Closed form expression for thermal resistance of a FET structure

Radar systems frequently place high RF output power demands on power amplifiers. High power amplification leads to increased heat dissipation in the devices which, in turn, leads to increased operating temperature. Knowing the operating temperature of these devices is of great interest because it significantly affects both their reliability and performance. In fact, the accurate determination (within a few percent) of the junction temperature in FETs and MMICs is critical for reliability assessment. This paper presents an original closed-form expression for determination of the thermal resistance of an FET device. It is based on the thermal resistance of a cylinder between two ground planes and it covers a wide range of practical gate lengths, and substrate thicknesses. The approach provides an assessment of the junction temperature of the device which will be a benefit to MMIC designers, packaging engineers and system designers.