Effect of Mg ionization efficiency on performance of Npn AlGaN/GaN heterojunction bipolar transistors

A drift-diffusion transport model has been used to examine the performance capabilities of AlGaN/GaN $\mathrm{Npn}$ heterojunction bipolar transistors (HBTs). Simulations using experimental mobility and lifetime reported in the literature closely reproduce the Gummel plot from the GaN-based HBT structure recently demonstrated. Numerical results have been explored to study the effect of the $p\mathrm{-type}$ Mg doping and its incomplete ionization in the base. The high base resistance induced by the deep acceptor level is found to be the cause of limiting current gain values. Increasing the operating temperature of the device activates more carriers in the base. An improvement of the simulated current gain by a factor of 2–4 between 25 and 300 °C agrees well with the reported experimental results. A preliminary analysis of high-frequency characteristics indicates substantial progress of predicted rf performances by operating the device at higher temperature due to a reduced extrinsic base resistivity.