Growth, fabrication and characterization of gallium nitride based bipolar transistors

GaN-based bipolar transistors promise applications in power electronics like the more-established AlGaN/GaN high electron mobility transistors (HEMTs), owing to the high breakdown fields, the large electron saturation velocity, and the large band offsets of the III-V nitride semiconductors. However, immature processing techniques and difficulties associated with controlled p-type doping of GaN has made it difficult to fabricate GaN based bipolar transistors. In this work, the technique of selective emitter regrowth by MOCVD was developed to solve the problems associated with ohmic contact to p-base layers and poor control of Mg-doping profile in the base.; Poor ohmic contacts and high leakage currents between terminals complicate the characterization of GaN based bipolar transistors. We have found that the anomalously high β at low current levels observed by many groups was not the gain of the intrinsic device, but an artifact of the effects of leakage coupled with non-ohmic contacts. Our work has cleared the ambiguity in the literature and provided a guideline for proper characterization of bipolar transistors with poor terminal contacts. The extensive analysis on device current gain reveals that currently GaN-based bipolar transistor performance is limited by a short minority carrier lifetime in the base at intermediate current levels and by the Kirk effect at high current levels.; Improved understanding gained from studies of growth and processing has led us to demonstrate record performance for GaN-based bipolar transistors. The output current is as high as ∼50 mA (corresponding to an average current density of 5 kA/cm2) at β ∼ 10 and the current gain is as high as ∼20 (I_C ∼ 20 mA, corresponding to an average current density ∼2 kA/cm2). The highest breakdown voltage of ∼470 V was demonstrated but it is still limited by premature surface breakdown.