No abstract

No abstract

Low-field mobilities for electrons in the channel of an Al 0.15 Ga 0.85 N/GaN heterostructure field-effect transistor are derived from direct current transistor characteristics. The dependencies of mobility on gate bias, sheet carrier concentration, and temperature are obtained. For negative gate bias voltages, mobility is found to increase monotonically with increasing sheet carrier concentration, which we interpret as a consequence of increased screening of carrier scattering. For positive gate bias voltages, mobility is found to decrease with increasing gate bias due to the onset of parallel conduction in the AlGaN barrier layer. The mobility varies approximately as T Ϫ␣ with ␣ Ϸ1.6-1.8 for temperature ranging from 200 to 400 K, indicating that phonon scattering is dominant in the two-dimensional electron gas in this temperature range.

The influence of surface chemical treatments and of deposition of a SiO 2 surface passivation layer on carrier distributions and mobility in Al x Ga 1Ϫx N/GaN heterostructure field-effect-transistor epitaxial layer structures is investigated. Surface chemical treatments are found to exert little influence on carrier distribution and mobility. Deposition of a SiO 2 surface passivation layer is found to induce an increase in electron concentration in the transistor channel and a decrease in mobility. These changes are largely reversed upon removal of the SiO 2 layer by wet etching. These observations are quantitatively consistent with a shift in Fermi level at the Al x Ga 1Ϫx N surface of approximately 1 eV upon deposition of SiO 2 , indicating that the Al x Ga 1Ϫx N/SiO 2 interface has a different, and possibly much lower, density of electronic states compared to the Al x Ga 1Ϫx N free surface.

GaAsSb is a low band gap, lattice matched to InP, alternative to GaInAs. Growth and doping using diethyltellurium and carbon tetrachloride were investigated. Hole concentrations up to 1.3×1020 cm−3 have been achieved in as-grown carbon-doped GaAsSb [i.e., no postgrowth annealing was necessary for dopant activation, a key requirement for n-p-n heterojunction bipolar transistor (HBT) structures]. This is a sevenfold improvement over the best carbon-doped InGaAs reported by metalorganic chemical vapor deposition. Hall measurements indicate that GaAsSb’s hole mobility is 55%–60% of GaInAs’s, for a given carrier concentration. InP HBTs with carbon-doped GaAsSb base are demonstrated.