Influence of Unsymmetrical Electrode structure on a-Si Photodiode Characteristics
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INFLUENCE OF UNSYMMETRICAL ELECTRODE STRUCTURE ON a-Si PHOTODIODE CHARACTERISTICS M.HAYAMA, K.KOBAYASHI, H.MIKI , AND Y.ONISHI Materials and Electronic Devices Lab., Mitsubishi Electric Corp., 1-1, Tsukaguchi-honmachi 8-chome, Amagasaki, Hyogo 661, JAPAN ABSTRACT Amorphous silicon(a-Si) stripe p-i junction photodiode array for contact-type image sensor of a facsimile has been developed and investigated This especially with regard to its unsymmetrical electrode structure. sensor consists of a p-i a-Si stripe layer, Cr separate electrodes, and an As a result, it is found that the photosensitive ITO common electrode. region of a-Si stripe type photodiode exists not only in a-Si sub-region having both upper and lower electrodes but also in a-Si sub-regions having that photocarrier It is considered upper or lower one-sided electrode. collecting mechanisms in the a-Si sub-regions are different and result to the different photodiode characteristics. INTRODUCTION Recently, hydrogenated amorphous silicon (a-Si) contact-type image sensors have been extensively developed for the applications to a facsimile reader and other systems. The structure of sandwitch type a-Si photodiode array for image sensors can be classified into two types. The one is the photodiode array in which a-Si layer is separated into photodiode elements. This typical example is p-i-n type photodiode array, named as p-i-n type.(l)(2) This type is very popular in the field of a-Si solar cells. The other is the photodiode array in which a-Si layer is fabricated continuously over the photodiode elements.(3)(4)(5)(6) This typical example is a-Si stripe schottky photodiode array, named as i stripe type.(3)(4) The p-i-n type has good photodiode characteristics but is high in cost. On the other hand, the i stripe type is lower cost than the p-i-n type because of simple structure and also simple fabrication process, but is inferior to the p-i-n type in photodiode characteristics. The details of the characteristics of i stripe type for practical application are not understood sufficiently. There, i stripe and p-i-n type were investigated and a-Si stripe p-i junction photodiode array, named as p-i stripe type,has been developed for the purpose of the application to the facsimile and investigated especially with regard to the unsymmetrical electrode structure. STRUCTURES AND FABRICATION PROCEDURES The photodiode array of the p-i stripe type is fabricated and measured on photodiode characteristics. For comparison, those of the i stripe type and the p-i-n type are also fabricated. The basic structure of three photodiode arrays is sandwitch type photodiode array in which a-Si layer is sandwitched between Cr and ITO electrode. These photodiode arrays are formed with the resolution of 8 dots/mm. The schematic structure of the p-i stripe type is shown in Fig.l. As shown in Fig.l-(a), a-Si layer can be divided into three sub-regions with regard to the unsymmetrical electrode structure. Region-I consists of i a-Si layer and Cr electrode and its length is about 0.7 mm. Region
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