We have measured the temperature- and field-dependent magnetoresistance in an undoped AlGaN/GaN two-dimensional electron gas sample at temperatures below 10 K. Well-resolved Shubnikov-de Haas oscillations have been observed, as well as spin splitting at magnetic fields above 8 T. The amplitude of the oscillations has been used as a thermometer for the electron temperature, Te, studies as a function of driving current have shown that under high applied electric fields the power input per electron follows a Te4.4 dependence. Comparison with numerical modeling indicates that in this heterostructure, electron-acoustic phonon scattering via the screened piezoelectric interaction is the dominant energy-loss mechanism at low temperatures.

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The magnetoresistance behaviour of a high-mobility AlGaN/GaN heterostructure has been studied at low temperatures under magnetic fields of up to 28 T. Clear dissipationless minima in the Shubnikov-de Haas curves, for both cyclotron-split and spin-split Landau levels, have been observed. The temperature dependence of the widths of these minima have enabled a phase diagram for breakdown of the integer quantum Hall effect to be partially mapped out, and compared with similar results obtained for AlGaAs/GaAs heterojunctions. A distinctive feature of this sample is the variation of low-field electron mobility between different voltage probes, and corresponding differences in the quantum Hall breakdown behaviour are seen.