Analysis of Electrothermal Reliability of Unipolar and Bipolar 4H-SiC Power Transistors

Abstract

Wide-bandgap semiconductor technologies, particularly Silicon Carbide (SiC), have revolutionized high-voltage power electronics by delivering significant improvements in efficiency, reliability, and thermal performance. This thesis has investigated the static and dynamic characteristics, reliability, and failure mechanisms of high-voltage power devices, focusing on Unipolar SiC MOSFETs and Bipolar SiC BJTs, in comparison with their Silicon counterparts, under a wide range of electrothermal stress conditions.

The thesis begins by exploring the reverse recovery behavior and bipolar degradation of SiC planar, symmetrical double-Trench, and asymmetrical trench MOSFETs. Experimental results have highlighted the superior reverse recovery performance of trench-based designs over planar devices, owing to their reduced charge storage and device size and improved switching capabilities. However, under prolonged DC current stress, the asymmetrical trench MOSFET has exhibited more pronounced degradation in I-V characteristics compared to the symmetrical double-trench structure, with trends of degradation seen in comparisons of the IV measurements before and after the stress.

A novel methodology for ‘high-energy’ avalanche testing has been applied to SiC and Silicon power MOSFETs, by incrementally increase of DC-link voltage, revealing their distinct failure mechanisms. SiC MOSFETs, despite their compact die size, have demonstrated resilience by handling higher energy densities due to their superior thermal conductivity and critical electric field strength, with failures predominantly driven by localized hotspots and thermal runaway. In contrast, Silicon MOSFETs have exhibited higher absolute energy tolerance for the same DC-link voltage because of their larger die size, which facilitates better heat dissipation, though fading away at the energy density per area.

A thorough analysis has been conducted on high-voltage NPN vertical BJTs, comparing the Silicon and 4H-SiC counterparts. Static and dynamic measurements have confirmed the superior DC gain, faster switching transients, and lower on-state resistance of the 4H-SiC BJTs. However, they have also revealed vulnerabilities, such as current collapse under high injection levels and sensitivity to base current fluctuations. Silicon BJTs, while less efficient in terms of DC gain and the transient response rate, have showed good stability under prolonged base bias electrothermal stresses given the larger die dimensions.

Experimental measurements, supported by advanced Technology Computer-Aided Design (TCAD) modellings, have been employed to evaluate the Silicon and SiC BJTs under electrothermal stress conditions, such as unclamped inductive switching (UIS) and short-circuit tests. These measurements are designed to assess the mechanisms leading to device degradation, failure, and thermal instability, with a particular focus on the roles of the device geometry, material properties, and the intended operating conditions.

By combining experimental measurements and modelling-based analysis, this thesis provides a systematic understanding of the factors influencing the reliability of Silicon and SiC unipolar and bipolar power devices. The findings offer guidance for optimizing device design, improving robustness, and enhancing the efficiency of power systems in high-stress applications, contributing to the advancement of modern power electronics.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Saeed Jahdi (Supervisor) & Xibo Yuan (Supervisor)