EMITTER UTILIZATION IN HETEROJUNCTION BIPOLAR-TRANSISTORS

We compare measured collector current densities, cutoff frequencies (f (t)), and transducer gains for thermally shunted heterojunction bipola r transistors with 2-16 mu m emitter dot diameters or 2-8 mu m emitter bar widths with models of the emitter utilization factors. Models tha t do not take emitter resistance into account predict that the d.c. ut ilization factors are below 0.7 for collector current densities greate r than 6 x 10(4) A cm(-2) and emitter diameters or widths greater than 8 mu m. However, because the current gains are compressed by the emit ter resistances at those current densities, the measured utilization f actors are close to 1, which agrees with models that include emitter r esistance. A.c. utilization factors are evident in the transistor Y pa rameters. For example, Re\Y-21\ drops off at high frequencies more ste eply in HBTs with large emitter diameters or widths than in small ones . However, measured data shows that the HBT a.c. current gains h(21) o r f(t) values are not influenced by the a.c. utilization factor. A.c. utilization effects on HBT performance parameters such as small signal and power gains, output power, and power added efficiency are also ex amined. Published by Elsevier Science Ltd.