EVAPORATED THIN FILM DEVICES

Evidence is presented to show that the dominant current control mechanism in the insulated-gate cadmium sulfide thin film transistor TFT is conductivity modulation in the semiconductor by field effect action of the gate. The characteristics of the coplanar-electrode TFT having overlying ohmic contacts were demonstrated to be equivalent to the earlier staggered-electrode structure having underlying gold contacts. The measured drift mobility as calculated from the ratio of transconductance to input capacitance may be either higher or lower than the measured Hall mobility depending upon the method of preparation of the semiconductor film. An increase in the Hall mobility as a function of positive gate bias was found, contrary to predictions based upon the effect of scattering at the surface of a homogeneous semiconductor. Tests on various processing procedures and electrode contacts were carried out for cadmium sulfide and other materials potentially useful for TFT fabrication.