(a) Schematic of a vertical amorphous Si photoconductive array. (b) Current–voltage characteristics of an individual synapse. The illumination intensity was appropriate 1 mW/cm². Bias was applied to the top Ti/Au electrode. Reproduced with permission from Ref. [79]. Copyright 1989, Optical Society of America. (c) Basic configuration of the optically controlled planar photoconductive synapses. (d) Block diagram of the overall hardware network. (e) Average errors number of learning trials for both the hardware network and its software equivalent for the example signal prediction problem. Reproduced with permission from Ref. [82]. Copyright 1991, Optical Society of America.