1. Field of the Invention
This invention generally relates to an image sensing device for converting light image information into electrical signals, and, in particular, to such an image sensing device which may be advantageously used as an image sensor of a facsimile machine and the like for reading the document information to be transmitted to a remote place. The present invention also relates to photoelectric elements which are particularly suited for use in an image sensing device and the method of manufacturing such photoelectric elements.
2. Description of the Prior Art
As is well known in the art, an image sensing device usually includes a plurality of light-sensitive or photoelectric elements arranged in the form of a single array, and the plurality of photoelectric elements are activated in timed sequence from one end of the array to the other in repetition thereby scanning a document to be read along scanning line sectors in a stepwise manner. In such an image sensing device, it is common practice to divide the plurality of photoelectric elements into a predetermined number of blocks and to provide a common electrode for each block thereby having one end of each of the photoelectric elements belonging to the same block connected to the corresponding common electrode mainly for simplification of wiring. With such a structure, it is true that wiring mav be simplified, but each of the photoelectric elements looses an operative independency so that the image sensing device would suffer from stray signals unless an adequate measure is taken
Such being the case, when it is so structured to group a plurality of light-sensitive elements into blocks, each of which includes a predetermined number of light-sensitive elements, then it is necessary to apply means for preventing the occurrence of stray signals. FIG. 1 illustrates one prior art approach for preventing the occurrence of strav signals in an image sensing device, which uses a plurality of blocking diodes one for each photoelectric element As illustrated, a plurality of photoelectric elements P.sub.ij (i=1-n and j=1-m) are disposed in the form of a single array, and these elements are grouped into n blocks B.sub.1, B.sub.2, . . . , B.sub.n, each comprising m photoelectric elements. In each of the blocks, the top ends of the photoelectric elements are commonly connected to the corresponding common block terminal. For example, in the leftmost block, the top ends of the photoelectric elements P.sub.11, P.sub.12, . . . , P.sub.1m are commonly connected to the corresponding block terminal B.sub.1.
On the other hand, the bottom end of each of the photoelectric elements defines an individual electrode which is connected to the anode of the corresponding blocking diode D.sub.ij. Each of the blocking diodes in one block, for example the diode D.sub.11 has its cathode interconnected to the cathodes of the corresponding blocking diodes, D.sub.21, . . . , D.sub.n1 for D.sub.11, in the other blocks through respective interconnection lines. These interconnection lines are connected to ground through respective load resistors R.sub.L and also to an output terminal V.sub.0 through analog switch S.sub.1, S.sub.2, . . . , S.sub.m, respectively. Provision of blocking diodes in this manner allows to establish operative independency for each of the photoelectric elements; however, it still suffers from various disadvantages since it is necessary to provide a relatively large number (m.times.n) of diodes corresponding in number to the photoelectric elements and the manufacture of such large number of diodes at a time tends to be expensive even if use is made of the thin film forming technology and moreover it tends to lower the yield.
FIG. 2 shows another prior art approach in which operational amplifiers are provided and their virtual ground is utilized to establish operative independency between the photoelectric elements. Similarly with the structure of FIG. 1, the top ends of the elements in each block in the arrangement of FIG. 2 are commonly connected, and the individual electrodes defined by the bottom ends of the photoelectric elements in one block are interconnected to the corresponding individual electrodes in the other blocks through the respective interconnections which are also connected to the inverting input of the respective operational amplifiers A.sub.1, A.sub.2, . . . , A.sub.m. These op amps have their outputs connected to the output terminal V.sub.0 through respective analog switches S.sub.1, S.sub.2, . . . , S.sub.m and their non-inverting inputs connected to ground. A resistor R.sub.f is connected between the output and the inverting input in each of the op amps thereby defining a feed-back loop. In such a structure, the inverting inputs of the op amps are, in effect, at virtual ground, so that the elements P.sub.ij may be operated independently one from another. However, the structure shown in FIG. 2 requires the provision of so many op amps corresponding to the number of the individual electrodes (m in the illustrated example), which also tends to push up the manufacturing cost partly because of difficulty in mounting of so many op amps.
A further prior art approach is illustrated in FIG. 3, in which case, use is made of a mxn bit shift register for directly driving an array of photoelectric elements. In this case, however, using a mxn bit shift register requires mxn number of connections to be made, which also pushes up the cost. Such a disadvantage can not be obviated even if the shift register is constructed in the form of an IC.