is effective to be set to be.

Further, the photoelectric conversion element comprises a first electrode layer, an insulating layer, a photoelectric conversion semiconductor layer, a semiconductor layer for preventing injection of carriers of the first conductivity type, and a second electrode on the substrate. A photoelectric conversion unit for driving the photoelectric conversion element, wherein the carrier of the first conductivity type generated by the signal light incident on the photoelectric conversion semiconductor layer is fixed to the photoelectric conversion semiconductor layer; An electric field is applied to the photoelectric conversion element in a direction to guide a carrier of a second conductivity type different from the second electrode layer to the second electrode layer, and refresh means for refreshing the photoelectric conversion element applies an electric field to the photoelectric conversion element. Giving the carrier of the first conductivity type from the photoelectric conversion semiconductor layer to the second electrode layer, and further, before the carrier is accumulated in the photoelectric conversion semiconductor layer during the photoelectric conversion operation by the photoelectric conversion means. The first conductive carrier, or, it is preferable that comprises a signal detector for detecting said second of said second conductivity type carrier guided to the electrode layer.

[0030]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a schematic plan view showing the structure of a photoelectric conversion core according to Embodiment 1 of the present invention. The same components as those shown in FIGS. 5 to 9 are denoted by the same reference numerals. The photoelectric conversion core here has a drain electrode (first electrode) 9a of a TFT 9 serving as a switch element in a U-shape, and a source electrode (second electrode).
The electrode 9b is L-shaped, the source electrode 9b is inserted in the middle of the drain electrode 9a, and the two rows of channel portions are connected.

Here, the total channel length is calculated at the center of the width of the channel portion and is 150 μm as a straight line in the case shown in FIG.
m, the overall length (vertical direction in the figure) of the transistor is also shortened. As a result, the area of the sensor 8 receiving the signal light is slightly increased in the upper section.
It is expanding. Thus, by forming the drain electrode 9a and the source electrode 9b of the transistor,
The inter-electrode capacitance (Cgs) shown in the equivalent circuit of FIG. 7 can be reduced to about half.

Therefore, as described above, 5 × 5 pixels per 1 mm in length and width are divided into 2000 × 2 pixels in the X and Y directions.
Two-dimensionally arranged as 000 pixels, for example, 40
For the case of producing an X-ray detector of cm × 40 cm,
The comparison between the present invention and the previous proposal is as follows.

That is, in the transistor of the pixel shown in FIGS. 5 and 6, the width of each of the source electrode and the drain electrode is 10 μm, the channel length is 150 μm, and the signal line width and the Vg line width are about 10 μm. Therefore, as described above, the following values are obtained in the case of manufacturing using a general amorphous silicon lamination process.

ΣCgs = 300 pF ΣC _cross = 20 pF On the other hand, in the case of the pixel shown in FIG. 1 (in the case of the present invention)
Takes the following values:

ΣCgs = 150 pF ΣC _cross = 20 pF However, the signal output Vout = Q / (ΣCgs + ΣCcr)
os + CAmp), the Vo of the pixel shown in FIG.
ut is about twice that of the pixels shown in FIGS.

In the embodiment of the present invention, the case where the L-shaped source electrode 9b is arranged in the middle of the U-shaped drain electrode 9a has been described.
The drain electrode 9a may have a comb shape, and may be arranged so as to be alternated. However, in this case,
It is necessary that the number of comb teeth of the drain electrode 9a be at least one more than the number of comb teeth of the source electrode 9b.

(Embodiment 2) FIG. 3 is an overall schematic circuit diagram of a photoelectric conversion device according to Embodiment 2 of the present invention. Note that the same components as those shown in FIG. 8 are denoted by the same reference numerals. Then, in the photoelectric conversion device illustrated in FIG. 3, its characteristic feature, the charges accumulated in the stray capacitance of the signal line _{SIG1~SIG3 [ΣCgs + ΣC cross + CAmp} ], a circuit for resetting a positive potential V _CRES, signal output The point is that a signal inversion processing circuit for inverting the positive and negative potentials of Vout is provided.

Here, V _CRES is, for example, the sensor S11
Is set higher than the potential on the G side. That is, the signal charge generated by the sensor S11 and accumulated in the capacitor C11 is
When transferring to the signal line SIG1 side, the transistor T11
The potential relationship between the two electrodes is such that the potential on the signal line side is higher.

As described above, when comparing the potential relationship at the time of transistor transfer between the case of the first embodiment of the present invention and the case of the second embodiment, the driving condition of the second embodiment is more suitable for driving the transistor. The resistance value in the ON state becomes smaller.

