Photoelectric converter provided with voltage dividing means

A photoelectric converter comprising a photoelectric conversion unit which includes a pair of main electrodes, spaced by a photoreception area over a semiconductor layer, and an auxiliary electrode. The semiconductor layer and the auxiliary electrode are laminated through the intermediary of an insulating layer in at least the photoreception area. It also includes a storage capacitor for storing electric charges flowing through the photoelectric conversion unit, a transfer transistor for transferring the charges stored in the capacitor, a discharge transistor for discharging the charges stored in the storage capacitor, and dividing device for dividing a switching voltage applied to the control electrode of the discharge transistor to apply a divided voltage to the auxiliary electrode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a photoelectric converter used with a bar 
code reader, a facsimile apparatus, a digital copier or the like and more 
particularly to a photoelectric converter of the type having an auxiliary 
electrode or of the thin film transistor (hereinafter called TFT) type. A 
gate electrode is provided on a semiconductor layer with an insulating 
layer interposed therebetween. 
2. Related Backgound Art 
Recently, as electronic office machines such as facsimiles or digital 
copying machines become popular, the demand for a small type of 
inexpensive image input device has increased. Coplanar photoelectric 
converter (photosensor) which use a-Si, CdS-CdSe or the like as the 
photoconductors to utilize a photoelectric conversion effect have the 
advantage that they can directly contact an original document, and 
requiring no focusing system. Furthermore, they have a short travelling 
distance of the focusing system. In order to stabilize the sensor 
characteristics, field effect type photosensors are provided which have an 
insulating layer and an auxiliary electrode above or below a semiconductor 
layer among the coplanar type photosensors. 
FIGS. 1(a) and (b) are schematics of the photoelectric conversion unit of 
the photosensor. In FIG. 1(a), an auxiliary electrode and insulating layer 
are formed on an insulating substrate 1 of glass, ceramic or the like The 
auxiliary electrode 2 and the insulating layer 3 on which is formed a 
semiconductor layer 4 are made of CdS.Se, a-Si: H or the like. A pair of 
main electrodes 6 and 7 are formed on the substrate through the 
intermediary of a doped semiconductor layer 5 for ohmic contact. A 
photoreception window 8 is formed between the main electrodes. 
The photosensor having a structure shown in FIG. 1(b) has the auxiliary 
electrode 2 provided over the main electrodes 6 and 7. The substrate 1 is 
made of a transparent material and receives light from the side of the 
substrate 1. A part having the same function as the corresponding one of 
the photosensor of FIG. 1(a) is given the same number. 
In the photosensor having the above structure, in order to get a large 
ratio of the output current to the dark current in response to incident 
light, an appropriate bias must be applied to the auxiliary electrode 2 in 
accordance with the kind of majority carriers of a current flowing through 
the semiconductor layer 4 to operate the photosensor. Namely, a negative 
bias must be applied when the majority carriers are electrons while a 
positive bias must be applied when the majority carriers are holes. Such 
operation will reduce the output current, so that a capacitor which stores 
the output current must be provided in the circuit which reads the signal 
from the photosensor. Correspondingly, switching elements such as a TFT 
which transfers as a signal the electric charges stored in the capacitor 
and a TFT which discharges the remaining charges after the transfer 
operation, and a matrix circuit which connects these switching elements 
are required. 
FIG. 2 is a schematic equivalent circuit diagram of a plurality of 
line-sensor type photoelectric conversion elements disposed in an array. 
In FIG. 2, reference characters Rs1, Rs2, . . . Rsn denote photosensors 
having an auxiliary electrode; Cs1, Cs2 . . . Csn storage capacitors; Tt1, 
Tt2 ... Ttn TFTs which transfer electric charges stored in the capacitors 
Cs1-Csn; Tr1, Tr2 . . . Trn discharge TFTx which discharge electric 
charges remaining in the capacitors Cs1-Csn after the transfer operation. 
Reference characters Lg1-Lgn denote leads connected to the control 
electrodes or gates of the transfer TFTs Tt1-Ttn and the control 
electrodes or gates of the discharge TFTs Tr1-Trn. Reference characters 
Vg1-Vgn denote switching voltages applied to the corresponding leads 
Lg1-Lgn. Reference character LD denotes a lead connected to the main 
electrodes of the photosensors Rs1-Rsn. Reference character VD denotes a 
voltage applied to LD. Reference character LR denotes a lead connected to 
the storage capacitors Cs1-Csn. Reference character VR denotes reference 
voltage for storage capacitors Cs1-Csn. If the storage capacitors Cs1-Csn 
are discharged via discharge TFTs Tr1-Trn, the voltages across the 
capacitors Cs1-Csn will become VR. 
