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- pi equivalent circuit is assumed valid up to the transition frequency f/sub t/. Peak f/sub max/ (maximum oscillation frequency) is examined as a function of the collector doping. Maximizing f/sub max/ at all costs leads to a design with an f/sub 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/sub max//f/sub t/ is derived and used to define the transistor design which gives some specified power gain at the highest possible frequency.<
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