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The microwave noise technique is used to estimate the hot-electron energy relaxation time in an AlGaN/GaN heterostructure containing a two-dimensional electron gas subjected to a strong electric field applied in the plane of electron confinement. Room-temperature data show that the energy relaxation time decreases monotonously from about 1 ps at 2 kV cm −1 to 0.4 ps at 10 kV cm −1 electric field. The estimated low-field value is 1.4 ps.

Decay of nonequilibrium longitudinal optical ͑LO͒ phonons is investigated at room temperature in two-dimensional electron gas channels confined in nearly lattice-matched InAlN/AlN/GaN structures. A nonmonotonous dependence of the LO-phonon lifetime on the supplied electric power is reported for the first time and explained in terms of plasmon-LO-phonon resonance tuned by applied bias at a fixed sheet density ͑8 ϫ 10 12 cm −2 ͒. The shortest lifetime of 30Ϯ 15 fs is found at the power of 20Ϯ 10 nW/ electron.

The experimental dependence of microwave noise temperature on supplied electric power is used to study hot phonons in an InP-based modulationdoped In 0.52 Al 0.48 As/In 0.53 Ga 0.47 As/In 0.7 Ga 0.3 As/In 0.52 Al 0.48 As heterostructure with a two-dimensional electron gas channel (n 2D = 2.3 × 10 12 cm −2 ). The effective hot-phonon temperature is extracted for two-subband and six-subband models treated in the electron-temperature approximation. The estimated value for the hot-phonon lifetime with respect to the longitudinal optical phonon conversion into other vibration modes is (0.9 ± 0.3) ps at room temperature.