Analytical model and experimental validation of critical dependency of pGaN gated AlGaN/GaN enhancement-mode high electron mobility transistor characteristics on trap-assisted tunneling

Enhancement-mode GaN-based high electron mobility transistors are essential for switching applications in power electronics. A heavily Mg-doped pGaN region is a critical feature of these devices. It pulls the Fermi energy level toward its valence band, depleting the two-dimensional electron gas region at the AlGaN/GaN interface at equilibrium. While a step profile of Mg doping in the pGaN region is desirable, it is difficult to achieve due to the out-diffusion of Mg-dopants, and the barrier AlGaN layer becomes unintentionally p-doped. This p-doping primarily leads to traps in the AlGaN barrier, leading to gate current through trap-assisted tunneling (TAT) and degradation of mobility due to the diffusion of the Mg-dopants to the channel region. The contribution of holes in the channel region and mobility degradation on the transistor characteristics are well understood. Here, we report the effect of TAT, which requires an improved understanding as it determines the key gate characteristics and transistor behavior. An increased TAT current increases the gate current and degrades the sub-threshold slope, which deteriorates transistor characteristics. However, TAT current makes the surface potential less sensitive to the change in gate voltage in the subthreshold regime, resulting in an increased transistor threshold voltage. Hence, an increase in the threshold voltage from the TAT current improves the fail-safe operation required for power-electronic applications. We show that the gate current and threshold voltage need to be tuned together for the desired performance of the enhancement-mode transistors.