In the photosensor, if a steady state bias is applied to the auxiliary 
electrode in accordance with the kind of majority carriers in the 
photosensor, namely, if a negative bias is applied when the majority 
carriers are electrons and if a positive bias is applied when the majority 
carriers are holes, the minority carriers will collect in the vicinity of 
the auxiliary electrode in the semiconductor layer and the majority 
carriers will collect in that portion of the semiconductor layer opposite 
the auxiliary electrode. In such condition, the majority and minority 
carriers are not re-combined smoothly, so that even if irradiation light 
to the photosensor may be cut off, the remaining photocurrent will flow as 
long as the minority carriers continue to exist. As a result, the optical 
response speed and hence the S/N ratio of the photosensor are lowered. 
In an attempt to cope with this, for example, as shown in FIG. 3, if a bias 
voltage V1 of a predetermined level, negative when the majority carriers 
are electrons and positive when the majority carriers are holes, is 
applied in advance to the auxiliary electrode when the photosensor is to 
be read and if a pulse voltage of V2 opposite in polarity to the bias 
voltage is applied to the auxiliary electrode during a non-reading 
interval provided immediately before reading, a rise in the optical 
response would be improved to provide a large ratio of the output current 
to the dark current in response to incident light. 
However, if such operation is tried on, for example, an image reading 
apparatus including a multiplicity of one-dimensionally arranged such 
photosensors, the respective sensor bits are read in a time series, so 
that the timings of applying the respective pulse voltages are shifted bit 
by bit. Thus, if voltages applied to the auxiliary electrodes of the 
photosensors are controlled in a circuit separated from the circuit in 
which the gate voltages of the transfer and discharge TFTs are controlled, 
a matrix circuit for control of the auxiliary electrodes of the 
photosensors and a matrix circuit for control of gate electrodes of the 
transfer TFTs would be required. Thus the entire circuit would become 
complicated. In addition, the timings of applying pulses, pulse widths, 
pulse magnitudes, etc., must be determined individually, so that the 
driving would become complicated. 
Next, an example of the structure of a photoelectric conversion section of 
a photosensor of TFT type is shown in FIGS. 4 and 5. FIG. 4 is a plan view 
of the photoelectric conversion section, and FIG. 5 is a cross section 
taken along line X--X' of FIG. 4. In the Figures, the photoelectric 
conversion section comprises a substrate 401 made of glass for example, a 
gate electrode 402, an insulating layer 403, a photoconductive 
semiconductor layer 404, source and drain electrodes 406 and 407, an 
n.sup.+ layer for ohmic contact between the semiconductor layer 404 and 
the source and drain electrodes 406 and 407. 
Dark current of a TFT type photosensor can be controlled by applying a bias 
voltage to the gate electrode to suppress the effect of the surface of the 
insulating layer. Thus, a favorable light quantity dependence 
characteristic (hereinafter called .lambda.) of a photoelectric conversion 
output is obtained which is as near as 1. The reproductiveness is also 
satisfactory with little manufacture deviation within a lot and among 
lots. 
These characteristics lead to advantageous effect under static (DC) drive 
conditions. However, under dynamic drive conditions commonly employed for 
image sensors or the like, i.e., under a charge storage mode, there may 
arise a problem which is described hereinafter. 
FIG. 6 is a circuit diagram of a readout circuit using a TFT type 
photosensor under a charge storage mode. Connected to a drain electrode is 
a sensor power supply V.sub.S and to a gate electrode a bias power supply 
V.sub.B. Connected to a source electrode is a storage capacitor C. The 
charge stored in the storage capacitor is discharged to a load resistor 
R.sub.L upon activation of a transfer switch SW. 
Operating waveforms of the circuit are shown in FIG. 7. The transfer switch 
SW repeatedly turned on and off at the period of a storage time T.sub.S. 
Namely, while the transfer switch SW. is being turned off, photocurrent 
i.sub.S of the photosensor is charged in the storage capacitor. Whereas 
while the transfer switch SW is made turned on, the charge stored in the 
storage capacitor C is caused to be discharged to the load resistor 
R.sub.L which in turn is read as an output of the photosensor. 
Voltage V.sub.C appearing across the storage capacitor C can be represented 
in terms of an integrated value of i.sub.S under the condition of V.sub.S 
&gt;&gt;V.sub.C :V.sub.C =.intg..sub.o.sup.t i.sub.S dt=i.sub.S xt . Thus, the 
voltage V.sub.C increases substantially linearly with respect to time t. 
This increase in voltage is shown by a broken line in FIG. 7. 
However, in practice, when the photosensor in the circuit of FIG. 6 is 
driven a distored waveform of the voltage V.sub.C as shown by a solid line 
in FIG. 7 is obtained. The reason for this is that the voltage V.sub.C 
quickly changes to zero when the transfer switch turns on and a gate bias 
voltage .DELTA. Vgs is made relatively small so that transient current 
i.sub.a (hatched portion in FIG. 7) flows through the source and drain. 
