A method of driving a thin film transistor type optical sensor and the drive unit

Abstract: The present invention relates to a method of driving a thin film transistor type optical sensor, having a gate electrode, a gate insulating layer, a thin film semiconductor layer, an ohmic contact layer, a source electrode and a drain electrode. The thin film transistor type optical sensor (S) is driven by providing the gate electrode (G) with the threshold voltage of a thin film transistor (T) adjacent to the thin film transistor type optical sensor (G) or a voltage based on this threshold voltage.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical sensor used for an image reader of an image processing device such as a facsimile apparatus and a digital copying machine and, more particularly to a method of driving a thin film transistor (hereinafter called "TFT") type optical sensor which is so constructed that a gate electrode is provided through an insulating layer for a thin film semiconductor layer, and the drive unit. Related Background Art As a construction example of the TFT type optical sensor, Fig. 1 shows a typical plan view, and Fig. 2 shows a sectional view along line X-X' in Fig. 1. In Figs. 1 and 2, numeral 1 denotes a substrate of glass, etc., numeral 2 denotes a gate electrode, numeral 3 denotes a gate insulating layer, numeral 4 denotes a photo conductive semiconductor thin film, and numeral 5 denotes an ohmic contact layer used to make an ohmic contact between a semiconductor thin film 4 and a source electrode 6 and a drain electrode 7. When such a TFT type optical sensor is normally used in charge accumulation mode as an image reader, a bias voltage VB is applied to the gate electrode as shown in Fig. 3 for stable operation. In Fig. 3, a sensor battery VS is connected to a drain electrode, and an accumulation capacitor C is connected to a source electrode. A charge stored in the accumulation capacitor C is discharged to a load resistance RL by a transfer switch SW. Even in this TFT type optical sensor, the electrical quality may change when the interface or the surface condition of the thin film semiconductor layer is affected by the film forming process or etching process in the production process. Also when it is produced on a large area of substrate, etc., the distribution in these processes may cause great distribution to the electrical quality. To give such an example, when a TFT type optical sensor has been formed at intervals of about 20 mm on a large area (320 x 256 mm) of glass substrate shown in Fig. 4, the light current Ip and dark current Id (when the voltage Vgs between the gate and the source = 0 V) of a TFT type optical sensor on the A - A' line in Fig. 4 are distributed as shown in Fig. 5, for example, and the S/N may also vary depending upon the location. Therefore even in a driving method in which a bias voltage is applied to the gate electrode for stable operation, there are the problems that the S/N varies and there is variation in the sensor output value. For this reason, a method to adjust the gate bias voltage in accordance with such an array position of TFT type optical sensor is used as shown in JP-A-63-092153. The equivalent circuit of this is shown in Fig. 6. In Fig. 6, Sl,l to Sm,n are TFT type optical sensors, CSl,l to CSm,n are accumulation capacitors, Ul,l to Um,n are transfer TFTs, and Vl,l to Vm,n are reset TFTs. The above-mentioned element group is divided into m blocks of n units, and is matrix connected to (m + 1) gate lines and n signal lines. A driver unit 11 applies a voltage to gate lines VGl to VGm+1 in order, a signal processing unit 12 takes out a signal voltage from signal lines SLl to SLn, and a control unit 13 applies a control voltage to sensor gate lines VBl to VBm. Also VS is a sensor bias voltage, VR is a reset voltage for the accumulation capacitor, and CLl to CLn are load capacitors. Although the above-mentioned problems can be almost solved by this system, new problems occur such as addition of out-going wiring for the gate bias and requiring an adjustable external power supply, preventing the cost of the image reader from being reduced. Further since the gate bias voltage value should be adjusted each time, it increases the production man-hours, hindering cost reduction. SUMMARY OF THE INVENTION The present invention provides a method of driving a TFT type optical sensor element as set out in claim 1, a drive circuit as set out in claim 3, an image processing device as set out in claim 5 and a photosensor as set out in claim 10. The remaining claims set out optional features. An embodiment of the present invention reduces the costs of an image reader and equipment, by adopting a driving method which does not require adjusting the gate bias voltage from outside, but allows the TFT type optical sensor to output at good S/N ratio and without variation in the sensor output value across the sensor area. In an embodiment of the invention, the thin film transistor type optical sensor is driven by setting the potential at the gate electrode of the thin film transistor type optical sensor to a threshold potential or a potential having a desired specified potential difference from the threshold potential. This enables variation in the sensor output to be reduced without requiring any external adjustment, and also enables the dark current to be reduced, the S/N ratio to be improved, and a low-priced, high performance image reader to be constructed. