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Electron concentration profiles have been obtained for Al x Ga 1Ϫx N/GaN heterostructure field-effect transistor structures. Analysis of the measured electron distributions demonstrates the influence of piezoelectric effects in coherently strained layers on III-V nitride heterostructure device characteristics. Characterization of a nominally undoped Al 0.15 Ga 0.85 N/GaN transistor structure reveals the presence of a high sheet carrier density in the GaN channel which may be explained as a consequence of piezoelectrically induced charges present at the Al 0.15 Ga 0.85 N/GaN interface. Measurements performed on an Al 0.15 Ga 0.85 N/GaN transistor structure with a buried Al 0.15 Ga 0.85 N isolation layer indicate a reduction in electron sheet concentration in the transistor channel and accumulation of carriers below the Al 0.15 Ga 0.85 N isolation layer, both of which are attributable to piezoelectric effects.

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Disorder induced semiconductor to metal transition and modifications of grain boundaries in nanocrystalline zinc oxide thin film J. Appl. Phys. 112, 073101 (2012) Control of normal and abnormal bipolar resistive switching by interface junction on In/Nb:SrTiO3 interface Appl. Phys. Lett. 101, 133506 (2012) Cross-plane electronic and thermal transport properties of p-type La0.67Sr0.33MnO3/LaMnO3 perovskite oxide metal/semiconductor superlattices J. Appl. Phys. 112, 063714 (2012) Polarization Coulomb field scattering in In0.18Al0.82N/AlN/GaN heterostructure field-effect transistorsThe temperature dependence of the current-voltage characteristics of Ni-GaN Schottky barriers have been measured and analyzed. It was found that the enhanced tunneling component in the transport current of metal-GaN Schottky barrier contacts is a likely explanation for the large scatter in the measured Richardson constant.

The role of spontaneous and piezoelectric polarization in III-V nitride heterostructures is investigated. Polarization effects and crystal polarity are reviewed in the context of nitride heterostructure materials and device design, and a detailed analysis of their influence in nitride heterostructure field-effect transistors is presented. The combined effects of spontaneous and piezoelectric polarization are found to account well for carrier concentrations observed in AlGaN/ GaN transistor structures with low to moderate Al concentrations, while the data for higher Al concentrations are consistent with defect formation in the AlGaN barrier. Theoretical analysis suggests that incorporation of In into the barrier and/or channel layers can substantially increase polarization charge at the heterojunction interface. The use of polarization effects to engineer Schottky barrier structures with large enhancements in barrier height is also discussed, and electrical characteristics of transistors with conventional and polarization-enhanced Schottky barrier gates are presented. The polarization-enhanced barrier is found to yield a marked reduction in gate leakage current, but to have little effect on transistor breakdown voltage.