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, T_(e), studies as a function of driving current have shown that under high applied electric fields the power input per electron follows a T_(e)~(4.4) 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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