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a typical plan view showing a construction example of a known TFT type optical sensor. Fig. 2 is a section view on X-X' in Fig. 1. Fig. 3 shows a driving circuit for a known TFT type optical sensor. Fig. 4 is a typical plan view showing a device obtained by forming the TFT type optical sensor on a large area on a glass substrate. Fig. 5 shows the relationship between light current Ip and dark current Id of the TFT type optical sensor on the A-A' line of a device in Fig. 4. Fig. 6 shows an equivalent circuit for a known example of TFT type sensor. Fig. 7 shows the relationship between gate voltage Vgs and light current Ip of the TFT type optical sensor on the A-A' line in Fig. 4. Fig. 8 shows the relationship between gate voltage Vgs and dark current Id of the TFT type optical sensor on the A-A' line in Fig. 4. Fig. 9 shows the relationship between threshold potential Vth and dark current Id. Fig. 10 shows the relationship between threshold potential Vth and light current Ip. Fig. 11 shows a read-out circuit in accumulation mode using a method of driving a TFT type optical sensor of a preferred embodiment according to the present invention. Fig. 12 is a typical plan view showing a construction example of a preferred circuit embodying the present invention shown in Fig. 11. Fig. 13 is a section view on X-X' in Fig. 12. Fig. 14 is a circuit diagram for a gate bias voltage supply unit in which the gate electrode of a TFT is connected to the drain electrode. Fig. 15 shows the relationship between the bias voltage VBB and the voltage Vd of the drain electrode in the circuit in Fig. 14, and the threshold potential Vth. Fig. 16 is a equivalent circuit diagram in an embodiment using TFT R in which the gate electrodes are connected to the source electrode of TFT R instead of the bias resistance RB in the circuit in Fig. 11 embodying the present invention. Fig. 17 is an equivalent circuit diagram of a line sensor type image reader which has been so constructed that the driving circuit shown in Fig. 16 is allocated in an n-by-m array. Fig. 18 is a typical construction pattern diagram for one bit of the circuit in Fig. 17. Fig. 19 is a section view on X-X' in Fig. 18. Fig. 20 is a section view on Y-Y' in Fig. 18. Fig. 21 is a section view on Z-Z' in Fig. 18. Fig. 22 is a typical section view showing an example of a complete lensless contact sensor. Fig. 23 is a typical section view showing an example of a complete contact sensor with a lens. DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> An embodiment of the present invention will be described in detail referring to the drawings below. First, the cause will be described of the distribution shown in Fig. 5 of the light current Ip and dark current Id values in the TFT type optical sensor in a large area of glass substrate shown in Fig. 4. When the relationship between the gate voltage Vgs and light current Ip and the relationship between the gate voltage Vgs and dark current Id have been investigated in the TFT type optical sensor at the points 1 ○, 2 ○ and 3 ○ (which correspond to the positions of 1 ○, 2 ○ and 3 ○ in Fig. 5 respectively) on the line A-A' in Fig. 4, such characteristics as shown in Figs. 7 and 8 have been obtained. When the gate voltage is less than a certain value as shown in Figs. 7 and 8, both light current and dark current show a small value because of an OFF condition in which no TFT channel is formed. When the gate voltage exceeds a certain value, a channel is formed and the TFT is on, and then a normal TFT switching characteristic, in which a large current flows, has been obtained. A gate voltage, at which this state changes from OFF to ON, is called "Threshold potential" hereafter. The relationship between the threshold potentials at the positions 1 ○, 2 ○ and 3 ○ of this TFT type optical sensor is 1 ○ < 2 ○ < 3 ○ for both light current and dark current. However, the slope in the current-voltage characteristic in OFF state and that in ON state are almost the same irrespective of the positions of 1 ○, 2 ○ and 3 ○. That is, the relationship between the gate voltage Vgs of the TFT type