High breakdown field of pnp GaN/InGaN/AlGaN DHBTs with AlGaN collector

We have investigated the characteristics of pnp GaN/InGaN/(Al)GaN double heterojunction bipolar transistors (DHBTs) grown by metalorganic vapour phase epitaxy. In GaN-collector DHBTs, the collector-base breakdown voltage under the open-emitter condition increased with increasing collector thickness. By fitting the experimental results to the theoretical model of the punch-through diode, we obtained the breakdown field of 3.1 MV/cm and the residual carrier concentration of 8.5×10 ¹⁶ cm ^-3 for the GaN collector layer. On the other hand, in a 500 nm thick AlGaN collector, the breakdown voltage was 190 V. Assuming that the bias was uniformly applied in the AlGaN collector, the corresponding breakdown field was calculated to be 3.8 MV/cm. The results indicate that the use of an AlGaN collector in HBTs is a promising way to increase the breakdown voltage without increasing the collector thickness.