The ability of operate at frequencies at and above 60 GHz is desirable in many applications to increase bandwidth, achieve more favorable tradeoffs between antenna beamwidth and size, and to exploit frequency dependent transmission properties of the atmosphere. Corresponding device and circuit requirements are delineated. One especially demanding requirement is to develop phased arrays at 60 and 94 GHz. At 94 GHz this requires an array of transmitreceive TR modules with center to center spacings of 1.5 mm between modules. Generating the required output power without exceeding thermal dissipation limits places stringent requirements on the solid state components of the TR module. The first 60 GHz transistor amplifier was developed in 1983 using GaAs MESFETs. The DARPA Strategic Technology Office quickly initiated a program to extend these capabilities to develop a 60 GHz TR module for satellite-to-satellite communications applications. Concurrently, in expectation that MESFET performance might prove inadequate for some EHF applications, the DARPA Defense Sciences Office initiated a program to develop alternative EHF transistor technologies. Various planar and vertical device structures have been fabricated and devices with measured gain at 94 GHz and projected values of the maximum frequency of oscillation, fmax up to 220 GHz have been developed under this program and others.