Abstract:
A photo diode array having improved linearity, wherein a plurality of photo diodes are arranged in an array with a plurality of electrodes connected to both ends of the light receiving surface of each photo diode to obtain current generated in each photo diode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a photo diode array; and more particularly, to such photo diode array having improved linearity. 
     2. Description of the Prior Art 
     A photo diode is formed, for example, by providing a PN junction with a p-type region on the light receiving surface side and an n-type region on the substrate side. A current generated by light applied to the photosensitive area flows from a cathode to an anode. 
     FIG. 1 is a sectional view of an exemplary conventional photo diode, wherein a p-type diffused region  2  is formed on one side of n-type layer  1  which is a substrate; n-contact  5  is formed on the opposite side of n-type layer  1 ; and passivation layer  3  is formed on the light receiving surface side, where p-type diffused region  2  is formed. A p-contact  4 , connected to p-type diffused region  2 , is formed by removing part of passivation layer  3  using an etching process, or the like. 
     The photo diode of FIG. 1 is operated as follows. Electrons, generated in the photo diode, by application of input light  100  to the photo sensitive area, are accumulated in n-type layer  1 , while holes are accumulated in p-type diffused region  2 . Accordingly, n-type layer  1  is negatively charged and p-type diffused region  2  is positively charged. Thus, when an external circuit is connected between p-type diffused region  2  and n-type layer  1 , electrons and holes flow toward opposite layers from n-type layer  1  and p-type diffused region  2 , respectively. 
     For example, an electron generated in p-type diffused region  2 , shown as “EL01” in FIG. 1 is accelerated towards n-type layer  1  by the electric field in the photo diode and accumulated there. In a similar manner, a hole generated in n-type layer  1 , shown as “HL01”, is accelerated towards p-type diffused region  2  and accumulated there. A photo diode array may comprise a plurality of such photo diodes arranged in an array. 
     FIG. 2 shows an example of a configuration of a plurality of photo diodes arranged in an array, wherein a plurality of photo diodes  6   a-   6   e  are arranged in the array on a substrate (not shown, for sake of convenience, but understood to be present. Same comments apply with the remainder of the drawing, namely, FIGS. 3-5) so that the photo sensitive areas thereof are oriented in a particular direction and electrodes  7   a-   7   e  are connected to one end of the photo sensitive area of each photo diode  6   a-   6   e , respectively. 
     Operation of the FIG. 2 array is as follows. When light is made incident to the photo sensitive area of photo diodes  6   a-   6   e,  currents are generated between each light receiving surface side and each opposite side. The currents are outputted to the outside via electrodes  7   a-   7   e,  respectively. For example, holes generated in photo diodes  6   a-   6   e  are outputted via electrodes  7   a-   7   e.    
     However, disadvantageously, in conventional arrays, current values outputted thereby may vary according to the location on the photo sensitive area at which the input light is applied. For example, when the input light is made incident to a portion-marked “P001”, the distance from that spot to electrode  7   e  is longer than when the input light is made incident to the portion marked “P002”. This causes a voltage drop due to the resistance that exists between the respective portions and the electrode. Hence, the current value which is outputted can vary. In other words, a problem exists in that in the prior art, linearity deteriorates depending on the location on the light sensitive area at which the input light is made incident. Specifically, when a high current is generated by the light being made incident to the spot marked “P001”, linearity deterioration increases, as compared to the light being made incident to spot “P002”,because the generated voltage drop increases further. 
     In order to reduce this linearity deterioration, a bias voltage which forms a reverse bias is generally applied to the photo diode. However, although this technique may improve frequency response and some amount of the linearity, in such a case, there is still the problem that this causes an increase in dark current which increases noise, and increases danger of destruction of the photo diode. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to overcome the foregoing and other deficiencies and disadvantages of the prior art. 
     Another object is to realize a photo diode array wherein linearity is greatly improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view depicting a conventional photo diode. 
     FIG. 2 is a diagram depicting arrangement of a conventional photodiode in an array. 
     FIG. 3 is a diagram of an illustrative embodiment of a photo diode array of the invention. 
     FIG. 4 is a diagram depicting another illustrative embodiment of the invention. 
     FIG. 5 is a diagram depicting a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 3, which shows an illustrative photo diode array of the invention, photo diodes  8   a-   8   e,  are arranged on a substrate (not shown) forming an array so that the photo sensitive areas thereof are oriented in the same predetermined direction. Electrodes  9   a-   9   e  are connected to one end of the photo sensitive areas of photo diodes  8   a-   8   e,  respectively, and also connected to the other end of the photo sensitive areas of photo diodes  8   a-   8   e,  respectively. The electrodes are extended so that the photo sensitive areas of two adjacent diodes are disposed therebetween, as shown. In addition, electrodes  9   a-   9   e  are connected to both ends of photo sensitive areas of photo diodes  8   a-   8   e,  respectively, and wired on the substrate in a direction downward, as depicted in FIG. 3, and led out to the outside. 
     Operation of the embodiment is as follows. Since the array of FIG. 3 is configured to obtain currents out of both ends of each photo sensitive area of photo diodes  8   a-   8   e,  the respective distances from the spots at which the input light are made incident to electrodes  9   a-   9   e  are shortened, compared with those of the prior art photo diode array of FIG. 2, for example. Thus, the resistance between the electrodes and the corresponding locations at which the input light is made incident is reduced. In other words, deterioration of linearity is reduced in the invention. 
