JPH065834A - Image sensor - Google Patents

Abstract

PURPOSE:To reduce the size of an image sensor by a method wherein a resetting thin film transistor for reducing an afterimage and a photodiode are unified. CONSTITUTION:A photoconducting layer 13 and a transparent electrode 11 are formed on the drain electrode 8R of a resetting thin film transistor part R to constitute a photodiode part P. The transparent electrode 11 is connected to a common power supply. The source electrode 9R of the resetting thin film transistor part R is connected to an Al electrode 12 through a contact hole 14 and grounded. The drain electrode 8R is so formed as to serve as the lower electrode 15 of the photodiode part P and a signal line as well.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor used in facsimiles, image scanners and the like.

[0002]

2. Description of the Related Art Conventionally, a TFT (thin film transistor) matrix driven image sensor is known as a contact type image sensor used for an image sensor. This is to project image information of a document or the like onto a light-receiving element array, transfer the charges generated in each light-receiving element to a capacitance between wirings for each block by a transfer transistor using a TFT switch, and perform chronological sequence. To read.

FIG. 6 is an equivalent circuit diagram of an example thereof. Drawing, P ₁₁ to P _MN photodiodes, C ₁₁ -C _MN photodiode unit volume, T ₁₁ through T _MN transfer transistor, C _L1 -C _LN is interline capacitance, S ₁ to S _N is the signal Line, G ₁
~G _M is a gate line.

The photodiodes P _{11 to} P _MN having the photodiode portion capacitances C _{11 to} C _MN are divided into M blocks, and N photodiodes are assigned to each block. The photodiodes P _{11 to} P _1N are the first block, the photodiodes P _{21 to} P _2N are the second block, and hereinafter, the photodiodes P _{M1 to} P _MN are the Mth block. The output of each photodiode is the transfer transistors T _{11 to} T _1N , T _{21 to} T _2N , ...
., T _{M1 to} T _MN are connected as switching elements to the respective drains, and the sources of the respective transfer transistors are
The blocks are connected to the signal lines S _{1 to SN} so that the blocks of the same rank are connected to the same signal line. Transfer transistors T _{11 to} T _1N , T _{21 to} T _2N , ..., T _M1
Gates of the through T _MN, the gate lines G ₁ in the common block by block, G _2, · · ·, are connected to the G _M. The signal lines S ₁ , S ₂ , ..., _SN have line capacitances C _L1 , C _L2 ,
..., _CL N exists. An additional capacitor may be connected to this.

The output of the drive pulse generating circuit (not shown) is
Are sequentially applied to the gate lines G ₁ , G ₂ , ..., G _M ,
By turning on the transfer transistors T _{11 to} T _1N , T _{21 to} T _2N , ..., T _{M1 to} T _MN which are switching elements for each block, the photodiodes P _{11 to} P _1N and P ₂₁ are turned on.
, P _2N , ..., P _{M1 to} P _MN output is the line capacitance C _L1 ,
, C _LN are transferred to and accumulated in C _L2 . The output signals are sequentially selected by sequentially reading the signal lines S ₁ , S ₂ , ..., _SN in synchronization with the drive pulse. First,
It is assumed that the drive pulse generating circuit outputs a gate pulse to the gate line G ₁ . Thereby, the transfer transistor T ₁₁ through T _1N of the first block are turned on to connect the output of the photodiode P ₁₁ to P _1N signal lines S _1, S _2, · · ·, to S _n, line capacity Transferred to C _L1 , C _L2 , ..., C _LN and stored. The output signals accumulated in the line capacitances C _L1 , C _L2 , ..., C _Ln are sequentially selected by the read signal.
Next, a gate pulse is output from the drive pulse generating circuit to the gate line G ₂ and the transfer transistor T of the second block is output.
_{21 to} T _2N are turned on, the outputs of the photodiodes P _{21 to} P _2N are transferred to the line capacitances C _L1 , C _L2 , ..., C _LN , and similarly, the signal lines S ₁ , S ₂ ,. .., S _N are sequentially selected by the read signal. Similarly, the output up to the Mth block is output as a time-series signal, the image information of one line in the original is read, the sub-scan is performed, the next line is read in the same manner, and the same is repeated. The entire image information can be read out as a time series signal.