FIG. 4 shows the on-resistance of the transistor shown in FIGS. 1 and 2 for the first embodiment and the second embodiment. Here, the horizontal axis indicates the potential difference between the source electrode 9b and the drain electrode 9a in each case. What is clear here is that the on-resistance in the second embodiment is small, and when transistors of the same size are arranged, the potential on the signal line side is increased as in the second embodiment. Thus, the signal charges can be transferred in a short time.

That is, when a signal charge is transferred to a signal line by a transistor, the on-resistance can be reduced by setting the potential of the electrode on the signal line side higher than the potential of the electrode on the sensor side. Therefore, not only a still image but also a moving image can be provided with a high S / N ratio.

(Embodiment 3) FIGS. 10 and 11 are configuration diagrams of a photoelectric conversion element according to Embodiment 3 of the present invention. 8, the drain electrode (first electrode) 9a has a U-shape, and the source electrode (second electrode) 9b has an L-shape, as in FIG. , A source electrode 9b is arranged in the middle of the drain electrode 9a, and two rows of channel portions are continuously connected in an arc shape.

Here, the total channel length is calculated at the center of the width of the channel portion, but is set so as to be the same as the 150 μm length shown in FIG. (Upward, vertical direction) is also shortened, and as a result, in the upper section, the area of the sensor 8 receiving the signal light can be slightly enlarged.

(Embodiment 4) FIG. 12 is a circuit diagram of a photoelectric conversion device according to Embodiment 4 of the present invention. For simplicity of description, the figure shows a case where a total of 9 pixels of 3 × 3 in the X and Y directions is used. S1-1 to S3-
Reference numeral 3 denotes a sensor (photoelectric conversion element) for receiving visible light and converting it into an electric signal, and includes switch elements T1-1 to T1-1.
Reference numeral 3-3 is for transferring the signal charges photoelectrically converted by the sensors S1-1 to S3-3 to the matrix signal wirings M1 to M3.

This switch element is shown in FIGS.
As shown in 0 and 11, the number of the comb-shaped source electrode 9b and the number of the drain electrode 9a in the channel portion are different,
In addition, the transistor has a structure in which the number of electrodes connected to the signal line is reduced. Here, G1 to G
Reference numeral 3 denotes a gate drive wiring of the switches connected to the switch elements T1-1 to T3-3 connected to the shift register (SR1).

To the matrix signal wiring M1, three capacitors of the inter-electrode capacitance (Cgs) of the switch element are added at the time of transfer, and are not shown as the capacitive elements in FIG. Other matrix signal lines M2, M3
The same applies to.

Sensors S1-1 to S3-3, switching elements T1-1 to T3-3, and gate drive lines G1 to G3
3 and the matrix signal wirings M1 to M3 are displayed in the photoelectric conversion circuit unit 101 in FIG. 12 and, although not shown, are each disposed on one insulating substrate. In addition, 102 is a switch element T1-1 to T3-
3 is a driving circuit unit including a shift register (SR1) for opening and closing the switching circuit 3.

A1 to A3 are operational amplifiers for amplifying the signal charges of the matrix signal wirings M1 to M3 and performing impedance conversion. In FIG. 12, only the buffer amplifiers constituting the voltage follower circuit are shown. is there. Sn1 to Sn3 are transfer switches for reading the outputs of the operational amplifiers A1 to A3, that is, the outputs of the matrix signal wirings M1 to M3, and transferring them to the capacitors CL1 to CL3.

The signals from the read capacitors CL1 to CL3 are supplied to buffer amplifiers B1 to B1 which constitute a voltage follower circuit.
The data is read by the read switches Sr1 to Sr3 via B3. Reference numeral 103 denotes a read switch Sr1.
To Sr3 (SR2)
It is.

Further, the parallel signals of CL1 to CL3 are S
The signals are serially converted by r1 to Sr3 and a shift register (SR2) 103, input to an operational amplifier 104 constituting a final-stage voltage follower circuit, and digitized by an A / D conversion circuit unit 105. Further, RES1 to RES3 are capacitors (three Cs) added to the matrix signal wirings M1 to M3.
gs) is a reset switch for resetting a signal component stored in the gs), and is reset to a certain reset potential (reset to a GND potential in FIG. 12) by a pulse from the CRES terminal.

