Two-dimensional electron gas transport properties in AlGaN/GaN single- and double-heterostructure field effect transistors

Two-dimensional electron gas transport properties have been investigated in both AlGaN/GaN single- and AlGaN/GaN/AlGaN double-heterostructure field effect transistors, by means of the Hall effect measurement under the gate-voltage application. In AlGaN/GaN single-heterostructures, the dependencies of the electron mobility on the electron density have systematically been examined from 30 to 400 K. Below room temperature, the mobility has been shown to assume a maximum value at a critical electron density where electrons begin to overflow into the AlGaN barrier layer. Above room temperature, even at high electron densities that exceed the channel electron capacity, the degradation in the mobility has been found to be small. This is a feature favorable for high-power and high-temperature device operation. In AlGaN/GaN/AlGaN double-heterostructures, a striking effect has been observed that the mobility is drastically enhanced compared with that in the AlGaN/GaN single-heterostructure at low temperatures. The dependencies of the mobility on the electron density measured at 4.2 K in both heterostructures have been analyzed from the viewpoint of the electron distribution in the channel. The observed mobility enhancement in the double-heterostructure has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Transport properties specific to AlGaN/GaN single- and double-heterostructures have thus successfully been revealed through the mobility-density relations that have been determined by the Hall effect measurement under the gate-voltage application.