Transistor design for predictable power gain at maximum frequency

Equations which define the neutral base width, collector doping, and epitaxial collector thickness of a bipolar transistor giving a specified unilateral power gain at the highest frequency, possible are derived. Emitter-base capacitance, emitter delay, emitter stripe width, base doping, and the operating base-collector voltage are assumed to be known and fixed. The hybrid-π equivalent circuit is assumed valid up to the transition frequency f_t. Peak f_max (maximum oscillation frequency) is examined as a function of the collector doping. Maximizing f_max at all costs leads to a design with an f_t which approaches zero. In designing a transistor, the two figures of merit must be traded off against each other. A simple expression giving maximum f_max/f_t is derived and used to define the transistor design which gives some specified power gain at the highest possible frequency