A Word-Organized Photodetector Array

A new word-organized photodetecting array is described, which is fast, sensitive, and easily integrated. The array consists of "digit" lines connecting common sides of p-i-n photodetecting diodes and "word" lines consisting of a series connection of p-i-n switching diodes. The word-organized character of the array results from the connection or disconnection of the photo-detecting diodes from ground by the forward- or reverse-biased state of the word line switching diodes. Photocurrent generated by light falling on a photodetector attached to a forward-biased word line will flow and be detected by a sense amplifier. Photocurrent in a photodetector attached to a reverse-biased word line can not be detected. An advantage of this array over existing arrays is that the photodetecting element, the p-i-n photodiode, is always back-biased; these diodes can be made with long carrier lifetimes to achieve a low dark current without slowing the switching time of the array. A complete analysis of the array operation is given, and in particular design criteria are found that relate the detector array discridmation ratio to the array size and signal rise time. The trade-offs between array simplicity and speed of operation are indicated, and it is demonstrated that the array is capable of submicrosecond cycle time. Fabrication procedures are discussed in detail and experimental results that justify the analysis are presented. Applications, such as in holographic computer memories, are briefly discussed.