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.

No abstract

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.

No abstract

The electrical properties of composite materials over a wide frequency range are of great interest, not only for experimental applications, but also for theoretical studies such as fractal analysis. This study presents comparative analysis of alternating current (ac) conductivity and fractal structure characteristics in standard and single walled carbon nanotube (SWCNT) reinforced polymer composites based unsaturated polyester resin (UPR). The electrical characteristics of polymer matrices at 320 K have been analyzed as a function of frequency by impedance analysis method. It was found that the conductivity of the nanotube doped material in the dc conductivity region, which is the low frequency region, is independent of the frequency and takes a constant value. It was proved that conductivity obeys Jonscher’s power law toward the high frequency region. The standard sample showed an insulating behavior that exhibits continuous increase with increasing frequency. The images of the samples were obtained by scanning electron microscope (SEM) to reveal the relationship between the conductivity of the materials and their fractal properties. All samples were converted to binary format for calculations. Cellular particle density for each sample was determined according to scaling theory. Then, the surface coverage ratio, fractal dimensions, cluster densities, average cluster sizes and critical interface exponent values of the samples were calculated and compared with different samples in the literature. It was determined that the coverage ratio and fractal dimension increased when carbon nanotubes were added. In addition, it was observed that the interface critical exponent decreased when the carbon nanotube was doped.