Measurement of drift mobility in AIGaN/GaN heterostructure field-effect transistor

Low-field mobilities for electrons in the channel of an Al_0.15Ga_0.85N/GaN heterostructure field-effect transistor are derived from direct current transistor characteristics. The dependencies of mobility on gate bias, sheet carrier concentration, and temperature are obtained. For negative gate bias voltages, mobility is found to increase monotonically with increasing sheet carrier concentration, which we interpret as a consequence of increased screening of carrier scattering. For positive gate bias voltages, mobility is found to decrease with increasing gate bias due to the onset of parallel conduction in the AlGaN barrier layer. The mobility varies approximately as T^-α with α ≈ 1.6-1.8 for temperature ranging from 200 to 400 K, indicating that phonon scattering is dominant in the two-dimensional electron gas in this temperature range.