Doctoral Thesis: Gate-geometry Dependence of Enhancement-mode p-GaN Gate High Electron Mobility Transistors

Ethan S. Lee

Abstract

The Enhancement-mode p-GaN Gate High Electron Mobility Transistor (HEMT) which matches the power, voltage, and efficiency of conventional GaN transistors but most importantly features a positive threshold voltage is increasingly gaining attention as the power transistor of choice. A positive threshold voltage is attained by the insertion of a Mg-doped p-GaN layer above the AlGaN/GaN heterostructure that is contacted by a Schottky metal. An undesirable aspect of this gate stack is a p-GaN intermediate node that is largely floating complicating device operation and understanding of device physics and reliability. This thesis carries out a detailed study of the impact of gate geometry design, in particular the relative area of the p-GaN/AlGaN interface and the gate metal/p-GaN interface, on the operation and reliability of industrially prototyped transistors. First, we confirm that the gate electrostatics is set by a gate current continuity requirement under steady state and results in unusual scaling of electrical characteristics like threshold voltage and subthreshold swing as well as an abnormally high gate current density in the ungated portion of the p-GaN. We then extend the study into pulsed conditions where we discover that immediately following pulsing, the gate stack initially responds in a capacitive manner before reaching steady state set by gate current continuity. This transition process is found to be dependent on gate current and thus show a strong gate geometry dependence. Finally, we investigate gate geometry dependence on positive-bias-temperature-instability of the devices and demonstrate that the ungated portion of the p-GaN further introduces reliability vulnerabilities. Overall, the results and understanding formed in this thesis have immediate applications on design and reliability of p-GaN Gate HEMTs.

Details

• Date: Monday, May 2
• Time: 10:00 am
• Location: via Zoom

Additional Location Details:

Thesis Committee:
Prof. Jesús A. del Alamo (Thesis Supervisor)
Prof. Tayo Akinwande
Prof. Tomás Palacios

For zoom details, please contact the doctoral candidate at ethanlee@mit.edu