PhD Oral Exam - Alireza Loghmany, 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.

Abstract

With excellent performance in high-frequency power amplifiers, AlGaN/GaN heterojunction field-effect transistors (HFETs) as next generation power amplifiers have drawn a great deal of attention in the last decade. These HFETs, however, are still quite limited by their inherently depletion-mode (D-mode: negative pinch-off voltage) nature, relatively poor gate-leakage, and questionable long-terms reliability. In addition, since AlGaN/GaN HFETs operate at extremely high-power densities, performance of these devices has so far remained quite limited by self-heating effects.

While a number of techniques have already been developed for realization of enhancement-mode (E-mode: positive pinch-off voltage) AlGaN/GaN HFETs, these techniques in addition to having a number of difficulties in achieving enhancement-/depletion-mode pairs, fall short of satisfying requirements such as low leakage-current, drain-current stability, and pinch-off voltage stability at the high operating temperatures and at elevated electric-fields. Among these techniques, fluoride-based plasma treatment is the most widely accepted. As an alternative to this mainstream technique, polarization-engineering of AlGaN/GaN HFETs through exploring the impacts of the mesa geometry is studied as a possible avenue for selective transformation of the D-mode nature of AlGaN/GaN HFETs to an E-mode character. Whereas limited experimental studies on the pinch-off voltage of HFETs realized on different isolation-feature geometries have indicated the presence of a certain correlation between the two, such observations lack the required depth to accurately identify the true culprit. This technique is expected to be ultimately capable of producing enhancement-/depletion-mode pairs without adding any extra steps to the microfabrication process.

In light of this requirement, microfabrication of AlGaN/GaN HFETs using a number of alternative isolation-feature geometries is explored in this study. In addition to developing an in-house microfabrication process, transistors designed according to these novel isolation-feature geometries have been fabricated through the services offered by Canadian Microelectronics Corporation (CMC). Investigation of the variation of pinch-off voltage among the devices fabricated through this latter means has conclusively indicated that the pinch-off voltage shift, rather than exclusively being caused by the surrounding-field effect, is also correlated to the perimeter-to-area ratio of the isolation-features.

In addition, through characterization and thermal modeling of these groups of devices, in this study a new approach is unveiled for reducing self-heating in AlGaN/GaN HFETs. According to finite element analysis (FEA) and electrical measurement of average channel temperature, an improved heat-dissipation was observed in HFETs enjoying a more distributed nature of the two-dimensional electron gas (2DEG) channel. This is observed to be the case especially for isolation features which offered the center of the channel a smaller distance to the side walls. Observations also indicate a more distinct gain in thermal management with reduction of the gate-length and also the surface area of the isolation pattern. Results suggest that self-heating in AlGaN/GaN HFETs can be substantially nullified by reducing the island-width below a certain threshold value, while maintaining the total width of the transistor constant.

In addition to exploring these alternatives on AlGaN/GaN HFET structures, in-house microfabrication of AlN/GaN MISFETs is also studied. The results of DC characterization of these novel transistors are also presented.