B.S.T.J. Briefs: A Camera Tube with a Silicon Diode Array Target

A variety of electronic cameras have been developed for television systems. 1 Among these the vidicon2 and the Plumbicon 3 have the inherent advantages of high sensitivity, small size, and simple mechanical construction. The operating principles of the vidicon and the Plumbicon are quite similar since they both utilize a thin photoconductive layer to convert the optical image to a stored charge pattern which is periodically scanned and erased by an electron beam. Erasing the charge pattern creates the video signal. However, there is a distinct difference in overall device performance since the photoconducting target in the Plumbicon (PbO) is deposited in a manner to form a single, large area, graded p-n junction, each layer having high resistivity. In the vidicon, the evaporated layers of Sb2S3 forming the target behave like a semiinsulating photoconductor. A new type of target consisting of an array of electrically isolated reverse-biased diodes, as first suggested by Reynolds, 4 later discussed by Heijne 5 and more recently by Wendland 6 , has several valuable attributes. (?·) The dark current and the light-induced current can be essentially independent of target (reverse bias) voltage and the response characteristic can have a gamma of unity as in the Plumbicon. (ii) The time constant associated with the charge leakage of an array of reverse-biased diodes can be very much larger than the intrinsic (dielectric relaxation) time constant of the bulk material. This implies that an infrared responsive camera operating at room temperature can be realized.