Image sensor

In the image sensor, thin film transistors are provided in the one-to-one correspondence to the photoelectric conversion element these thin film transistors being divided into blocks of adjoining ones whose gate electrodes are mutually connected, and the switching signal is sequentially supplied to each mutually connected gate electrodes of the same block. Further, the output electrodes of those thin film transistors located in the corresponding positions in each odd and even numbered blocks are mutually connected, and the charge transfer circuits comprising the MOS transistors are provided corresponding to respective common connection line groups of odd numbered block and even numbered block. By this construction, at the time of picking up the outputs of the photoelectric conversion elements, the switching period for the thin film transistors can be retained as long as the original reading time of a plurality of the photoelectric conversion elements corresponding to a plurality of the thin film transistors whose gate electrodes are mutually connected, and therefore the speed of outputting read signal is determined soley by the MOS transistor switching speed. In addition, the number of the connection lines required may be as small as the number of the common connection lines.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of image sensors in use for 
facsimile or the like equipment, and in paticular, to the field of 
elongated image sensors now under development instead of IC type image 
sensors such as MOS photodiode arrays or CCD image sensors. 
2. Description of the Prior Art 
An elongated image sensor is so constructed that a photoelectric conversion 
array comprising a plurality of photoelectric conversion elements and a 
circuit for sequentially switching those elements are formed on a single 
insulative substrate, and is designed such that the width of the 
photoelectric conversion element array is sized the same as that of an 
original. The image on the original is read in the one-to-one image 
forming manner by an optical system such as an optical fiber array or a 
lens array. As a result, the optical path length for focusing the image 
can be shortened, thus enabling the reduction in size of an image reading 
apparatus. 
FIG. 1 shows a circuit diagram of a prior art elongated image sensor, in 
which the image sensor 1 comprises photoelectric conversion elements 10 of 
a photoconductive film respectively represented equivalently by a 
photodiode PD and a capacitor C, switch elements 20 for picking up a 
signal generated at the photo-electric conversion element 10, capacitance 
30 comprised of a parasitic capacitance formed by stray capacitance of the 
wiring connecting the photoelectric conversion element 10 and the switch 
element 20 and an input capacitance of the switch element 20, a charge 
transfer circuit 40, and a load resistor 50. Reference numeral 60 
indicates a power supply for biasing. The charge transfer circuit 40 is 
comprised of charge storage capacitance 41, charge transfer switch 
elements 42, and clock lines 43 and 44. 
When an image on the original is focused on the photoelectric conversion 
element 10, a photocurrent is generated at the photodiode PD corresponding 
to the light intensity, and a signal charge is thus stored in capacitance 
30. When the switch element 20 is closed by applying a suitable signal to 
a terminal T1, the signal charge stored in the capacitance 30 is fed to 
the charge storage capacitance 41. Then as the charge transfer switch 
elements 42 are sequentially actuated by an alternating voltage applied 
via terminals T2 and T3 to the clock lines 43 and 44, the signal charge 
stored in the charge storage capacitance 41 is shifted to the right-hand 
direction of FIG. 1, eventually applied to the load resistor 50, and 
outputted through an output terminal T4 as a voltage signal. 
As the switch element 20 and the charge transfer switch element 42, MOS 
transistors produced by ordinary IC technology may be used. The charge 
transfer circuit 4 of this case is BBD (bucket brigade device). 
A MOS transistor is suitable as a switch element of the image sensor due to 
its features of comparatively high switching speed and the capability of 
being operated at a low voltage. For realizing the above-mentioned 
elongated image sensor, it is necessary to mount the switch elements 10 
and an IC chip on which the switch elements 20 and the charge transfer 
circuit 40 are formed on a single substrate and further to connect the 
above two with each other by such means as wire bonding, causing the 
increase in the number of connection lines for each image sensor. This 
fact is a serious problem in producing highly reliable devices at low 
cost. 
As a means for solving the aforementioned problem, it has been proposed 
that the switch element 20 and the charge transfer circuit 40 be formed of 
thin film transistors employing semiconductor film. According to this 
method, since the switch element 20 and the charge transfer circuit 40 are 
formed by the same film forming process as that for the photoelectric 
conversion element 10, the above-mentioned connection related problem can 
be solved favorably. 
However, because of slow switching speed compared with the MOS transistor 
and high voltage required for the actuation of switching operation, the 
thin film transistor is not satisfactory as a switching element to be 
applied to such an image sensor, leaving the method imperfect for 
practical application. 
SUMMARY OF THE INVENTION 
The present invention, taking the situation given above well into 
consideration, is directed to provide an image sensor capable of high 
reliability, high speed original reading through ingenious utilization of 
the advantages of the thin film transistors and those of the MOS 
transistors. 
