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The experimental evaluation of cadmium sulfide field effect transistors indicates that, although the material does not compete with germanium for general transistor application, useful power gain was achieved and certain advantages exist for CdS in phototransistor applications. Many unique circuit functions can be performed by the device. The active electronic properties vary greatly with the intensity and wavelength of incident light, and with photoconductivity quenching infrared radiation. Voltage gains greater than 500, mutual transconductances up to 20 Â¿mho and input resistances near 100 megohms have been observed. Electron-hole pairs are generated photoelectrically in the highly pure, single crystal CdS. The holes are virtually immobile and form a space charge when an applied electric field removes the mobile electrons from a region. Modulation of the space charge boundary by application of a potential to one terminal of the device produces modulation of the conductance between two other terminals and signal amplification results. Attempts to correlate the illumination sensitivity of the terminal characteristics with theoretical field effect parameters indicates the carrier distribution in the crystal is complex and varies with illumination wavelength and quenching intensity.