Investigation of Threshold Voltage Instability in Normally Off GaN on Si HEMT

Abstract

Wide bandgap semiconductor by virtue of its high critical electric field, low intrinsic carrier concentration, high thermal conductivity is considered a promising material for enabling high density power conversion systems with a smaller form factor. Gallium Nitride (GaN) in particular, with its ability to form heterointerfaces naturally forms a high mobility 2-dimensional electron gas (2-DEG) at hetero interfaces like AlGaN/GaN making it an apt candidate for making power devices with lower on state resistance and higher frequency of operation. With the recent advances in epitaxial growth of GaN on Silicon (Si) substrates, AlGaN/GaN on Si High Electron Mobility Transistors (HEMT) have gained unprecedented commercial attention. Normally off HEMT devices are essential to design robust power systems. Of the various normally off HEMT architectures p- GaN HEMT has significant traction. Despite the advancements, reliability of HEMTs remains a challenge. Significant work has been done in the recent few years to mitigate reliability issues like Dynamic on resistance (D-RON), by continuously improving the quality of the GaN buffer and transition layers. However, threshold voltage instability exists as a key reliability issue which is not well understood. In this thesis, we investigate on threshold voltage instability of normally off 600-650V p-GaN AlGaN/GaN on Si HEMT. This thesis aims to advance the physical understanding of the threshold voltage (VTH) instability arising during the VTH measurement, nominal ON state and OFF state stresses by novel measurement techniques and a comprehensive TCAD modelling. This thesis starts by analytically understanding the design parameters across the gate stack which have a significant control on the threshold voltage (VTH). The measurement induced VTH instability is quantified for the first time and the best practices to mitigate them are proposed. Physical models explaining the instability mechanism are proposed and validated using detailed TCAD simulations. The effect of Schottky gate and ohmic gate contacts on the VTH instability and the challenges in making reliable measurements of VTH are summarised. Subsequently the nominal ON state, OFF state stress induced effect on threshold voltage is studied in detail. It has been shown that OFF state stress voltages (600V) at the drain terminal creates a dynamic threshold voltage in addition to creating a D-RON. The physical mechanisms behind the OFF-state stress induced dynamic threshold voltage are proposed and validated using TCAD simulations. Threshold voltage is a key parameter for the designer to design power systems and its instability is a crucial issue. The implications of the VTH instability at the system level are reviewed and a technique to extract the recovery time of the threshold voltage, post instability is developed. This thesis investigates and advances the science around the threshold voltage instability of the normally off p-GaN AlGaN /GaN on Si HEMT. In summary, this work has quantified the measurement induced VTH instability and addressed the challenges of the VTH measurement in the Ohmic/Schottky type p-GaN gate with detailed TCAD analysis. This work also demonstrates the existence of the OFF-state stress induced dynamic threshold voltage with TCAD validated physical models. In addition to the above this work also showcases a novel measurement technique to extract the recovery time post threshold voltage instability.