In the image sensor described in Japanese Unexamined Patent Publication No. 4-133350, a photodiode in which a photoconductor layer and a transparent conductor layer are laminated in this order on the source electrode or the drain electrode of the transfer thin film transistor described above. Is disclosed, and a structure in which a transfer thin film transistor and a photodiode are integrated is disclosed. In such an image sensor, the photodiode portion capacitance C ₁₁
The accumulated charges of C _MN are the line capacitances C _L1 , C _L2 , ..., C
When transferred to the _LN , the capacitance of the photodiode section C ₁₁ ~
There is a problem that charges remain in the photodiode parts capacitances C _{11 to} C _{MN due} to the ratio of C _MN to the line capacitances C _L1 , C _L2 , ..., C _LN , which causes an afterimage.

In order to reduce this afterimage, Japanese Patent Laid-Open No. 1-94
In the image sensor shown in FIG. 6 of Japanese Patent No. 655 publication,
An image sensor including a thin film transistor for resetting the photodiode part capacitances C _{11 to} C _MN has been considered.

FIG. 7 is an equivalent circuit diagram of a conventional example using a reset transistor. In the figure, the same parts as those in FIG. 6 are designated by the same reference numerals and the description thereof will be omitted. R _{11 to} R _MN are reset transistors. It is assumed that each block is driven in order from the first block to the Mth block as described with reference to FIG. The reset transistors R _{11 to} R _1N of the first block have their gates connected in common, and are connected to the gate line G ₂ so as to be driven by the same gate signal as the transfer transistors T _{21 to} T _{2N of} the second block. There is. Similarly, the gate of the reset transistor of each block is
It is commonly connected to the gates of the transfer transistors of the next block. However, the gate of the reset transistor of the Mth block, which is the last block, is connected to the gate of the transfer transistor of the first block, which is the first block.

Therefore, when the transfer transistor of a certain block is driven, the reset transistor of the preceding block is turned on and the residual charge of the photodiode portion capacitance is reset.

The structure of the sensor portion of the image sensor of FIG. 7 having the reset transistor will be described. 8 is a plan view, and FIG. 9 is a cross-sectional view taken along the line AA of FIG. In the figure, P is a photodiode portion, T is a transfer thin film transistor portion, R is a reset thin film transistor portion, 1 is an insulating substrate, 2T and 2R are gate electrodes, 3 is a gate insulating film,
4 is a semiconductor layer, 5 is an ohmic contact layer, 6 is a signal lead wire, 7T and 7R are top insulating layers, 8T and 8R are drain electrodes, 9T and 9R are source electrodes, 10 is a photoconductor layer, and 11 is a transparent electrode. , 12 is an Al electrode, 13 is a polyimide layer, 14 is a contact hole, and 15 is a lower electrode.

A glass substrate is used as the insulating substrate 1, and the gate electrodes 2T and 2R have Cr deposited thereon and are commonly patterned for each block. A gate insulating film 3 made of a silicon nitride film is provided thereon, and a semiconductor layer 4 using hydrogenated amorphous silicon (a-Si: H) is deposited. Top insulating layers 7T and 7R are provided on the channel regions above the gate electrodes 2T and 2R of the semiconductor layer 4 and insulated by the top insulating layers 7T and 7R.
Through the drain electrodes 8T and 8R and the source electrode 9
T and 9R are formed. The layer structure for forming the gate insulating film 3, the semiconductor layer 4, the ohmic contact layer 5, and the drain electrodes 8T, 8R and the source electrodes 9T, 9R is divided into regions of a transfer thin film transistor portion T and a reset thin film transistor portion R. The photoconductor layer 10 and the transparent electrode 11 are formed on the layer structure of the intermediate region, and the photodiode part P is constituted. The layer on which the drain electrode and the source electrode are formed is the lower electrode 15 and is connected to a common power supply terminal, for example, the potential of + 5V. Transparent polyimide layer 1 on each of these layer structures
3 is layered and the contact hole 14 is patterned. The transparent electrode 11 is formed of, for example, ITO, and the drain electrode 8T of the transfer thin film transistor section T, the drain electrode 8R of the reset thin film transistor section R, and the Al electrode 12 are respectively formed through a contact hole 14 provided thereon. It is connected. Further, the source electrode 9T of the transfer thin film transistor portion T is connected to the signal line S of FIG. 7 by the Al electrode 12 through the contact hole 14.
₁ , S ₂ , ..., _SN , and the source electrode 9R of the reset thin film transistor section R is connected to the ground potential by the Al electrode 12 through the contact hole 14.