More particularly, in the image sensor of the present invention, the thin 
film transistors are provided in the one-to-one correspondence to the 
photoelectric conversion element, these thin film transistors being 
divided into blocks of adjoining ones whose gate electrodes are mutually 
connected, and the switching signal is sequentially supplied to each of 
the mutually connected gate electrodes of the same block. Further, the 
output electrodes of those thin film transistors located in the 
corresponding positions in each of the odd and even numbered blocks are 
mutually connected, and the charge transfer circuits comprising the MOS 
transistors are provided corresponding to respective common connection 
line groups of odd numbered block and even numbered block. By this 
construction, at the time of picking up the outputs of the photoelectric 
conversion elements, the switching period for the film transistors can be 
retained as long as the original reading time of a plurality of the 
photoelectric conversion elements corresponding to a plurality of the thin 
film transistors whose gate electrodes are mutually connected, and 
therefore the speed of outputting read signal is determined soley by the 
MOS transistor switching speed. In addition, the number of the connection 
lines required may be as small as the number of the common connection 
lines, thus making it possible to achieve the above-mentioned object of 
the present invention. 
In sum, the image sensor of the present invention has the following 
features: 
(1) Capable of stable, high speed reading of image by compensating 
unsatisfactory characteristics of the thin film resistor. 
(2) Enabling the reduction of the number of connection lines required, 
whereby high operational reliability and low production cost are realized.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 shows an example of circuit configuration of an embodiment of the 
image sensor of the present invention, in which for the sake of easy 
explanation, an image sensor 1 has twelve photoelectric conversion 
elements and the same or corresponding reference numerals and symbols as 
in FIG. 1 are assigned to the elements or circuits having the same 
functions as those of FIG. 1. 
Referring to FIG. 2, photoelectric conversion elements 10a to 10l are 
connected in one-to-one correspondence to switch elements 20a to 20l 
formed of film transistors. These switch elements 20a-20l are divided into 
four blocks (20a-20c, 20d-20f, 20g-20i, 20j-20l), gate electrodes of the 
switch elements in the same block are mutually connected and then 
connected to the output lines 71-74 of a shift register 70 respectively. 
The output electrodes of the switch elements 20a-20c and 20g-20i which are 
in the odd numbered blocks are connected to common connection lines 81-83 
respectively, while the output electrodes of the switch elements 20d-20f 
and 20j-20l which are in the even numbered blocks are connected to common 
connection lines 84-86 respectively. The other ends of these common 
connection lines 81-83 and the common connection lines 84-86 are connected 
to charge transfer circuits 40A and 40B respectively. 
The device of this embodiment operates basically in such manner that the 
signal charges stored in capacitance 30a-30l are transferred to charge 
storage capacitance 41a-41c or 41f-41h of the charge transfer circuits 40A 
and 40B respectively by the actuation (ON) of the switch elements 20a-20l, 
and signals are outputted from output terminals T4A and T4B by driving the 
charge transfer circuits 40A and 40B. In the charge transfer circuits 40A 
and 40B, MOS transistors are used as charge transfer switch elements 
42a-42j. 
The above operation will now be described in detail with reference to the 
timing chart shown in FIG. 3. The operation given below is that to be 
performed after the signal charge is stored in the capacities 30a-30l. 
Firstly, a switching signal S1 is applied to the output line 71 by 
actuating a shift register 70 (refer to FIG. 3(a)). As a result, the 
switch elements 20a-20c become ON, and charge accumulated in the 
capacitance 30a-30c are transferred to the charge storage capacitance 
41a-41c through the common connection lines 81-83. It is necessary that 
the time duration t of the switching signal S1 is sufficiently long for 
the charge to be transferred. The capacity of the charge storage 
capacitance 41a-41j is sufficiently large compared with that of the 
capacitance 30a-30l. Accordingly, after the charge transfer, almost all 
signal charge can be transferred to the capacitance 41a-41c. Then, the 
switch elements 20d-20f are turned on by applying a switching signal S2 to 
the output line 72 of the shift register 70 (refer to FIG. 3(b)), and the 
signal charge accumulated in the capacitance 30d-30f are transferred to 
the charge storage capacitance 41f-41h. Simultaneously, clock signals S5 
and S6 are supplied from terminals T2A and T2B to clock lines 43A and 44A 
(refer to FIG. 3(e) and (f)) causing the charge transfer circuit 41A to 
operate, and the charge accumulated at the capacitance 41a-41c are 
transferred to the output terminal T4A. That is, the charge transfer 
switch elements 42a-42c formed of MOS transistors are turned on at the 
rise of the clock signal S5 at the time t1, the charges in the capacitance 
41b and 41c are transferred to the capacitance 41d and 4e, the charge in 
the capacitance 41a is discharged, and a signal S9 at the time t1 is 
output from the output terminal T4A. (Refer to FIG. 3(i)). Then, the 
charge transfer switch elements 42d and 42e are turned on at the rise of 
the clock signal S6 at the time t2, the charges in the capacitance 41d and 
41e are transferred to the capacitance 41a and 41b, and a signal S9 at the 
time t3 is output from the output terminal T4A by the rise at the time t3 
of the clock signal S5. In the same manner, an output signal S9 at the 
time t5 may be obtained. Then, when the charge transfer circuit 40B is 
operated similar to the above output signals S10 at times t6-t8 are 
obtained (refer to FIG. 3(j)), and all signals are read out by the 
repetition of the similar operation. 