This transient current adversely effects the light dependence 
characteristic of an output from a photoelectric converter constructed of 
the above circuit, to thereby lower the gamma .lambda. value to 0.4 to 
0.5as shown in FIG. 8, which is considerably small as compared to the 
gamma .lambda. value 1 calculated based on the static drive conditions, 
and thereby deteriorates an S/N ratio. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a photosensor 
apparatus which provides an improved optical response speed and has a 
simple structure. 
It is another object of the present invention to provide a photoelectric 
converter capable of eliminating the above-mentioned problem associated 
with a dynamic operation and positively utilizing the characteristic 
performance of a TFT type photosensor. 
It is a further object of the present invention to propose a drive circuit 
which can be easily fabricated on a same substrate as that of a 
photosensor and provide a photoelectric converter of low cost and high 
yield by positively utilizing the characteristic performance of a TFT type 
photosensor having a high S/N ratio and low manufacture deviation. 
It is another object of the present invention to provide a photoelectric 
converter of the type that the photoelectric conversion section is 
constructed of a photoconductive layer, first and second electrodes formed 
on a same plane and facing to the photoconductive layer, and a third 
electrode formed on the photoconductive layer with an insulating layer 
interposed therebetween and that a supply voltage is applied to the first 
electrode to obtain a photoelectric conversion output from the second 
electrode, wherein the second electrode is connected commonly to the third 
electrode. 
It is a further object of the present invention to provide a photoelectric 
converter of the type that the photoelectric conversion section is 
constructed of a photoconductive layer, first and second electrodes facing 
to the photoconductive layer, and a third electrode formed on the 
photoconductive layer with an insulating layer interposed therebetween and 
that a supply voltage is applied to the first electrode to obtain a 
photoelectric conversion output from the second electrode, wherein a 
capacitor is formed between the second electrode and the third electrode. 
It is a still further object of the present invention to provide a 
photoelectric converter which comprises a photoelectric conversion section 
constructed of a photoconductive layer, first and second electrodes facing 
to the photoconductive layer, and a third electrode formed on the 
photoconductive layer with an insulating layer interposed therebetween; a 
capacitor formed between the second and third electrodes; a storage 
capacitor electrically connected to the second electrode; first switch 
means electrically connected to the second electrode for discharging the 
storage capacitor; gate bias switch means electrically connected to the 
third electrode and actuated in cooperation with first switch means; and 
second switch means electrically connected to the second electrode for 
charge transfer. 
It is another object of this invention to provide a photoelectric converter 
comprising a photoelectric conversion unit which includes a pair of main 
electrodes, spaced by a photoreception area over a semi-conductor layer, 
and an auxiliary electrode laminated with the semiconductor layer through 
the intermediary of an insulating layer in at least the photoreception 
area, a storage capacitor for storing electric charges flowing through the 
photoelectric conversion unit, a transfer transistor for transferring the 
charges stored in the capacitor, and a discharge transistor for 
discharging the charges stored in the storage capacitor, and dividing 
means for dividing a switching voltage applied to the control electrode of 
the discharge transistor to apply a divided voltage to the auxiliary 
electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now an embodiment of this invention will now be described in detail with 
reference to the drawings. 
Embodiment 1 
FIG. 9 is a circuit diagram of one embodiment of a photoelectric conversion 
apparatus according to this invention. FIG. 10 is a timing chart for 
explaining the operation of the apparatus. In FIG. 9, as in FIG. 2, 
reference characters Rs1, Rs2 . . . Rsn denote photosensors (photoelectric 
conversion units) having an auxiliary electrode; Cs1, Cs2 ... Csn storage 
capacitors; Tt1, Tt2, Ttn transfer TFTs for transferring the charges 
stored in the capacitors Cs1-Csn; Tr1, Tr2 . . . Trn discharge TFTs for 
discharging the charges remaining in the capacitors Cs1-Csn after the 
transfer operation; and Cx1, Cx2 . . . Cxn dividing capacitors for 
applying divided voltages to the auxiliary electrodes of photosensors 
Rs1-Rsn. Reference characters Cgs1, Cgs2, . . . Cgsn denotes capacitances 
fromed between the auxiliary electrodes and the corresponding main 
electrodes on the connection side of the storage capacitors in the 
photosensors Rs1-Rsn. Reference characters Lg1-Lgn denote leads connected 
to the control electrodes or gates of the transfer TFTs Tt1-Ttn and the 
control electrodes or gates of the discharge TFTs Tr1-Trn, and the 
dividing capacitors Cx1-Cxn. Reference characters Vg1-Vgn denote switching 
voltages applied to the corresponding leads. Reference character LD 
denotes a lead connected to the main electrodes of the photosensors 
Rs1-Rs2, and VD a voltage applied to that lead. Reference character LR 
denotes a lead connected to the storage capacitors Cs1-Csn, and VR a 
reference voltage for the storage capacitors Cs1-Csn. When the storage 
capacitors Cs1-Csn are discharged via the discharge TFTs Tr1-Trn, the 
voltages across the storage capacitors Cs1-Csn all become VR. The leads 
Lg1-Lgn, LD, and LR constitute a matrix circuit. 
