Improved microwave-transistor structure

An improved microwave-transistor structure, in which the active and inactive base regions are formed by separate processes, is described. In this structure, a low base resistance is obtained by the use of a heavily doped inactive base region, which has a low sheet resistance and which also, by lateral diffusion, reduces the effective width of the emitter. At the same time, the active base region, formed by ion implantation, is separately optimised to improve the current-gain cutoff frequency. Transistors with the improved structure have noise factors of 2.3 dB at 4 GHz, compared with 3.6 dB for transistors with the same geometry but fabricated by the conventional double-diffusion process.