A unified static-dynamic analytic model for ultra-scaled III-nitride high electron mobility transistors
This paper presents an analytic model to calculate nodal charges and their corresponding internodal capacitances in ultra-scaled III-nitride high electron mobility transistors (HEMTs) in which carrier transport is expected to be quasiballistic. The dynamic model is combined with our previously published static model [K. Li and S. Rakheja, J. Appl. Phys. 123, 184501 (2018)] to provide a unified physical description of the HEMT. In the dynamic model, the position-dependent channel charge is obtained using a realistic potential profile in the channel, obtained from the solution of the Poisson equation. The effect of electric field lines penetrating from the ungated access regions into the channel (gated region) is included. Fringing electric field lines from the gate to the channel, which are especially critical in the off-region of the device, are included in the charge description. The channel charges are obtained self-consistently with the transport model and introduce 16 additional input parameters, which are necessary to explain the bias dependence of internodal capacitances in the off-state and in the off-to-on transition region of the device. Using the model, we elucidate the difference in the capacitance-voltage behavior of drift-diffusive and quasiballistic devices. The sensitivity of channel charges to the specific formulation of the potential profile is also examined. The unified model is applied to the experimental capacitance-voltage data of 42-nm and 105-nm gate-length InAlN/GaN HEMTs with an InGaN backbarrier. Additionally, the model is validated against hydrodynamic simulations of a 50-nm gate-length AlGaN/GaN HEMT with significant Joule heating. The model yields an excellent agreement with the measured and simulated data sets over a broad range of bias and temperature conditions. The model is also used to elucidate the role of contact resistance, Joule heating, and nodal capacitances on the RF performance, i.e., cut-off frequency, third-order intermodulation current, and the input third-order intercept point, of scaled III-nitride HEMTs.