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Fig. 1: Microwave current gain h21 and unilateral power gain U vs frequency for carbon-doped non-selfaligned HBT (pB=ae 19cm-3). spacers, and lower base thicknes s, 2) greater freedom in temperature of growt[ and ,syb.sequent processing. With carbon-doping, high performance MOCVD-grown wafers are available. Fig.1, illustrates microwave gain resulting from such a wafer [4]. Tailoring the electric field profile with doping spikes has produced structures with ultrahigh speed (Ballistic Collection Transistor-of Ishibashi et al. tS]) as well as controllable conduction band steps. Pseudomorphic base regions have been produced with the addition of In, or more recently, Sb to the base layer. This can lead to graded composition base without the complication of Al alloys.

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