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Thermal conductivity in non-metallic crystalline materials results from cumulative contributions of phonons that have a broad range of mean free paths. Here we use high frequency surface temperature modulation that generates non-diffusive phonon transport to probe the phonon mean free path spectra of GaAs, GaN, AlN, and 4H-SiC at temperatures near 80 K, 150 K, 300 K, and 400 K. We find that phonons with MFPs greater than 230 ± 120 nm, 1000 ± 200 nm, 2500 ± 800 nm, and 4200 ± 850 nm contribute 50% of the bulk thermal conductivity of GaAs, GaN, AlN, and 4H-SiC near room temperature. By non-dimensionalizing the data based on Umklapp scattering rates of phonons, we identified a universal phonon mean free path spectrum in small unit cell crystalline semiconductors at high temperature.

High electron mobility was achieved in Al1−xInxN∕AlN∕GaN (x=0.20–0.12) heterostructure field effect transistors (HFETs) grown by metal-organic chemical vapor deposition. Reduction of In composition from 20% to 12% increased the room temperature equivalent two-dimensional-electron-gas density from 0.90×1013to1.64×1013cm−2 with corresponding electron mobilities of 1600 and 1410cm2∕Vs, respectively. The 10K mobility reached 17600cm2∕Vs for the nearly lattice-matched Al0.82In0.18N∕AlN∕GaN heterostructure with a sheet carrier density of 9.6×1012cm−2. For comparison, the AlInN∕GaN heterostructure without the AlN spacer exhibited a high sheet carrier density (2.42×1013cm−2) with low mobility (120cm2∕Vs) at room temperature. The high mobility in our samples is in part attributed to ∼1nm AlN spacer which significantly reduces the alloy scattering as well as provides a smooth interface. The HFETs having gate dimensions of 1.5×40μm2 and a 5μm source-drain separation exhibited a maximum transconductance of ∼200mS∕mm with good pinch-off characteristics and over 10GHz current gain cutoff frequency.

A microwave noise technique has been used for experimental investigation, at room temperature, of power dissipation in the voltage-biased two-dimensional electron gas channel located in the GaN layer of a lattice-matched Al 0.82 In 0.18 N/AlN/GaN heterostructure. No saturation of the relaxation time is found in the investigated electron temperature range up to ∼2800 K: the hot-electron energy relaxation time decreases from ∼6 ps at near equilibrium to 75 ± 20 fs at ∼200 nW/electron. The electron drift velocity reaches ∼1.8 × 10 7 cm s −1 at 65 kV cm −1 electric field. The hot-phonon effect on power dissipation is discussed.

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