The operation of the photoelectric conversion apparatus will now be 
described. Assume that the majority carriers in the photosensors are 
electrons. 
In FIG. 9, if positive and negative voltages VD and VR are applied to the 
leads LD and LR, respectively, currents will flow through the photosensors 
Rs1-Rsn in accordance with a quantity of irradiated light, so that 
electric charges are stored in the storage capacitors Cs1-Csn to raise the 
main electrode side potentials of the storage capacitors Cs1-Csn in the 
photosensors Rs1-Rsn. 
If switching voltage Vgi-Vgn are applied at the timing shown in FIG. 10, to 
the gates of the transfer and discharge TFTs Tt1-Ttn and Tr1-Trn via the 
matrix circuit constituted by the respective leads in FIG. 9, electric 
charges will be stored in the storage capacitors Cs1-Csn during the time 
interval T11, and the transfer TFT Tt1 will be turned on during the time 
interval T12 to thereby cause the electric charges in the storage 
capacitor Cs1 to be transferred as a signal. The discharge TFT Tr1 is 
turned on during the time interval T13 to thereby cause the storage 
capacitor Cs1 which has completed the transfer operation to be 
short-circuited to thereby discharge the remaining charges. Thus one cycle 
of reading the photosensor Rs1 is completed. At the same time, the 
transfer TFT Tt2 is turned on and the charges which are being stored in 
the storage capacitor Cs2 are transferred as a signal during the time 
duration T21. A reading cycle similar to that effected in the photosensor 
Rs1 is effected successively in the subsequent photosensor bits. 
In the circuit shown in FIG. 9, reading, transfer and discharge are 
performed as described above, and, simultaneously, a voltage in phase with 
the switching voltage applied to the gates of the discharge TFTs Tr1-Trn 
is applied to the auxiliary electrodes of the photosensors Rs1-Rsn to 
improve their optical response speed. In more detail, the photosensor Rs1 
is especially considered as an example. A divided voltage between the 
switching voltage Vg2, applied to the discharge TFT Tr1, and the reference 
voltage VR obtained by the dividing operation of the dividing capacitor 
Cx1, the capacitance Cgs1 formed between the auxiliary electrode and the 
main electrode (on the side of the storage capacitor Cs1) of the 
photosensor Rs1, and the storage capacitor Cs1 is applied to the auxiliary 
electrode of Rs1. In the photosensor Rs1, electrons are drawn in by a 
pulse voltage applied during the discharge interval T13 during which a 
large current flows, the re-combination of holes is expedited at the same 
time, the optical information remaining in the photosensor Rs1 before the 
discharge interval is cancelled, and especially improves a fall in the 
optical response during the reading duration T11 to thereby provide a 
large ratio of the output current to the dark current in response to 
incident light. On the other hand, while a large current flows through the 
photosensor Rs1 during the discharge duration T13, electric charges are 
not stored in the storage capacitor Cs1 because the storage capacitor Cs1 
is in its discharge state, so that there is no probability that 
unnecessary information is mixed. Further, in the charge storage type 
reading circuit, the potential of the auxiliary electrode of the 
photosensor changes relative to that of the main electrode on the signal 
reading side as electric charges are stored in the storage capacitor, so 
that as shown in FIG. 11, the output current changes. In contrast, in the 
circuit of this invention, fluctuations in the relative potential of the 
auxilairy electrode are suppressed to within the ratio in capacitance 
valued of Cgs1 to Cx1 due to the above dividing effect, so that 
fluctuations in the output current are suppressed to thereby provide 
stabilized driving of the apparatus as shown in FIG. 12. 
An image reading device using the above circuit structure will now be 
described. FIG. 13 is a schematic view of the photosensor device. Like 
reference characters are used to denote like parts in FIGS. 13 and 9. FIG. 
14 is a schematic view of the image reading device using the photosensor 
device. In FIG. 13, photosensor Rs1 is constituted by an amorphous silicon 
hydride semiconductor layer 4 (not shown in FIG. 13) as the 
photoconductive layer, insulating layer 3 (also not shown in FIG. 13), 
main electrodes 6 and 7, and auxiliary electrode 2. A lead 9 comprising 
lead Lg2, insulating layer 3 and electrode 12 constitute dividing 
capacitor Cx1. Electrodes 13 and 14 and insulating layer 3 consitute the 
storage capacitor Cs1. Semiconductor layer 4, insulating layer 3 and 
electrodes 16 and 17 and 19 constitute discharge TFT Tr1. The 
semiconductor layer 4, insulating layer 3 and electrodes 15, 16 and 18 
constitute transfer TFT Tt1. These elements are formed on a transparent 
insulating substrate. 
