The present invention relates to an image sensor for use in facsimile systems and the like.
As an image sensor for use in facsimile systems and the like, an IC type image sensor has been known in which a MOS photodiode array, a CCD image sensor or the like is employed.
In addition to such IC type image sensor, there has been proposed a so-called contact type image sensor in which a plurality of photoelectric conversion elements are formed on an insulator substrate and a circuit for scanning these elements is provided either on the same substrate or outside of the substrate. In such contact type image sensor, the width of the array of the photoelectric conversion elements is same as that of a document to be read, and therefore the image sensor of this type can read the document with one picture element to one photoelectric conversion element correspondence by the use of an optical system composed of an optical fiber array, lens array or the like.
FIG. 1(a) is a circuit diagram of the aforesaid contact type image sensor, FIG. 1(b) is a fragmentary plan view showing the contact type image sensor of FIG. 1(a), and FIG. 1(c) is a sectional view taken along line A--A of FIG. 1(b). In the drawings a photoelectric conversion element 1 is composed of a photoelectric film. This element is equivalently represented by a photodiode PD and a condenser CD as shown in FIG. 1(a). A switching circuit 7 consists of MOS transistors 5 and a shift register 6. These MOS transistors are for switching the photoelectric conversion elements 1, and the shift register 6 is for scanning the MOS transistors 5. Furthermore, reference numeral 2 designates electrodes, 8 signal wire, 9 a load resistor, and 10 a power source for biasing respectively.
Referring to FIGS. 1(b) and (c), the photoelectric conversion element 1 is constructed in such a manner that a photoconductive film 3 is laminated on the separated electrodes 2 provided on a substrate 11, and an electrode 4 is further superposed thereon, in other words, the photoconductive film 3 is sandwiched between the separated electrodes 2 and the electrode 4.
The switching circuit 7 is constructed by integrating the MOS transistors 5 and the shift register 6 onto a semiconductor substrate, and the switching circuit 7 thus constructed is placed on the substrate 11 and connected to the photoelectric conversion elements 1 by lead wire in wire bonding or the like manner.
In such construction as mentioned above, when image on a document to be read is focused on the photoelectric conversion elements 1 by means of an optical system (not shown), photoelectric current corresponding to the light intensity is generated in the photodiodes PD, so that electric potentials of the electrodes 2 vary in accordance with the photoelectric current. These electric potentials are transmitted to the signal wire 8 through the MOS transistors which is in ON state, and as the result, they are taken out by the aid of the resistor 9 as picture signals. However, in such contact type image sensor the switching circuit 7 is formed with a substantially same width with that of the array of the photoelectric conversion elements 1. As a consequence, length of the signal wire 8 unfavorably becomes longer so that various noises are induced on the signal wire 8. Such noises principally include noise derived from a clock pulse for driving the shift register 6, spike noise generated at the time when a MOS transistor 5 is turned ON and OFF. These noises are a cause for deteriorating signal to noise ratio of the contact type image sensor and therefore serve as an obstacle to improvement of performance such as speeding up in reading speed of the image sensor.