The Temperature-dependent Electrical Properties Of AlGaN/GaN Heterostructures

AlGaN/GaN heterostructures have recently been attracting much attention because of their great potential for high-voltage, high-power and high-temperature microwave applications. As the growth technique is developing, it will be not only aiming to improve the electron characteristic to AlGaN/GaN heterostructures, but to find and explain the mechanisms in a wide temperature range, and to study the feasibility at high-temperature, model the degenerated mechanisms, and so on. In the paper, the electron characteristic of AlGaN/GaN heterostructures at variable temperatures will be studied. By the use of capacitance-voltage (CV) effect, Hall effect at low-fields and the capacitance-voltage, conductance-voltage (CC) measurement on FAT HFET device, the electron systems, the relationship between two-dimensional-electron-gas (2DEG) density and its mobility, and the high-temperature electron transport characteristic of AlGaN/GaN heterostructures have been examined and analyzed. The main works and results on the research are listed as following:1 Research on the 2DEG interface characteristic at variable temperatures by the use of theory to polarization and self-consistently solving Schrodinger's and Poisson's equations. At the high temperature of 700K, the confinement to 2DEG was still well, which is good reason to the high-temperature applications. At low temperature 2DEG mobility was mainly dominant by interface roughness (IFR) scattering, alloy disorder (Alloy) scattering and piezoelectric (PE) scattering. At room temperature almost every scatterings contributed to 2DEG mobility, but at high temperature it becomes to accoustic phonon (AC) scattering and polarized optical phonon (PO) scattering, as a result of the modeling to 2DEG scattering mechanisms.2 It was found that the electron system could be devided into 2DEG in the interface, electron ionized by donors in AlGaN barrier layer, and the background electron in GaN buffer layer, through the examination of AlGaN/GaN heterostructures by using CV effect at variable temperatures. The electron density in AlGaN increased by twice at 673K, compared to 77K, while it was increased by magnitudes in GaN buffer layer. 2DEG density showed in rules to the temperatures which was firstly decreased and then increased. The decreasing at lower temperature was caused by the decrease of the conduction discontinuity, which meant that the triangle quantum well could not contain more electrons. The increasing at higher temperature was because of the contribution of the electron ionized by the donors in the interface and at both sides. 3 The relationships between the background electron density with the temperature was studied to the AlGaN/GaN heterostructures grown by MOCVD-320 using Hall effect. It was found that the lower the room temperature depletion capacitance of AlGaN/GaN heterostructures, the few the increasing of background electron density, even not appared at every temperature. But when he depletion capacitance was higher, the increasing of background electron density was more acute, even exceed to 2DEG. This result will be more important to grow high quality AlGaN/GaN heterostructures.4 The relationships between 2DEG densities and mobilities at different temperatures had been examined using the CC measurements and mechanism theory. And found that at high temperature,2DEG mobility at low density was weaken badly by coulomb scattering, which should be include another source:background electrons. Because the mobility added by the background electron coulomb scattering was much lower, so the 2DEG mobility was much lower compared to the higher 2DEG density's.5 High-temperature electron transport properties were researched on different Al-contents AlGaN/GaN heterostructures using Hall effect. It was found that the electron mobility of lower Al-content heterostructure was lower than that of higher Al-content, they were 154,182cm2/V.s respectively. Theoretical calculation showed that electron occupations in the first subband were 75% and 82% at 700K, respectively, for the 15% and 40% Al-contents heterostructures. The 2DEG ratios in the whole electron system were 30% and 60%, respectively. Indicated that at high temperature, the higher Al-content AlGaN/GaN heterostructure had, the stronger to prevent electron from moving to higher subbands, and the more effective for 2DEG dominant. This found agreed well with the results of theoretical calculations, indicating that higher Al-content AlGaN/GaN heterostructure was more suitable for high-temperature applications.