The photosensor device is provided in the image reading device, as shown in 
FIG. 14, for example. Shown in FIG. 14, an incident, light window is 
provided in a mount 24 so that the photosensor device is irradiated with 
light from a light source 23 through the window. Provided on the mount 24 
are the photosensor device 22 and an IC 25 which processes the optical 
signal from the photosensor device 22. An original document 21 fed by a 
paper feed roller 20 is irradiated with the light passing through the 
window and photosensor device 22. The light reflected from the document is 
sensed by a photosensor of the photosensor device 22. The image reading 
device having the above structure is able to read the document at high 
speed to provide an image of high quality. 
Embodiment 2 
FIGS. 15 and 16 are circuit diagrams showing other embodiments of the 
photosensor device according to this invention. Like reference characters 
are used to denote like parts in FIGS. 9, 15 and 16 and further 
description of those parts and the operation thereof will be omitted. In 
FIG. 15, the first bit is especially considered as an example as in the 
previous embodiment. A voltage obtained by dividing the switching voltage 
Vg2, applied to the discharge TFT Tr1, by dividing capacitors Cx11 and 
Cx12 with the VR as the reference voltage is applied to the auxiliary 
electrode of the photosensor Rs1. A pulse voltage is applied to the 
auxilairy electrode immediately before the reading interval as in the 
previous embodiment to especially improve a fall in the optical response 
speed of the photosensor Rs1 to thereby provide a large ratio of the 
output current to the dark current in response to incident light. Since in 
this embodiment the potential of the auxiliary electrode is fixed during 
the reading interval. however, fluctuations in the relative potential of 
the signal outputting side one of the main electrodes cannot be 
compensated. While in the particular embodiment the switching voltage Vg2 
of the discharge TFT Tr1 is divided by the capacitors to be applied to the 
auxiliary electrode of photosensor Rs1, the dividing manner is not limited 
to the capacitor division. Alternatively, for example, as shown in FIG. 
16, resistance division which uses dividing resistors Rx11 and Rx12 may be 
used. When these embodiments are applied to the image reading device of 
FIG. 14 as in the previous embodiment, high quality image reading is 
performed at high-speed. 
As described above in detail, in the photosensor device of the above 
embodiment, a fall in the optical response speed of the photosensor is 
greatly improved and a high ratio of the output current to be dark current 
in response to incident light is provided. 
By forming the dividing means with the storage capacitor, the capacitance 
between the auxiliary electrode and the storage capacitor connection side 
one of the main electrodes of the photosensor, and the dividing capacitor 
connected between the control electrode of the discharge transistor and 
the auxilairy electrode, or by forming the dividing means with series 
connected capacitors or series connected resistors connected across the 
control electrode of the discharge transistor and the storage capacitor, 
fluctuations in the potential of the auxiliary electrode, occurring when 
electric charges are stored in the storage capacitor, relative to that of 
the main electrode from which the signal of the photosensor is taken are 
reduced and hence fluctuations in the output current are suppressed to 
within a small limit. 
The image reading device using the photosensor device according to this 
invention is able to read a document at high speed with high quality. 
Embodiment 3 
An equivalent circuit of a third embodiment of the photoelectric converter 
according to the present invention is shown in FIG. 17. 
A sensor electrode V.sub.S is connected to a drain electrode D, while a 
gate bias electrode V.sub.B is connected to a gate electrode G via a 
resistor R. A bias capacitor Cgs is connected between the gate electrode G 
and a source electorde S. Connected to the source electrode S are a 
transfer switch SW and a load resistor R.sub.L. 
The operation of the photoelectric converter constructed as shown in FIG. 
17 will be described. 
The gate bias power supply V.sub.B charges the bias capacitor Cgs through 
the resistor R to thereby make the gate electrode potential at V.sub.B. 
When the transfer switch SW turns off from its on-state, photocurrent 
i.sub.S starts charging a storage capacitor C.sub.S. As the storage 
capacitor C.sub.S is charged, the potential V.sub.C thereof rises. 
Assuming that the time constant R x Cgs&gt;&gt;T.sub.S (which is an ON/OFF 
period of the transfer switch), the gate voltage V.sub.G follows the 
potential V.sub.C of the storage capacitor C.sub.S. 
Namely, the gate-source voltage .DELTA.V.sub.GS =V.sub.G -V.sub.C becomes 
constant which is maintained nearly constant irrespective of the ON/OFF 
operation of the transfer switch. Thus, transient current as described 
with the conventional problem does not flow. 
FIG. 18 shows waveforms at various circuit points of the photoelectric 
converter. 
An effective .DELTA.V.sub.GS becomes equal to (-V.sub.B)-V.sub.C during 
operation wherein V.sub.C is a mean value of V.sub.C..DELTA.V.sub.GS 
varies with V.sub.C but it does not change transiently so that there 
occurs no problem. 
Embodiment 4 
A fourth embodiment of the present invention is shown in FIG. 19. 
In the circuit shown in FIG. 19, in addition to a transfer switch sw2, a 
reset switch sw1-a is provided which serves to discharge the residual 
charge after charge transfer in a storage capacitor C.sub.S. Connected to 
the gate electrode of a photoelectric conversion section is a gate bias 
switch sw1-b which is activated in cooperation with the switch sw1-a. 