optical sensor and current Ip,Id varies between the positions 1 ○, 2 ○ and 3 ○. only in that the respective threshold potentials have shifted. As a result, it has become evident that there is a correlation between the threshold potential of dark current Id and light current Ip as shown in Figs. 9 and 10. From the foregoing, it is considered that such a distribution of characteristics as shown in Fig. 5 occurs because the actual operating points differ across the sensor since the threshold potentials differ while the TFT type optical sensor operates under the condition that Vgs = constant. It has been found from numerous experiments that it is possible to make the distribution of characteristics more uniform by driving the sensor so as to correct for the shift in the threshold potential. This correction method will be described. Fig. 11 shows an example of a read-out circuit in accumulation mode using a method of driving a TFT type optical sensor embodying the present invention. This circuit consist of a TFT type optical sensor and a portion, which supplies a gate bias voltage. A source bias battery VS is connected to the source electrode of the TFT type optical sensor S, and an accumulation capacitor C is connected to the drain electrode. The accumulation capacitor C is connected to a load resistance RL and sensor battery VD through a transfer switch SW. The gate electrode of the TFT sensor S is connected to the gate electrode of TFT T for supplying a gate bias voltage. Values of the sensor battery VD and source bias battery VS are so set that VD > VS. In a gate bias voltage supply unit, one end of a bias resistance RB is connected to a bias battery VBB, and the other end is connected to the drain electrode of the bias TFT T. Further the drain electrode of the TFT T is also connected to the gate electrode, and the source electrode is grounded. The operation of the gate bias voltage supply unit for this circuit is shown in Figs. 14 and 15. In a state (so-called diode connection), in which the TFT gate electrode is connected to the drain electrode as shown in Fig. 14, by properly setting the bias resistance RB value, the voltage VD of the drain electrode changes almost at an inclination of 1 to the bias voltage VBB up to the TFT threshold potential Vth as shown in Fig. 15, and thereafter the VD value becomes almost Vth value irrespective of the VBB value because TFT shifts to ON state. That is, the TFT drain electrode becomes almost a potential equivalent to Vth by setting the bias resistance RB value properly and setting the bias battery VBB value to a value higher than the TFT threshold potential Vth value in the circuit in Fig. 14. By supplying the potential occurring in the above-mentioned gate bias voltage supply unit to the TFT type optical sensor as the gate potential as shown in Fig. 11, the voltage Vgs between the gate and source of the TFT type optical sensor will be set to Vgs = Vth - VS. Although this threshold potential Vth is Vth of the bias TFT, the threshold potential can be expected to be almost equal to the threshold potential of the TFT type optical sensor because it is easy to locate the bias TFT very near the TFT type optical sensor. That is, the operating point of the voltage Vgs between gate and source of the TFT type optical sensor is always set to a value only VS lower than its own Vth. By this driving method, the shift of the threshold potential Vth is corrected in the relationship between gate voltage Vgs and light current Ip, and in the relationship between gate voltage Vgs and dark current Id as shown in Figs. 7 and 8. As a result, the distribution of light current Ip and dark current Id has been improved as compared with the distribution in the case of ordinary driving shown in Fig. 5. As a construction example of a circuit embodying the present invention shown in Fig. 11, Fig. 12 shows a typical plan view, and Fig. 13 shows a typical section view on X-X' in Fig. 12. In Figs. 12 and 13, S is a TFT type optical sensor, T is a bias TFT, and RB is a bias resistance. Numeral 1 is a substrate of glass, etc., numerals 2, 3, 4, 5, 6, and 7 are a gate electrode, a gate insulating layer, a photo conductive semiconductor thin film, an ohmic contact layer, a drain electrode, and a source electrode of the TFT type optical sensor respectively, and numerals 8 and 9 are bias TFT source, and drain electrode respectively. Numeral 10 is a bias resistance, and 11 is a bias battery wiring. In the embodiment according to the present invention, the circuit is composed of a gate electrode layer comprising the gate electrode for TFT type optical sensor, the gate electrode for bias TFT, and a conductive layer with the afore-mentioned bias resistance. Fig. 16 shows an equivalent circuit in an embodiment using a TFT R, in which the gate electrode is connected