This paper describes the structure and performance of a silicon-on-sapphire (SOS) dual-gate MOSFET-the SOS MOS tetrode. Extremely low-drain capacitance, negligible parasitics, and the absence of a semiconducting substrate make the device very attractive as a building block in UHF integrated systems. I-V characteristics are shown, and a simple small-signal model that is useful up to tens of megahertz is described. The expressions for the output resistance and gain functions for both gates are given. The UHF y-parameters for the tetrode are derived, based upon Hopkins' accurate UHF model for the MOS triode, utilizing the cascode configuration. The important parameters and the unilateral power gain are examined, and compared with measured values. Measurements confirm the extended UHF performance, negligible reverse transadmittance, and high-unilateral power gain. Variations of the y-parameters with bias are examined; low-frequency voltage gain factor, µ, and UHF power gain, Gp, are compared as functions of gate voltage. All derivations assumed f = 500 MHz as the frequency of interest but appear to be valid up to low gigahertz range.