The operating timings are shown in FIG. 20. 
When the switches sw1-a and sw1-b are turned on, the charge in the storage 
capacitor C.sub.S is discharged to thereby cause V.sub.C to become equal 
to 0. At the same time, the bias capacitor Cgs is charged to V.sub.B. When 
the switches are turned off, photocurrent produced at the photoelectric 
conversion section charges the storage capacitor C.sub.S. As it is 
charged, the potential V.sub.C of the storage capacitor V.sub.C rises. On 
the other hand, since the switch sw1-b is maintained turned off, there is 
no current path for the bias capacitor Cgs. 
As a result, the potential V.sub.G at the gate electrode G operates to 
follow the potential V.sub.C of the storage capacitor C.sub.S while 
maintaining .DELTA.V.sub.GS constant. 
The transfer switch sw2 becomes turned on after a storage time T.sub.S. At 
this time, the charge voltage V.sub.C of the storage capacitor C.sub.S is 
discharged via the load resistor R.sub.L. In the circuit shown in FIG. 19, 
however, since the storage capacitor C.sub.S is discharged via the 
discharge switch sw1-a, it is not necessary to completely effect the 
discharge through the load resistor R.sub.L. In other words, there occurs 
no problem even if the voltage V.sub.C across the storage capacitor 
C.sub.S directly read out is used. In this embodiment, the potential Vgs 
between the gate electrode G and the source electrode S is maintained 
constant (-V.sub.B). Therefore, it is possible to set the gate-source bias 
of the photoelectric conversion section at an optimum value under any 
conditions of incident light quantity, storage time and so on. Further, 
transient current to be caused by a change in the potential .DELTA.Vgs 
will not be produced. Furthermore, the effect of the surface of the 
insulating layer is suppressed to large extent. 
FIG. 21 is an equivalent circuit of a line sensor type photoelectric 
converter composed of n x m photoelectric converters of FIG. 19 disposed 
in array. 
In the Figure, S1 to Sn+m denote TFT type photoelectric conversion 
sections, Cgs1 to Cgsn+m denote gate bias capacitors, R1 to Rn+m denote 
gate bias TFTs, Csl to Csn+m denote storage capacitors, U1 to Un+m denote 
reset TFTs, and Tl to Tn+m denote transfer TFTs. 
The line sensor type photoelectric converter is divided into m blocks each 
having n elements, m block being matrix connected to m +1 gate lines and n 
signal lines. In the Figure, reference numeral 2111 represents a driver 
section for sequentially applying voltage to gate lines V.sub.Gl `to 
V.sub.Gm+1, 2121 a signal processing section for picking up signal voltage 
on signal lines S1 to Sn. V.sub.S denotes a sensor bias, V.sub.R denotes a 
reset voltage for the storage capacitor. 
In the photoelectric converter, the gate electrodes of a reset TFT U and a 
gate bias TFT R are connected in common to the gate electrode of a next 
block transfer TFT T. Simultaneously when a next block signal is 
transferred upon shifting of voltage pulses at the driver section 2111, 
the preceding block is reset. 
The circuit shown in FIG. 21 can be fabricated in a same substrate. More 
particularly, by using as the photoconductive semiconductor material an 
a-Si:H film (amorphous silicon hydride film) formed by glow discharge, the 
TFT photoelectric conversion section, storage and bias capacitors, 
transfer, reset and bias TFTs, wiring leads and so on can be realized by 
simultaneous processes using a laminated structure of a lower electrode, 
an SiNH insulating layer, an a-Si:H layer, an n.sup.+ layer, and an upper 
electrode. The photoelectric converter of this invention is suitable for 
application to a line sensor type photoelectric converter made through 
simultaneous processes on a same substrate. 
An example of patterning a line sensor type photoelectric converter made 
through simultaneous processes of the above type will be described. 
FIG. 22 shows a patterning of one bit of the circuit shown in FIG. 21, 
wherein only the upper and lower wiring leads and contact hole portions 
are shown in order not to make the drawing complicated. In the Figure, 
2213 represents a signal matrix section, 2214 a photoelectric conversion 
section, 2215 a gate bias capacitor, 2216 a storage capacitor, 2217 a gate 
bias TFT, 2218 a transfer TFT, 2219 a reset TFT and 2220 a gate drive 
wiring section. In this example, a so-called lens-less structure is 
employed wherein an original is read by directly contacting it on a sensor 
section without using a focusing lens. Therefore, a window 2221 for 
applying light on an original is provided, and in addition the lower 
electrode of the photoelectric conversion section is made of transparent 
material which electrode serves also as a light shielding film. The 
transfer and reset TFTs are disposed in mirror symmetrical relation to 
each other. The reason for this is that in case the alignment for the 
upper and lower electrodes is displaced in the longitudinal direction of 
the substrate, the gate-soruce capacitance of a pair of TFTs is 
compensated and maintained unchanged. 
