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A thin-film transistor, TFT, fabricated by evaporation of all components on to an insulating substrate has been developed. Operation is based upon the control of injected majority carriers in a wide-band-gap semiconductor by means of an insulated control gate. Experimental units using microcrystalline layers of cadmium sulfide have yielded voltage amplification factors greater than 100, transconductances greater than 10,000 , Â¿mho, input impedances greater than 106 Â¿ shunted by 50 pf and gain-bandwidth products greater than 10 Mc. Switching speeds of less than 0.1 Â¿sec have been observed. Simple evaporated thin-film circuits incorporating the TFT have been built. Direct coupling between stages is permitted since the insulated gate electrode can be biased positively as well as negatively without drawing appreciable gate current. Modified forms of the TFT have been built for use as a flip-flop, an AND gate and a NOR gate in computer applications.