AlGaN/GaN-based power semiconductor switches

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 209-219).AlGaN/GaN-based high-electron-mobility transistors (HEMTs) have great potential for their use as high efficiency and high speed power semiconductor switches, thanks to their high breakdown electric field, mobility and charge density. The ability to grow these devices on large-diameter Si wafers also reduces device cost and makes them easier for wide market adoption. However, the development of AlGaN/GaN-based power switches has encountered three major obstacles: the limited breakdown voltage of AlGaN/GaN transistors grown on Si substrates; the low performance of normally-off AlGaN/GaN transistors; and the degradation of device performance under high voltage pulsed conditions. This thesis studies these issues and presents new approaches to address these obstacles. The first part of the thesis studies the breakdown mechanism in AlGaN/GaN-on-Si transistors. A new quantitative model-trap-limited space-charge impact-ionization model- is developed. Based on this model, a set of design rules is proposed to improve the breakdown voltage of AlGaN/GaN-on-Si transistors. New technologies have also been demonstrated to increase the breakdown voltage of AlGaN/GaN-on-Si transistors beyond 1500 V. The second part of the thesis presents three technologies to improve the performance of normally-off AlGaN/GaN transistors. First, a dual-gate normally-off MISFET achieved high threshold voltage, high current and high breakdown voltage simultaneously by using an integrated cascode structure. Second, a tri-gate AlGaN/GaN MISFET demonstrated the highest current on/off ratio in normally-off GaN transistors with the enhanced electrostatic control from a tri-gate structure. Finally, a new etch-stop barrier structure is designed to address low channel mobility, high interface density and non-uniformity issues associated with the conventional gate recess technology. Using this new structure, normally-off MISFETs demonstrated high uniformity, steep sub-threshold slope and a record channel effective mobility. The thesis concludes with a new dynamic on-resistance measurement technique. With this method, the hard- and soft-switching characteristics of GaN transistors were measured for the first time.by Bin Lu.Ph.D

Design, Microfabrication, and Characterization of Polar III-Nitride HFETs

ABSTRACT Design, Microfabrication, and Characterization of Polar III-Nitride HFETs Alireza Loghmany, Ph.D. Concordia University, 2016 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