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Microwave noise technique is applied to study energy dissipation in an AlN/GaN heterostructure containing a two-dimensional electron gas channel. Measurements of the dissipated power and the noise temperature are performed at 80 K lattice temperature in the electric field range up to 40 kV/cm. The energy relaxation time is found to decrease from 40 ps to 0.55 ps when the bias is increased. The experimental data are discussed in the electron temperature approximation assuming electron energy dissipation on optical phonons and hot-phonon effects. Dependencies of the hot-phonon number and the hot-phonon temperature on the hot-electron temperature are deduced. The frequency cutoff imposed by the limited energy dissipation through optical phonons is estimated.

This paper presents high performance device results using an ultra-thin AlN/GaN structure on sapphire substrate with a 100-nm T-gate. Excellent dc and RF characteristics are reported, including an extrinsic transconductance of 500 mS/mm and an extrinsic f_t/f_max(U) ratio of 78/111-GHz which is among the highest reported for AlN/GaN HFETs. Low gate leakage results are also presented despite the small barrier thickness and absence of a gate dielectric. Modeling of the small signal parameters is also discussed to gain an understanding of the limiting and contributing performance factors.