Field effect investigations in thin cadmium sulfide films.

Field effect investigations on thin cadmium sulfide films have yielded a method of controlling the surface potential. The control of the surface potential is achieved by the successive evaporation of two dielectrics, GaF2 and SiO. The SiO produces donor-like surface states at the CdS surface,, while CaF2 produces acceptor-like surface states. By evaporating a thin layer of CaF2 between the CdS and SiO., the effect of the donor-like surface states on the surface conduction of CdS may be reduced. Thus, by proper choice of the CaF3 thickness, any surface potential between the limits of SiO and CaF2 may be obtained. The analysis, construction, and performance of a new evaporated, thin-film transistor capable of withstanding over 300 V is described. This thin-film transistor will switch a current of 100p.a with a gate voltage of less than 50 V, and is particularly suited as a transistor controlled switch. The large increase in operating voltage is achieved, by changes in the device geometry, material characteristics, and through careful control of the CdS surface potential by 'the method, described above. Devices were constructed with an incremental saturation resistance of 200Mi7, and a maximum operating voltage in excess of 350 V. These transistors were used to switch electro- luminescent lamps with gate voltages of 50 V in less than 0.2 msec. The transistors with semiconducting layers of CdS, utilizing CaF3 _, Si03 and Ge03 insulators and Al electrodes^ were fabricated on glass substrates by vacuum evaporation. In related experiment it was found that an evaporated CaP3 layer could be used to vary the surface potential of germanium,, and the effects of controlling the surface potential of a Ge(Li) p-i-n diode were observed. A thickness of 150 to 200 A of CaF3 appeared to restore the surface potential in the intrinsic region to the bulk value,, and thus resulted in a lower surface leakage current. - 11 -