Reference numeral 106 denotes photoelectric conversion elements S1-1 to S1-1.
This is a power supply for applying a bias to 3-3. The read circuit unit 107 includes buffer amplifiers A1 to A3, transfer switches Sn1 to Sn3, read capacitors CL1 to CL
3, buffer amplifiers B1 to B3, readout switch Sr
1 to Sr3, a shift register SR2, an operational amplifier 104 at the last stage, and reset switches RES1 to RES3.

[0053]

As described above, according to the present invention, the source electrode and the drain electrode of the transistor have different lengths (channel lengths) in contact with the channel portions, and the electrodes connected to the signal lines have different lengths. By shortening the channel length, the stray capacitance of the signal line is reduced, and as a result, the signal output of the photoelectric conversion element can be increased, and the effect that the SN ratio can be increased can be obtained. Can be

When signal charges are transferred to a signal line by a transistor, the on-resistance can be reduced by setting the potential of the electrode on the signal line side higher than the potential of the electrode on the sensor side. Therefore, not only a still image but also a moving image can be provided with a high SN ratio.

[Brief description of the drawings]

FIG. 1 is a plan view of each photoelectric conversion cell for describing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along line AA of FIG.

FIG. 3 is an overall circuit diagram according to a second embodiment of the present invention.

FIG. 4 is a diagram showing the on-resistance of the transistor shown in FIG.

FIG. 5 is a plan view of each photoelectric conversion cell in the photoelectric conversion device previously proposed by the present inventors.

6 is a cross-sectional view along the line BB in FIG.

FIG. 7 is an equivalent circuit of one photoelectric conversion cell.

FIG. 8 is an overall circuit diagram of the photoelectric conversion device.

FIG. 9 is a timing chart showing the operation of FIG.

FIG. 10 is a plan view of each photoelectric conversion cell for explaining a third embodiment of the present invention.

FIG. 11 is a cross-sectional view taken along the line CC of FIG. 10;

FIG. 12 is an overall circuit diagram for explaining a fourth embodiment of the present invention.

[Explanation of symbols]

 S11 to S33 Sensor (photoelectric conversion element) T11 to T33 Transistor C11 to C33 Capacitor SR1, SR2 Shift register IC detection integrated circuit 8 Sensor (photoelectric conversion element) 9 TFT (switch element) 9a Drain electrode 9b Source electrode

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 5/335 H01L 31/10 EF term (Reference) 2G088 FF02 GG19 GG20 GG21 JJ05 JJ31 JJ33 4M118 AB01 AB10 BA05 BA12 CA40 CB06 CB11 FA22 FA34 FB23 5C024 CX03 CX41 DX04 GX02 GX16 GX18 HX17 HX23 5F049 MA14 MB05 NB05 RA02 UA01 5F110 AA02 AA07 BB10 CC07 GG28 HM12

Claims (3)

[Claims] 1. A plurality of photoelectric conversion cells each having a photoelectric conversion element and a switch element connected thereto in a two-dimensional array in the X and Y directions are provided on a substrate, and the switch element is provided for each line in the X direction. In the photoelectric conversion device that sequentially drives the drive lines, transfers signal charges to the signal lines in the Y direction, and sequentially extracts signals, the switch element is configured by a transistor, Each of the source electrode and the drain electrode of the transistor has a different length (channel length) in contact with a channel portion,
Further, a channel length of an electrode connected to the signal line is shorter than a channel length of the electrode. 2. The method according to claim 2, wherein, when transferring the signal charge to the signal line, the potential of the electrode on the signal line side is set to be higher than the potential of the electrode on the photoelectric conversion element side. The photoelectric conversion device according to claim 1, wherein: 3. The photoelectric conversion element includes a first electrode layer, an insulating layer, a photoelectric conversion semiconductor layer, a semiconductor layer for preventing injection of carriers of a first conductivity type, and a second electrode on the substrate. A photoelectric conversion unit for driving the photoelectric conversion element, wherein the first conductivity type carrier generated by the signal light incident on the photoelectric conversion semiconductor layer is retained on the photoelectric conversion semiconductor layer, and the carrier An electric field is applied to the photoelectric conversion element in a direction in which a carrier of a second conductivity type different from the above is introduced to the second electrode layer, and refresh means for refreshing the photoelectric conversion element applies an electric field to the photoelectric conversion element. Giving the carrier of the first conductivity type from the photoelectric conversion semiconductor layer to the second electrode layer, and further, the carrier accumulated in the photoelectric conversion semiconductor layer during the photoelectric conversion operation by the photoelectric conversion means. 3. The photoelectric conversion device according to claim 1, further comprising a signal detection unit configured to detect a carrier of one conductivity type or the carrier of the second conductivity type guided to the second electrode layer. 4. Conversion device.