A changed in gate-source capacitance in the longitudinal direction results 
in the offset components of a signal output. With the above-described 
patterning, however, it is possible to remove the offset components. The 
load capacitor C.sub.Li (i=1 to n) in the equivalent circuit of FIG. 21 is 
not shown in FIG. 22. The value of the load capacitor is set at a value 
ten to several hundred times as large as the stray capacitance between 
signal lines S1 to Sn present at an signal matrix section 2213. It is also 
possible to directly read a signal output in the form of current without 
using the load capacitor. 
FIG. 23 is a cross section taken along line X--X'of FIG. 22, and FIG. 24 is 
a cross section taken alone line Y--Y' of FIG. 22. In the Figures, 2301 
represents a substrate made of glass for example, 2302 represents a lower 
electrode which corresponds to a lower electrode of a capacitor in FIG. 23 
and a gate electrode of a TFT in FIG. 24. An insulating film 2320 is made 
of SiNxH, SiO.sub.2 or the like. A photoconductive semiconductor layer 
2304 is made of a-Si:H or the like. An n.sup.+ layer 2305 is used for 
ohmic contact with the upper electrode. 2306 and 2307 represent upper 
electrodes which correspond to an upper electrode of a capacitor in FIG. 
23 and source and drain electrodes of a TFT in FIG. 24. 
According to the embodiments 3 and 4 described so far, a capacitor is 
formed between the gate and source electrodes of a TFT serving as the 
photoelectric conversion section so that the potential between gate and 
source electrodes can be always maintained constant. As a result, various 
advantageous effects can be obtained some of which are: 
(1) Transient current does not flow during a storage operation. Thus, the 
gamma .lambda. value becomes near 1 and a same S/N ratio as with a static 
operation can be obtained. 
(2) Since a stable negative bias can be applied between the gate and source 
of a photosensor, the effect of the surface of the insulating layer on the 
side of the photosensor can be eliminated, to thereby realize a 
photoelectric converter with little manufacture deviation among lots and 
good reproductiveness and productivity. 
(3) A TFT photosensor, gate bias TFT, transfer and reset TFTs, storage 
capacitor and the like can be fabricated using simultaneous processes on a 
same substrate, to thereby realize a line sensor type photoelectric 
converter having a high performance and of low cost. 
The above-mentioned first and second electrodes are preferably formed on a 
same plane on the photoconductive layer. 
Further, the bias voltage is applied to the third electrode via a resistor 
or a switch. 
Embodiment 5 
An equivalent circuit of a fifth embodiment of the photoelectric converter 
according to the present invention is shown in FIG. 5. 
A sensor power supply V.sub.S is connected to a drain electrode D, and a 
gate electrode is connected in common to a source electrode S. Connected 
also to the source electrode A are a storage capacitor C and a load 
resistor R.sub.L via a transfer switch SW. 
Next, the operation of the photoelectric converter whose equivalent circuit 
is shown in FIG. 25 will be described. 
When the transfer switch turns off from its on-state, photocurrent i.sub.S 
flows into the storage capacitor C to start a charge operation. As the 
charge operation is performed, the voltage V.sub.C of the storage 
capacitor C rises. 
On the other hand, since the gate and source electrodes are connected 
together, the gate-source voltage V.sub.GS is always maintained at 0 
potential. Therefore, transient photocurrent described before to be caused 
while the transfer switch SW is turned on does not flow. 
Embodiment 6 
FIG. 26 is an equivalent circuit of a line sensor type photoelectric 
converter composed of n x m photoelectric converters of FIG. 25 disposed 
in array. 
In the Figure, S1 to Snxm denote TFT type photoelectric conversion 
sections, C.sub.S1 to C.sub.Snxm denote storage capacitors, U1 to Unxm 
denote rest TFTs, and T1 to Tnxm denote transfer TFTs. 
The line sensor type photoelectric converter is divided into m blocks each 
having n element, m blocks being matrix connected to m +1 gate lines and n 
signal lines. In the Figure, reference numeral 2611 represents a driver 
section for sequentially applying voltage to gate lines V.sub.G1 to 
V.sub.Gm+1, 2612 a signal processing section for picking up signal voltage 
on signal lines S1 to Sn. V.sub.S denotes a sensor bias, V.sub.R denotes a 
reset voltage for the storage capacitor, and C.sub.L1 to C.sub.Ln denote 
load capacitors. 
In the circuit, a rest TFT U is provided which can fully rest the residual 
charge after the charge transfer into the storage capacitor C.sub.S. The 
gage electrode of a rest TFT U is connected together to a gate electrode 
of a next block transfer TFT T. Simultaneously when a next block signal is 
transferred upon shifting of voltage pulses at the driver section 2611, 
the preceding block can be reset. 
