The Impact of Band Non-Parabolicity on the Sheet Resistance of GaN Channel-Based HEMTs

The effective mass m∗ is an important parameter for 2-dimensional electron gases (2DEG) in high-electron-mobility transistors (HEMTs). Since m∗ is difficult to measure, we proposed a new method by fitting the temperature-dependent electron mobility. Our fits and literature data from other methods confirm the systematic increase in m∗ with the carrier density Ns. Poisson-coupled k·p calculations show that the ground-state energy E0 of the 2DEG increases linearly with Ns, allowing us to construct the dispersion of the conduction band (CB) around the Γ point perpendicular to [0001] with high accuracy. Density functional theory (DFT) of strained and unstrained GaN channels resulted in comparable m∗(E) dependence. The resulting linear increase in m∗ with Ns due to the CB non-parabolicity explains the observed maximum room temperature mobilities of HEMTs and fundamentally limits the sheet resistance above ≈150 Ω/□ for an ideal single-channel HEMT.