Influence of AlGaN n-type doping and AlN thickness on the two-dimensional electron gas density (ns) and resistance (R2DEG)

In this paper, electrical characterizations by C(V-G) and I-D(V-G) and comparison with 1D Schrodinger-Poisson simulations are carried out to investigate the effects of AlN layer thickness and n-type AlGaN barrier doping on the two-dimensional electron gas resistance (R-2DEG). Specifically, these effects are related to either the electron sheet density (n(s)) or the mobility (mu). We show that varying the AlN thickness from 0.7 nm (n(s) = 7.8 x 10(12) cm(-2)) to 1.5 nm (n(s) = 9.0 x 10(12) cm(-2)) leads to a linear increase of n(s). However, simultaneous degradation of the transport properties probably due to intensifying roughness mechanism at high n(s) tends to limit the improvement of the device properties. Furthermore, we show that the heavily doped AlGaN barrier slightly increases n(s) and reduces the 2DEG mobility to similar to 1770 cm(2).V-1.s(-1) leading to a degraded 2DEG resistance. However, the overall resistance is improved (R = 323 Omega/square) compared to the undoped case (R = 380 Omega/square) due to the simultaneous contribution of two conducting channels at V-G = 0 V. To go further, the polarization charges at the AlGaN/AlN and AlN/GaN interfaces are computed in the 1D Schroinger-Poisson simulations to account for the presence of the AlN layer. A reduced polarization for very thin AlN layers is considered to account for the experimental results. Finally, a very simple empirical model is proposed that predicts the enhancement of polarization charges (sigma) as a function of AlN thickness.