Abstract

Over the past two decades AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) have been the target of many studies on their suitability for high-power and high-temperature applications. Due to the sizable inherent polarization effects, present in these heterostructure-based devices, the built-in sheet charge density at the AlGaN/GaN heterointerface is remarkably high, which makes these devices fall into the category of the depletion-mode field effect transistors. Despite the suitability of this wide-bandgap material system for switching power applications, the depletion-mode character of these HFETs has been acting as an obstacle against the adoption of AlGaN/GaN HFETs to these applications. As a result, a vibrant research on the development of techniques capable of reliably changing the depletion-mode character of AlGaN/GaN HFETs into an enhancement-mode character is currently being pursued by many investigators. Towards this end, the proposed approach of this thesis has been based on modifying the piezoelectric component of the polarization sheet charge density through studying its correlation with the size of the isolation mesa.
The impact of the size of the isolation-mesa on the sweeping- and pulsed-mode drain current-voltage characteristics of AlGaN/GaN HFETs has been studied. Investigations reveal that while by implementing AlGaN/GaN HFETs on array of islands or mesas of smaller dimensions, rather than one continuous-mesa, same values for the maximum drain current level can be maintained, pinch-off voltage can be made less negative. Also, it is shown that the maximum gate transconductance is improved by island-type isolation. In addition, it is shown that the proportionally larger surface area available for power dissipation in fin- and island-isolated HFETs can reduce the impact of self-heating on AlGaN/GaN HFETs.
Modeling the drain current of AlGaN/GaN HFETs at high-temperature ambient was also another objective of this thesis. A Monte Carlo-based temperature-dependent mobility model, with incorporation of steady-state velocity overshoot, is employed in modeling the drain current-voltage characteristics of AlGaN/GaN HFETs at 300, 400, and 500K. One of the major merits of this model is that it employs a very small set of fitting parameters. The model takes into account the temperature-dependence of the electron transport through the gated-channel of an AlGaN/GaN HFET and also its access regions. This model is validated with regards to the experimentally measured drain current characteristics. Results confirm that the temperature dependency of the drift electron velocity is the cause of the degradation of drain current at elevated temperatures.