Towards Higher Voltages : Optimization of 4H-SiC Devices

QC 20240522

Abstract

Silicon carbide (4H-SiC) devices find extensive use in power applications due to the wide bandgap and high thermal conductivity. High-voltage Schottky diodes and MOSFETs have been available for commercial use for more than a decade. Much effort is now focused on reducing production costs and increasing the reliability and long term operation for 1-3 kV devices. This thesis focuses on three key problems for next generation of higher voltage and larger current 4H-SiC devices: i) reaching the ideal breakdown voltage of the material under reverse bias by a proper designed termination structure, ii) obtaining the best balance between on-state and switching losses of bipolar devices by localized lifetime control, iii) testing the radiation tolerance of MOSFETs and understanding the physical mechanisms of single event effects. 

This thesis starts with an introduction after which the experimental work and methodology for lifetime control and radiation tests are described (Chapter 2). The theoretical base and the physical models specified for 4H-SiC are discussed (Chapter 3). The results generated during the thesis work are summarized in the following three chapters. Chapter 4 reviews the current status of termination designs and different types of structures are evaluated from reliability and fabrication considerations. A buried junction termination extension (buried-JTE) structure, where implanted JTE zones are buried under a thin field buffer layer, has been proposed and simulated, resulting in a uniform field profile at the semiconductor/oxide interface. Simulation results also show that this structure will improve the area efficiency and reduce the effects of surface charge. In Chapter 5, a design process of lifetime control by MeV proton implantation for bipolar devices has been presented. The improvement on turn-off losses of 10 kV PiN diodes has been tested by both reverse recovery measurements and simulations. A process flow of physical simulations, based on deep level transient spectroscopy (DLTS) data for defects, has also been proposed, in which the lifetime profiles can be tailored by adjusting proton energies and fluences to reach the optimized device trade-off between on-state and turn-off losses. In Chapter 6, alpha particles of low doses have been used to irradiate 3.3 kV 4H-SiC MOSFETs to trigger single event burnout (SEB) failures. In this experiment, SEB threshold voltage, output characteristics and leakage current are characterized before and after the irradiation. The MOSFET cell used for the experiment has also been modelled to study the mechanisms of SEB, which typically occurs within nanoseconds after the irradiation and cannot be captured experimentally. The recently initiated radiation hardness experiments clearly indicate an SEB effect, but the linear energy transfer (LET) from the alpha particles in experiments cannot generate an SEB in the simulations. This discrepancy, and possible roads for future research are discussed in the concluding Chapter 7.

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