Date of Award

Summer 2014

Document Type


Degree Name

Doctor of Philosophy (PhD)


Electrical & Computer Engineering

Committee Director

Ravindra P. Joshi

Committee Member

Jiang Li

Committee Member

Frederic D. McKenzie

Committee Member

Linda Vahala


It has recently been postulated that high voltage stress can result in the degradation of nanoscale structures that are made up of piezoelectric materials. The inverse piezoelectric effect (IPE) is believed to be the likely reason for this degradation mechanism. Basically, the IPE leads to the creation of high internal stresses driven by the presence of an electric field. Consequently, devices based on piezoelectric materials are postulated to undergo defect formation induced by the large mechanical stress arising from the inverse piezoelectric effect in the presence of an applied bias. GaN based devices are mostly observed to show this degradation mechanism, in particular AlGaN/GaN HEMTs. The key feature of this mechanism is the sudden increase in leakage currents due to defect induced energy levels. The leakage currents can contribute to local heating or electromigration, and further enhance defect creation leading to an irreversible device degradation cycle. Given this possibility, and the need to mitigate such deleterious effects, it becomes important to understand and model this degradation mechanism in nanoscale devices.

The aim of this dissertation research is to focus on the particular aspect of the inverse piezoelectric effect, understand its role in potential device degradation of GaN-based High Electron Mobility Transistors (HEMTs), and evaluate the possibilities of minimizing the inverse piezoelectric effect by optimizing the GaN-HEMT geometry and design parameters. The possible modifications of the parameter space include changing device dimensions, varying the Al composition, and employing high-k insulating cap layers. The effect of such changes on various device aspects such as the carrier density, strain, internal electric fields and the related stored energy etc. were carefully and systematically evaluated in this dissertation research. Details on the salient results, potential summarizing conclusions, and scope for future work are also presented.