to the source electrode, instead of the bias resistance RB in Fig. 11. The TFT R is always turned off as it is an enhancement type TFT in which the threshold potential is positive. When the resistance R OFF in the OFF state is set to the desired bias resistance RB value by appropriately designing the channel width and channel length of the TFT R, the same operation as the gate bias voltage supply unit in Fig. 11 can be performed. <Second Embodiment> Fig. 17 shows an equivalent circuit of a line sensor type image reader which has been constructed in an n-by-m array using the driving circuit shown in Fig. 16. In Fig. 17, Sl,l to Sm,n are TFT type optical sensors, CSl,l to CSm,n are accumulation capacitors, Ul,l to Um,n are transfer TFTs, and Vl,l to Vm,n are reset TFTs. The above-mentioned element group is divided into m blocks of n units, and is matrix connected to (m + 1) gate lines and n of signal lines. Tl,l to Tm,n are bias TFTs, and Rl,l to Rm,n are bias loading TFTs. A driver unit 11 applies a voltage to gate lines VGl to VGm+1 in order, and a signal processing unit 12 takes out a signal voltage from signal lines SLl to SLn. Also VS is a sensor bias voltage, VR is a reset voltage for the storage capacitor, VBB is a bias voltage, and CLl to CLm are readout capacitors. In this circuit, the gate electrodes of the transfer TFTs U of a block are connected in common to a gate line and to the gate electrodes of the reset TFTs V for the previous block in common. By shifting the voltage pulse of the driver unit 11, the previous block can be reset at the same time as a signal of the next block is transferred. The above-mentioned circuit can be all constructed on the same substrate. Especially by using an a-Si:H film as the photo conductive semiconductor material, the construction can be performed in a simultaneous process by means of the following lamination construction: a lower electrode layer for a TFT type optical sensor, an accumulation capacitor, a transfer TFT, a reset TFT, a bias TFT, a loading TFT, wiring, etc., an SiNH insulating layer, an a-Si:H layer and n layer for the upper electrode. A pattern example of an image reader using this type of process will be described. Fig. 18 shows a construction pattern diagram for one bit of the circuit in Fig. 17. However, only the upper and lower electrode patterns and contact holesare shown to avoid the complexity of the figure. In Fig. 18, numeral 14 is a signal line matrix, numeral 15 is a TFT type optical sensor, numeral 16 is an accumulation capacitor, numeral 17 is a transfer TFT, numeral 18 is a reset TFT, numeral 19 is a wiring area for gate driving lines, numeral 20 is a bias TFT, numeral 21 is a contact hole for connecting a gate and a drain, and numeral 22 is a bias loading TFT. Although read-out capacitors CL to CLn are not shown in Fig. 18, their capacitance is set to 10 to several hundreds times the stray capacitance between signal lines which occur in a signal line matrix 14. The image may be, of course, read out in the form of a direct current using a load resistance as shown in the above-mentioned embodiment without using the stray capacitance. Fig. 19 shows a section view on X-X' in Fig. 18. Fig. 20 shows a section view on Y-Y' in Fig. 18. Fig. 21 shows a section view on Z-Z' in Fig. 18. In Figs. 19, 20 and 21, numeral 1 is a substrate of glass, etc., and numeral 30 is a lower electrode, which is a gate electrode of the TFT type optical sensor in Fig. 19, a gate electrode of the reset TFT in Fig. 20, and a gate electrode of the bias and bias loading TFTs in Fig. 21. As shown in Fig. 22 in this example, an abrasion resistant layer 24 is formed above the TFT type optical sensor, and a window 23 is provided to illuminate an original 26 by a light source 25 through the rear surface of the sensor, and further the gate electrode of the TFT type optical sensor is made of an opaque material in order to enable complete close-contact reading without a lens. This can be also used for a close-contact read type image reader using such an equal magnification image-formation lens 27 as shown in Fig. 23. In the above-mentioned example, a bias TFT is allocated for each bit of the TFT type optical sensor to correct the variation in the threshold potential at each bit of the TFT type optical sensor. When, however, this threshold voltage varies more slowly across the array of the TFT type optical sensor, it is, of course, possible to correct in units of blocks by dividing the TFT type optical sensor into blocks and allocating a bias TFT for each block. Since it is possible to correct the lowered S/N of the TFT type optical sensor and the sensor output distribution without requiring any external adjustment according to the embodiments described above, a low-priced, high performance image reader can be constructed.

