Modeling small-signal response of GaN-based metal-insulator-semiconductor high electron mobility transistor gate stack in spill-over regime: Effect of barrier resistance and interface states.

We provide theoretical and simulation analysis of the small signal response of SiO₂/AlGaN/GaN metal insulator semiconductor (MIS) capacitors from depletion to spill over region, where the AlGaN/SiO₂ interface is accumulated with free electrons. A lumped element model of the gate stack, including the response of traps at the III-N/dielectric interface, is proposed and represented in terms of equivalent parallel capacitance, C_p, and conductance, G_p, . C_p, -voltage and G_p, -voltage dependences are modelled taking into account bias dependent AlGaN barrier dynamic resistance R_br, and the effective channel resistance. In particular, in the spill-over region, the drop of C_p, with the frequency increase can be explained even without taking into account the response of interface traps, solely by considering the intrinsic response of the gate stack (i.e., no trap effects) and the decrease of R_br, with the applied forward bias. Furthermore, we show the limitations of the conductance method for the evaluation of the density of interface traps, Dit, from the G_p,/ω vs. angular frequency ω curves. A peak in G_p/ω vs. ω occurs even without traps, merely due to the intrinsic frequency response of gate stack. Moreover, the amplitude of the G_p/ vs. ω peak saturates at high Dit, which can lead to underestimation of D_it. Understanding the complex interplay between the intrinsic gate stack response and the effect of interface traps is relevant for the development of normally on and normally off MIS high electron mobility transistors with stable threshold voltage. [ABSTRACT FROM AUTHOR]