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The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/ AlN/GaN two-dimensional electron gas ͑2DEG͒ heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm 2 / V s along with 8.44ϫ 10 12 cm −2 electron density, which resulted in a two-dimensional sheet resistance of 435 ⍀ / ᮀ for the Al 0.2 Ga 0.8 N / AlN/ GaN heterostructure. The sample designed with an Al 0.88 In 0.12 N barrier exhibited very high sheet electron density of 4.23ϫ 10 13 cm −2 with a corresponding electron mobility of 812 cm 2 / V s at room temperature. A record two-dimensional sheet resistance of 182 ⍀ / ᮀ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al 0.88 In 0.12 N / AlN/ GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al 0.88 In 0.12 N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors ͑HEMTs͒, after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.

The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN twodimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm 2 (V s) −1 with corresponding sheet carrier densities of 2.3-1.3 × 10 13 cm −2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm 2 (V s) −1 with corresponding sheet carrier densities of 1.3 and 1.1 × 10 13 cm −2 , respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.

Abstract. In this work, we investigated the hot-electron dynamics of AlGaN/GaN HEMT structures grown by MOCVD on sapphire and SiC substrates at 80 K. High-speed current-voltage measurements and Hall measurements over the temperature range 27-300 K were used to study hot-electron dynamics. At low fields, drift velocity increases linearly, but deviates from the linearity toward high electric fields. Drift velocities are deduced as approximately 6.55 × 10 6 and 6.60 × 10 6 cm/s at an electric field of around E ∼ 25 kV/cm for samples grown on sapphire and SiC, respectively. To obtain the electron temperature as a function of the applied electric field and power loss as a function of the electron temperature, we used the so-called mobility comparison method with power balance equations. Although their low field carrier transport properties are similar as observed from Hall measurements, hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. We found that LO-phonon lifetimes are 0.50 ps and 0.32 ps for sapphire and SiC substrates, respectively. A long hot-phonon lifetime results in large nonequilibrium hot phonons. Non-equilibrium hot phonons slow energy relaxation and increase the momentum relaxation. The effective energy relaxation times at high fields are 24 and 65 ps for samples grown on sapphire and SiC substrates, respectively. They increase as the electron temperature decreases.

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