Electron mobility in an AlGaN/GaN two-dimensional electron gas. I. Carrier concentration dependent mobility

The transport properties of two-dimensional electron gas (2-DEG) at the AlGaN/GaN interface were studied by characterizing the 2-DEG mobility dependence on carrier concentration, n_s, and temperature. High-quality AlGaN/GaN heterostructures were grown, and heterostructure field effect transistors (HFETs) using a Fat FET geometry were fabricated. Measurements of 2-DEG mobility were performed by magnetoresistance and capacitance-conductance. In order to understand the dominant transport factors, the mobility was modeled using different scattering mechanisms and compared to our results. It is found that mobility dependence on n_s shows a bell-shape behavior over the whole temperature range. For low n_s the mobility is dominated by Coulomb interaction from interface charge, and at high n_s the mobility is dominated by interface roughness. Using previously reported experimental values of interface charge and interface roughness in our modeling, we show good agreement with mobility measurement results. Scattering from interface states in AlGaN/GaN heterostructures, seems to be related to the high polarization field in the heterointerface. At temperatures higher than 200K polar optical phonon scattering dominates the transport, yet both interface charge and roughness affect the mobility at the low and high n_s, respectively.