PhD Oral Exam - Hassan Rahbardar Mojaver, Electrical and Computer Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Since their advent, polar AlGaN/GaN hetero-junction field effect transistors (HFETs) have drawn a great deal of attention especially in high frequency/high power applications. However, the superb prospects of these transistors are affected by a few drawbacks such as aging/crack formation under strain, presence of high gate-leakage, and challenging realization of enhancement-mode (normally-off) devices.
Quite recently, study of quaternary AlInGaN barriers has been presented as a promising avenue for fulfilling various design demands including: lattice matching, polarization matching, and positive shifting the inherently negative threshold voltage of AlGaN/GaN HFETs. However, thus far only a limited scope of theoretical studies on AlInGaN/GaN hetero-structure characteristics has been reported. As part of this thesis, the two dimensional electron gas (2DEG) characteristics of gated metal-face wurtzite AlInGaN/GaN hetero-junctions as function of physical and compositional properties of the hetero-junction are theoretically evaluated using the variational method. According to this study, a considerable shift in the positive direction for the threshold voltage of AlInGaN/GaN HFETs can be achieved by engineering both the spontaneous and the piezoelectric polarization (using a quaternary AlInGaN barrier-layer of appropriate mole-fractions). Succeeding this study, a novel quaternary lattice-match layer structure based on employing a bilayer barrier for improving the carrier confinement in the channel of enhancement-mode AlInGaN/GaN HFETs is for the first time proposed. It is shown that while the proposed layer structure substantially improves the carrier confinement in the GaN channel layer, it also upholds the merits of employing a lattice-match barrier towards achieving an enhancement-mode operation.
One of the most important device characteristics of AlGaN/GaN HFETs which is often poorly understood is the gate-leakage current. As part of this thesis, reverse gate-leakage of AlGaN/GaN HFETs is studied over a wide range of lattice-temperatures. While unveiling an obscure path for gate leakage through the mesa sidewall, a model considering different leakage paths, including the identified sidewall leakage, is presented. It is illustrated that the sidewall path to the 2DEG is associated with the Poole-Frenkel electron emission. The novel contribution of the present analysis is that it postulates that in absence of absolute uniformity, Fowler-Nordheim (FN) tunneling takes place through only a small portion of the surface of the barrier, which boasts the highest electric field or the smallest Schottky barrier height. This consideration, allows the model to avoid unrealistic values for quantities such as effective electron mass (that has plagued many of the existing models).
Also as part of this thesis work, process recipe for microfabrication of submicron gate AlGaN/GaN HFETs using electron beam lithography was developed at McGill’s nano-tools micro-fabrication facilities. The results of DC characterization of the fabricated transistors along with the results of the DC stress test are presented.