Bare Surface Barrier Height: A New Assessment ofStructural Reliability in AlGaN/GaN Heterostructures

AlGaN/GaN based HEMTs are becoming one among the favorite choices for future highfrequency, high-power, high temperature electronics applications. This can be attributed to the excellent physical properties such as a wide band gap (more than 3.4 eV), a high critical field for breakdown, and a good thermal conductivity of these nitride based heterostructures. The most interesting feature of these devices is the presence of a high mobility, two-dimensional electron gas (2DEG) with a sheet density of the order of 1013 cm-2 at the AlGaN/GaN interface, even in the absence of both an AlGaN barrier layer doping and a gate metal (bare surface). This phenomenon is attributed to the strong piezoelectric as well as spontaneous polarization effects in these nitride based devices, allowing an efficient, gate-controlled charge transport between the source and drain electrodes of the device.To design these devices, it is very important to understand their fundamental properties, including the mechanism responsible for the formation of the 2DEG, and how this is influenced by the formation of defects in the AlGaN layer at large thicknesses and Al contents in the layer. Another important issue is to derive modeling tools suitable for device characterization and for reliability analyses. In this thesis, a comprehensive mathematical framework to address these issues is presented. A main focus has been on the fundamental surface properties of the AlGaN layer, especially those related to the surface donor density and the surface donor level, the effect of gate metal deposition on the donor states, and the effects of defect formation in the AlGaN layer at high strain levels. The present analysis allows important design parameters to be described mathematically in terms of these properties. The present modeling is verified by comparisons with experimental results for all practical ranges of AlGaN thicknesses and Al concentrations in the AlGaN layer.Traditionally, silicon based LDMOS devices have been an important work horse in less demanding commercial high-frequency electronics for RF applications. As part of the present investigations, a new design is proposed for LDMOSFETs where the incorporation a Zener diode in the drift region of the device is shown to improve the on-state performance characteristics of the device.