As seen in the above, with the device of this embodiment, the operation 
period t of the switch element 20a-20l may be lengthened sufficiently, 
therefore, unfavorable characteristics of the film transistor such as slow 
switching speed and high ON resistance can be compensated. In addition, 
since MOS transistors whose operation speed is high are employed in the 
charge transfer circuits 40A and 40B, eventual signal outputting to 
external circuits can be made quickly and all bits can be read within a 
short period of time. In addition, Since the switch elements 20a-20l and 
the common connection lines 81-86 can be formed on the same substrate 
using the same film as that for the photoelectric conversion element 10 or 
through the thick film forming process, aforementioned process for 
connecting with the photoelectric conversion element in one-to-one 
correspondence is not required. Although connection between the switch 
elements 20a-20l and the output lines 71-74 of the shift register 70 and 
between the common connection lines 81-86 and the charge transfer circuits 
40A and 40B should be made by wire bonding as so far been practiced, the 
number of such connections is very small and therefore the above-mentioned 
problem is unlikely to occur. For example, when the number of the 
photoelectric conversion elements 10 is 2048 and the photoelectric 
conversion elements 10 are divided to 32 blocks by common connecting 
adjoining 64 gates of switch elements 20, the number of the connection 
lines becomes 128, i.e., about 1/20 of the prior art device shown in FIG. 
1. 
With reference to FIGS. 4(a) and 4(b) which show the construction of the 
embodiment device described above, the procedure for fabricating the image 
sensor will be described in which FIG. 4(b) shows a section taken along 
line A--A' of FIG. 4(a). 
For this device, glass, ceramics, or surface glazed ceramics each having an 
insulative property is used typically as a substrate 101. Firstly, a metal 
such as chrome (Cr), gold (Au), aluminum (Al) or nickel (Ni) is deposited 
in a layer of the thickness about 1000.ANG. to 5000.ANG. on the substrate 
typically by the electron beam evaporation method, and then a given 
pattern is formed by photo lithography, thus electrodes 102-104 are 
obtained. For an better adhesion onto the substrate 101, it is preferred 
to form the electrode using chrome (Cr). Then, a photoconductive film 105 
and a semiconductor film 106 are formed depositing a semiconductor film of 
amorphous silicon, polycrystalline silicon, selenium (se)-tellurium (Te), 
selenium (Se)-arsenic (As), selenium (Se)-arsenic (As)-tellurium (Te), 
cadmium sulfide (CdS), Cadmium selenide (CdSe), zinc (Zn)-Selenium (Se), 
or zinc (Zn)-Cadmium (Cd)-tellurium (Te) on the electrodes 103 and 104 by 
mask evaporation or other suitable method. For this device, it is 
desirable to form the photoconductive film 105 and the semiconductor film 
106 using 0.1 to 1 .mu.m thick amorphous silicon provided by glow 
discharge of silane gas (SiH4). Thereafter, a film of ITO (In2O3, SnO2) 
1500.ANG. thick is deposited as a transparent conductive film 107 by the 
DC spatter or any other suitable method, and is formed into a given shape 
typically by mask evaporation or photolithography. Thereby the 
photoelectric conversion element 10 (refer to FIG. 2) is formed at a 
portion where the electrode 105 and the transparent conductive film 107 
overlap. An insulation film 108 typically of silicon dioxide (SiO2), 
silicon tetranitride (Si3N4) or glass is formed typically by evaporation 
photolithography or thick film printing so that through holes are formed 
to enable multilayer wiring at a given part. Further, by forming a given 
pattern of thin or thick film typically of chrome (Cr), gold (Au), 
aluminum (Al) or nickel(Ni) thereon, a gate electrode 109 and common 
wirings 110 are provided. As a result, a film transistor is formed under 
the gate electrode 109. To the end of the gate electrode 109 and to the 
ends of the common wirings 110 the shift register 70 and the charge 
transfer circuit 40 are connected respectively, thus completing the image 
sensor of the present invention. 
It is apparent that though BBD is used for the embodiment shown in FIG. 2 
as the charge transfer circuit 40, CCD (charge coupled device) may 
likewise be employed. 
In addition, though as a method for connecting the shift register 70 and 
the charge transfer circuit 40, wire bonding alone has been shown, other 
methods such as soldering connection and tape carrier pressure adhesion 
(but not limited to) may be applied to this invention.