     For example, in FIG. 3, if the input light is made incident to the portion marked “P101”, the effective distance from the portion “P101” to electrode  9   e  is that from the portion “P101” to the connecting portion marked “ED01”. On the other hand, if the input light is made incident to the portion marked “P102”, the effective distance from the portion “P102” to electrode  9   e  is that from portion “P102” to the connecting portion marked “ED02”. That is to say, in both cases, where the input light is made incident to the portions “P101” and “P102”, the effective distance to electrode  9   e  are similar. In addition, if the input light is made incident to the portion marked “P103” (which is located at the center), the effective distance between portion “P103” and electrode  9   e  is approximate ½, when compared to the conventional array of FIG.  2 . Since this effective distance, from the center to the end, represents the maximum, even in the worst case, deterioration of linearity is reduced by ½. 
     As a result, the currents attained by the ends ED01 and ED02 of electrodes  9   a-   9   e , out of both ends of the photo sensitive areas of photo diodes  8   a - 8   e  are from shortened distances between the input light applied portions and electrodes  9   a-   9   e , respectively. Hence, deterioration of linearity is reduced by the invention. 
     In the embodiment of FIG,.  3 , there is shown only one lead out terminal to electrodes  9   a-   9   e  (i.e. towards the bottom). However, the lead out terminals can be provided in both directions (ie. towards the bottom and top). FIG. 4 shows a second embodiment, wherein the lead out terminals are lead out in different directions and alternately. 
     The photo diodes  8   a-   8   e  are arranged similar to the array in FIG.  3 . Electrodes  10   a  and  10   e  are connected to both ends of the photo sensitive areas of photo diodes  8   a - 8   e , respectively, as in FIG.  3 . However, the lead out terminals or electrodes  10   a,    10   c  and  10   e  are lead out in a direction directed toward the bottom of FIG. 4, while lead out terminals or electrodes  10   b  and  10   d  are lead out in an opposite direction towards the top of FIG.  4 . 
     The basic operation of FIG. 4 is the same as that of FIG. 3 with the following differences. By alternately changing the lead out direction, advantageously, wiring to the Electrodes is easier. Also, another advantage is attained in that the alternate lead out terminals can be used in situations where it is difficult to secure wiring spaces for the electrodes because the photo diodes are arranged in a highly dense manner. Although the lead out terminals are alternately changed in direction, such lead out direction is not limited to the embodiment depicted. For example, the lead out direction may be changed regularly, such as for every two, three, etc, lead out terminals, or may be changed irregularly. 
     As a result, according to the invention, wiring of the terminals and electrodes is made easier and the entire process is simplified. The wiring spaces for the electrodes are secured by leading out terminals from both ends of the photo sensitive areas. 
     In another embodiment, the electrode extensions which interconnect the end portions connected to the ends of the photo sensitive areas, may be made of transparent material to extend the photo sensitive areas, as shown for example, in FIG.  5 . 
     In FIG. 5, photo diodes  11   a-   11   e  are arranged to form an array on a substrate (not shown) so that the light receiving surfaces face to the side. Electrodes and lead out terminals  12   a-   12   e  are connected to one end of the photo sensitive area of photo diodes  11   a-   11   e,  respectively, and electrodes  14   a-   14   e  are connected to the other ends of the photo diodes  11   a-   11   e,  respectively, as depicted. Furthermore, electrodes  12   a  and  14   a  are connected to each other by transparent extension  13   a , located between adjacent photo diodes; and electrodes and lead out terminals  12   b  and  14   b  are connected to each other by transparent electrode extension  13   b  located between adjacent photo diodes. Also, electrodes  12   c ,  12   d , and  12   e , and electrodes  14   c ,  14   d , and  14   e  are connected to each other by corresponding transparent extensions  13   c ,  13   d  and  13   e , extending between adjacent photo diodes. Furthermore, electrodes  12   a ,  12   c  and  12   e  have lead out terminals which lead out to the outside in a direction directed towards the bottom of FIG.  5 . Electrodes  12   b  and  12   d  are wired to have lead out terminals which lead out to the outside in a direction directed toward the top of FIG.  5 . 
     Operation of the embodiment of FIG. 5 is as follows. The basic operation is the same as in FIG. 3 with the following differences. By making the electrodes which extend between adjacent photo diodes transparent, the light receiving areas of the photo diodes are increased, compared with the embodiments of FIG.  3  and FIG. 4, due to the transparent electrode extensions  13   a-   13   e  being overlapped with the photo diodes. As a result, by employing the transparent electrode extensions  13   a-   13   e  between adjacent photo diodes, the photo sensitive area of each photo diode is enlarged. 
     The transparent material may be an oxide of indium-tin. 
     Although the embodiments of FIGS.  3 , 4  and  5  show five photo diodes, the number of photo diodes is not limited thereto. Any number of photo diodes may be used to form the array. 
     Moreover, in the embodiments of FIGS.  3 , 4  and  5 , the electrode extensions connecting the end electrodes are disposed between the respective photo diodes. These extensions may be disposed on either the right or left side of any particular photo diode. Moreover, the lead out terminals may be directed in any one or more directions in any pattern, such as alternately, in multiples, or singly, or in regular or irregular patterns. 
     The invention provides the following advantages and effects. The distance between the locations at which the input light is made incident and the electrodes is shortened by using a configuration wherein current produced by the diode is outputted from both ends of the photo sensitive area of each photo diode. Thus, deterioration of linearity is reduced. Also, the photo sensitive area of the photo diode is increased by making the the electrode extension which interconnect the electrode ends out of transparent material. Also, the lead out terminals may be directed in one or more directions in any desired pattern. 
     The foregoing description is illustrative of the principles of the invention. Numerous extensions and modifications thereof would be apparent to the worker skilled in the art. All such modifications and extensions are to be considered to be within the spirit and scope of the invention.