Claims: A method of driving a thin film transistor type optical sensor element, having on a substrate (1), a gate electrode (2), a gate insulating layer (3), a thin film semiconductor layer (4), a source electrode (7), a drain electrode (6), and an ohmic contact layer (5) between said thin film semiconductor layer and said source and drain electrodes, wherein said thin film transistor type optical sensor element is driven by providing said gate electrode with the threshold voltage of a thin film transistor (T) provided on said substrate adjacent to said thin film transistor type optical sensor element (S). The method of driving a thin film transistor type optical sensor element (S) according to claim 1, wherein a predetermined voltage is applied to said source electrode. A drive circuit of a thin film transistor type optical sensor element (S) having on a substrate (1) a gate electrode (2), a gate insulating layer (3), a thin film semiconductor layer (4), a source electrode (7), a drain electrode (6), and an ohmic contact layer (5) between said thin film semiconductor layer and said source and drain electrodes, wherein the voltage at said gate electrode is set to a predetermined value, the drive circuit comprising a thin film transistor (T) provided on said substrate adjacent to said thin film transistor type optical sensor element (S), the drain electrode (9) of said thin film transistor being electrically connected to the gate electrode of said thin film transistor to provide the threshold voltage of said thin film transistor as the voltage at said gate electrode of the thin film transistor type optical sensor element. A drive circuit according to claim 3 including a voltage source (V BB ) electrically connected via a bias resistance to the drain electrode of the thin film transistor. An image processing device, comprising on a substrate (1): a plurality of thin film transistor type optical sensor elements (S) each having a gate electrode, a gate insulating layer, a thin film semiconductor layer, a source electrode, a drain electrode and an ohmic contact layer between said thin film semiconductor layer and said source and drain electrodes; and a plurality of thin film transistors (T) each being provided on said substrate adjacent to a corresponding one of said thin film transistor type optical sensor elements, wherein the drain electrode (9) and the gate electrode (2) of each thin film transistor (T) are electrically connected to the gate electrode (2) of said corresponding thin film transistor type optical sensor element (S) and a voltage source is electrically connected via a bias resistance (10) to the drain electrode (9) of said thin film transistor to provide the threshold voltage of said thin film transistor as the voltage at said gate electrode of said corresponding thin film transistor type optical sensor element. The image processing device according to claim 5 wherein the source electrodes (8) of said thin film transistors are grounded. The image processing device according to claim 5 wherein a desired voltage is applied to the source electrodes (7) of said thin film transistor type optical sensor elements. The image processing device according to claim 5 wherein a capacitor is electrically connected to the drain electrode (6) of each of said thin film transistor type optical sensor elements to store charge in proportion to light incident on the corresponding sensor element. The image processing device according to claim 5 wherein a plurality of second thin film transistors are provided on a said substrate and the source electrode and the gate electrode of each of said second thin film transistors are electrically connected to the drain electrode and the gate electrode of a corresponding one of said thin film transistors to form said bias resistance. A photosensor comprising: an optical sensor element (S) of thin film transistor type comprising, on a substrate (1), a gate electrode (2), a gate insulating layer (3), a thin film semiconductor layer (4), a source electrode (7), a drain electrode (6), and an ohmic contact layer (5) between the source and drain electrodes and the thin film semiconductor layer (4); and a thin film transistor (T) comprising, on said substrate (1), a gate electrode (2), a gate insulating layer (3), a thin film semiconductor layer (4), a source electrode (8), a drain electrode (9), and an ohmic contact layer (5) between the source and drain electrodes and the thin film semiconductor layer; wherein said optical sensor element (S) and said thin film transistor (T) are adjacent each other, and the gate electrode of said optical sensor element and the gate and drain electrodes of said thin film transistor are electrically connected together to provide the gate electrode of the optical sensor element (S) with the threshold voltage of the thin film transistor (T). A photosensor according to claim 10, wherein the gate insulating layer of said optical sensor element (S) and the gate insulating layer of said thin film transistor (T) are a continuous common layer (3), the thin film semiconductor layer of said optical sensor element (S) and the thin film semiconductor layer of the thin film transistor (T) are a continuous common layer (4), and the gate electrode of said optical sensor element (S) and the gate electrode of the thin film transistor (T) are a continuous common layer (2). A photosensor according to claim 10, wherein the thin film semiconductor layer (4) is amorphous silicon. A photosensor according to claim 10, wherein the gate insulating layer (3) is an SiNH insulating layer.

Inventors: ENDO TADAO, KAIFU NORIYUKI, KOBAYASHI ISAO, SAIKA TOSHIHIRO, ENDO, TADAO, KAIFU, NORIYUKI, KOBAYASHI, ISAO, SAIKA, TOSHIHIRO, ENDO, TADAO, C/O CANON KABUSHIKI KAISHA, KAIFU, NORIYUKI, C/O CANON KABUSHIKI KAISHA, KOBAYASHI, ISAO, C/O CANON KABUSHIKI KAISHA, SAIKA, TOSHIHIRO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L27
IPC codes: H01L29, H04N1, H04N3, H01L27