1 HBTs: high-frequency attributes LECTURE 16 Figures of merit f T : definition and derivation Design for high f T f max : definition and derivation Design for high f max

2 HF figures of merit HF figures of merit Sec Represent transistor by its small-signal equivalent circuit Consider frequency dependence of some current gain Consider frequency dependence of some power gain What are the associated figures of merit?

3 f T is measured under AC short-circuit conditions. We seek a solution for |ic/ib| 2 that has a single-pole roll-off with frequency. Why? Because we wish to extrapolate at -20 dB/decade to unity gain. f T from hybrid-pi equivalent circuit f T from hybrid-pi equivalent circuit What should this be? Sec. 14.4

4 Extrapolated f T Extrapolated f T Assumption: Conditions: Current gain: Extrapolated f T : Sec. 14.4

5 Improving f T Improving f T III-V for high g m Highly doped sub-collector and supra-emitter to reduce R ec Dual contacts to reduce R c and R B Lateral shrinking to reduce C's What is required to get f T ≈ 300 GHz ? Sec. 14.5

6 Circuit for derivation of f max Sec Justify these omissions Model with Thévenin equivalent circuit Add source and load

7 Developing an expression for f max Further conditions: Conjugately match at the output Conjugately match at the input Sec. 14.6

8 Improving f max Pay even more attention to R b and C  Sec. 14.6

9 Base-spreading resistance What is rho? Do you now see why HBTs have helped enable portable wireless products? What is R B,QNB ? Sec