The circuit shown in FIG. 26 can be fabricated in a same substrate. More 
particularly, by using as the photoconductive semiconductor material an 
a-Si:H film formed by flow discharge, the TFT photoelectric conversion 
section, storage capacitor, transfer and reset TFTs, wiring leads and so 
on can be realized by simultaneous processes using a laminated structure 
of a lower electrode, an SiNH insulating layer, an a-Si:H layer, an 
n.sup.+ layer, and an upper electrode. The photoelectric converter of this 
invention is suitable for application to a line sensor type photoelectric 
converter made through simultaneous processes on a same substrate. An 
example of patterning a line sensor type photoelectric converter made 
through simultaneous processes of the above type will be described 
hereinbelow. 
FIG. 27 shows a patterning of one bit of the circuit shown in FIG. 26, 
wherein only the upper and lower wiring leads and contact hole portions 
are shown in order not to make the drawing complicated. In the Figure, 
2713 represents a signal matrix section, 2714 a photoelectric conversion 
section, 2715 a contact hole for gate and source, 2716 a storage 
capacitor, 2717 a transfer TFT, 2718 a reset TFT, 2719 a wiring lead for a 
gate drive line. In this example, a so-called lens-less structure is 
employed wherein an original is read by directly contacting it on a sensor 
section without using a focussing lens. Therefore, a window 2720 for 
applying light on an original is provided, and in addition the lower 
electrode of the photoelectric conversion section is made of transparent 
material which electrode serves also as a light shielding film. The 
transfer and reset TFTs 2717 and 2718 are disposed in mirror symmetrical 
relation to each other. The reason for this is that in case the alignment 
for the upper and lower electrodes is displaced in the longitudinal 
direction of the substrate, the gate-source capacitance of a pair of TFTs 
is compensated and maintained unchanged. A change in gate-source 
capacitance in the longitudinal direction results in the offset components 
of a signal output. With the above-described patterning, however, it is 
possible to remove the offset components. The load capacitor C.sub.Li (i=1 
to n) in the equivalent circuit is not shown in FIG. 27. The value of the 
load capacitor is set at a value ten to several hundred times as large as 
the stray capacitance between signal lines S1 to Sn present at the signal 
matrix section 2713. It is also possible to use a load resistor as in the 
preceding embodiment and directly read a signal output in the form of 
current without using the load capacitor. 
FIG. 28 is a corss section taken along line X--X' of FIG. 27, and FIG. 29 
is a cross section taken along line Y--Y' of FIG. 27. In the Figures, 2801 
represents a substrate made of glass for example, 2821 represents a lower 
electrode which corresponds to a gate electrode of the photosensor in FIG. 
28 and a gate electrode of a TFT in FIG. 29. An insulating film 2803 is 
made of SiNxH, SiO.sub.2 or the like. A photoconductive semiconductor 
layer 2804 is made of a-Si:H or the like. An n.sup.+ layer 2805 is used 
for ohmic contact with the upper electrode. 2806 and 2807 represent upper 
electrodes which correspond to a source electrode of the photosensor in 
FIG. 28 and source and drain electrodes of a TFT in FIG. 29. 
According to the embodiments 5 and 6 described so far, the gate and source 
electrodes are connected in common to each other. Therefore, the following 
and other effects can be obtained: 
(1) Transient current does not flow during a storage operation. Thus, tha 
gamma .lambda. value becomes 1 and a same and high S/N ratio as with a 
static operation can be obtained. 
(2) A TFT photosensor, transfer and reset TFTs, storage capacitor and the 
like can be fabricated using simultaneous processes on a same substrate, 
to thereby realize a line sensor type photoelectric converter having a 
high performance and of low cost. 
As described above, in the photoelectric converter according to this 
invention, dividing means for dividing the switching voltage applied to 
the control electrode of the discharge transistor to apply the resulting 
voltage to the auxiliary electrode is provided, so that a bias voltage is 
applied to the auxiliary electrode during the non-reading time interval of 
the photosensor, and a pulse voltage opposite in polarity to the bias 
voltage is added to the same to thereby eliminate the previous remaining 
read output in preparation for the next reading process. 
Further, according to the photoelectric converter of this invention, since 
a negative bias can be applied to the third electrode, i.e., the gate 
electrode, the above-mentioned transient current can be eliminated and the 
light dependence characteristic .lambda. can be made as near as 1, to 
thereby realize a photoelectric converter with a high S/N ratio and a good 
reproductiveness. 
Furthermore, the photoelectric conversion section as well as the other 
elements can be fabricated on a same substrate using simultaneous 
processes, to thereby realize a line sensor type photoelectric converter 
with a high performance and of low cost. 
Also, according to the photoelectric converter of this invention, since 
there is no transient current as discussed before, it is possible to have 
a gamma .lambda. value of approximately 1, a high S/N ratio, and an 
excellent reproductiveness. Further, a drive circuit can be fabricated on 
a same substrate as of the photoelectric conversion section at a same 
time, to thereby realize a photoelectric converter with a high performance 
and of low cost.