As described above, in the image sensor provided with the reset thin film transistor, since the transfer thin film transistor portion T and the reset thin film transistor portion R are provided for each bit, it is difficult to miniaturize the image sensor.

In the image sensor described in the above-mentioned Japanese Patent Laid-Open No. 1-94655, a photodiode is arranged on the source electrode or the drain electrode of the transfer thin film transistor to reduce the size of the image sensor. However, if this structure is adopted and a reset thin film transistor is separately provided, an increase in size cannot be avoided, and a problem arises in that the wiring between the photodiode and the reset thin film transistor becomes long.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described circumstances, and integrates a reset thin film transistor for reducing an afterimage and a photodiode, and further, a reset thin film transistor and a transfer thin film transistor section. By integrating the and photodiode,
The purpose is to reduce the size of the image sensor.

[0015]

SUMMARY OF THE INVENTION According to the present invention, a plurality of photodiodes are set as one block, a photodiode array formed by the plurality of blocks, and a plurality of blocks for transferring signal charges generated in the photodiodes in each block unit. 2. An image sensor having a transfer thin film transistor and a reset thin film transistor for resetting residual charges of the photodiode, wherein the photodiode is formed on a source electrode or a drain electrode of the reset thin film transistor. In the invention according to claim 2, the reset thin film transistor and the transfer thin film transistor have a common source electrode or drain electrode, and a photodiode is formed on the common source electrode or drain electrode. And also It is.

[0016]

According to the first aspect of the invention, since the photodiode is formed on the source electrode or the drain electrode of the reset thin film transistor, the connection electrode between the photodiode and the reset thin film transistor can be omitted and the size can be reduced. The obtained image sensor is obtained.
According to the invention of claim 2, the source electrode or the drain electrode of the reset thin film transistor and the transfer thin film transistor are shared, and the photodiode is formed on the common source electrode or drain electrode, thereby connecting the photodiode and the reset thin film transistor. The electrodes can be omitted, and a more compact image sensor can be obtained.

[0017]

FIG. 1 is an equivalent circuit diagram of an embodiment of the image sensor of the present invention. 7, those parts that are the same as those corresponding parts in FIG. 7 are designated by the same reference numerals, and a description thereof will be omitted. In this embodiment, the transparent electrode side of the photodiode can be set to the common potential by using the power source having the polarity opposite to that in FIG. Thereby, as will be described later, the lower electrode 15 of the photodiode is
The potential of can be set to the same potential as the drain electrode potential or the source electrode potential of the reset thin film transistor or the transfer thin film transistor, and the layer structure described later can be adopted.

2 and 3 are views for explaining the structure of an embodiment of the sensor portion of the image sensor for realizing the equivalent circuit of FIG. 1, FIG. 2 is a plan view, and FIG. 2 is a cross-sectional view taken along line AA of 2. In the figure, the same parts as those in FIGS. 8 and 9 are designated by the same reference numerals, and the description thereof will be omitted. In this embodiment, the drain electrode 8 of the reset thin film transistor section R is
The photoconductor layer 10 and the transparent electrode 11 are formed on the R to form the photodiode portion P. The transparent electrode 11 is
As can be seen from FIG. 3, it extends in the main scanning direction in common, and as shown in FIG.
It is connected to a potential of -5V. The source electrode 9R of the reset thin film transistor section R has a contact hole 14
It is connected to the Al electrode 12 through and is grounded to the ground potential. The drain electrode 8R has a photodiode portion P.
Is formed so as to also serve as the lower electrode 15 and the signal line 6 to the transfer thin film transistor, and is connected to each of the transfer transistors T _{11 to} T _MN in FIG.

