Performance predictions for n-p-n Al/sub x/Ga/sub 1-x/N/GaN HBTs

Predictions of the attainable current gain and cut-off frequency of n-p-n Al/sub x/Ga/sub 1-x/N/GaN HBTs are made using compact models. This analytical approach allows the minority-carrier lifetime in the base to be readily identified as a critical parameter in the determination of the gain. For realistic values of lifetime, room-temperature gains in the region of 200-2000 should be attainable, with a graded-base being necessary for highest performance. Gains around 100 are predicted for operation at 600 K. Design of the base is shown to be also important in attaining high cutoff frequencies: a value around 30 GHz would appear to be the upper limit, and its realization would need a graded-base device. Spontaneous polarization is shown to be unlikely to have a major impact on device performance. On the other hand, incomplete ionization of the base dopant is shown to be an important factor.