Explanation of anomalously high current gain observed in GaN based bipolar transistors

The potential applications of GaN-based bipolar transistors have suffered a setback from poor ohmic contacts and leakage currents. We show in this work that the extrinsic current gain (beta) /sub EXT/ measured at a low current level can be erroneously attributed to the gain of the intrinsic transistor. By accounting for leakage current coupled with poor ohmic contacts, we show that the observed very high (beta)/sub EXT/ at low current levels can be modeled accurately. The real gain of the intrinsic transistor (beta)/sub INT/ is generally much lower. As the current is increased, the effect of leakage currents is diminished, and (beta)/sub EXT/(longrightarrow)(beta)/sub INT/. This model is satisfactorily applied to explain our experimental results.