4 and 5 are for explaining the structure of another embodiment of the sensor portion of the image sensor for realizing the equivalent circuit of FIG. 1. FIG. 4 is a plan view and FIG. FIG. 5 is a sectional view taken along the line AA of FIG. In the figure, the same parts as those in FIGS. 8 and 9 are designated by the same reference numerals, and the description thereof will be omitted. In this embodiment, the drain electrode 8 of the transfer thin film transistor portion T
T and the drain electrode 8 of the reset thin film transistor section R
R is a common electrode, and the photoconductor layer 10 and the transparent electrode 11 are formed on the common electrode to form a photodiode portion P. Also in this embodiment, the transparent electrode 11 extends in the main scanning direction in common, as can be seen from FIG.
As shown in FIG. 1, they are connected to a common power supply terminal, for example, a potential of −5V. The source electrode 9R of the reset thin film transistor section R is connected to the Al electrode 12 through the contact hole 14 and is grounded to the ground potential. The source electrode 9T of the transfer thin film transistor section T is
From the signal line 6 to the signal lines S ₁ , S ₂ , ..., S _{N of FIG.}
Connected to each.

Although the photodiode portion is provided on the drain electrode in the above-mentioned embodiments, the drain electrode and the source electrode of the thin film transistor may be replaced with each other and provided on the source electrode.

[0021]

As is apparent from the above description, according to the present invention, the connection resistance between the photodiode and the reset thin film transistor can be reduced, and the image sensor can be miniaturized. This has the effect of facilitating downsizing of the image sensor and higher density of the two-dimensional image sensor.

[Brief description of drawings]

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

FIG. 2 is a plan view of an embodiment of a sensor unit.

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

FIG. 4 is a plan view of another embodiment of the sensor unit.

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

FIG. 6 is an equivalent circuit diagram of an example of a conventional image sensor.

FIG. 7 is an equivalent circuit diagram of a conventional example using a reset transistor.

FIG. 8 is a plan view of an example of the sensor unit of FIG.

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

[Explanation of symbols]

P _{11 to} P _MN photodiode, C _{11 to} C _MN photodiode part capacitance, T _{11 to} T _MN transfer transistor, C
_L1 -C _LN line capacitance, S ₁ to S _N signal lines, G ₁ ~G _M
Gate line, P photodiode section, T transfer thin film transistor section, R reset thin film transistor section,
1 insulating substrate, 2T, 2R gate electrode, 3 gate insulating film, 4 semiconductor layer, 5 ohmic contact layer,
6 signal lead wire, 7T, 7R top insulating layer, 8
T, 8R drain electrode, 9T, 9R source electrode, 1
0 photoconductor layer, 11 transparent electrode, 12 Al electrode, 1
3 polyimide layers, 14 contact holes, 15 lower electrodes.

Claims (2)

[Claims] 1. A photodiode array comprising a plurality of photodiodes as one block, a plurality of transfer thin film transistors for transferring signal charges generated in the photodiodes in each block, and the photodiode array. An image sensor having a reset thin film transistor for resetting residual charge of a diode, wherein a photodiode is formed on a source electrode or a drain electrode of the reset thin film transistor. 2. A plurality of photodiodes as one block, a photodiode array formed by the plurality of blocks, a plurality of transfer thin film transistors for transferring signal charges generated in the photodiodes in each block unit, and the photodiodes. An image sensor having a reset thin film transistor for resetting a residual charge of a diode, wherein the reset thin film transistor and the transfer thin film transistor have a common source electrode or drain electrode, and a photodiode is formed on the common source electrode or drain electrode. .