Date of Award

Summer 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Ravindra Joshi

Committee Member

Linda Vahala

Committee Member

Jiang Li

Committee Member

Duc Nguyen

Abstract

Since the early studies by Auston, photoconductive semiconductor switches (PCSSs) have been investigated intensively for many applications owing to their unique advantages over conventional gas and mechanical switches. These advantages include high speeds, fast rise times, optical isolation, compact geometry, and negligible jitter. Another important requirement is the ability to operate at high repetition rates with long device lifetimes (i.e., good reliability without degradation). Photoconductive semiconductor switches (PCSSs) are low-jitter compact alternatives to traditional gas switches in pulsed power systems. The physical properties of Silicon Carbide (SiC), such as a large bandgap (3.1-3.35 eV), high avalanche breakdown field (~3 MV/cm), and large thermal conductivity (4-5 W/cm-K) with superior radiation hardness and resistance to chemical attack, make SiC an attractive candidate for high voltage, high temperature, and high power device applications.

A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC was carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, were comprehensively included. Results of our model matched the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters were also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples could be discounted up to applied fields as high as 275 kV/cm.

In addition, calculations of electric field distributions in a SiC photoconductive semiconductor switch structure with metal contacts employing contact extensions on a high-k HfO2 dielectric were carried out, with the goal of assessing reductions in the peak electric fields. For completeness, analysis of thermal heating in a lateral PCSS structure with such modified geometries after photoexcitation was also included.

The simulation results of the electric field distribution show that peak electric fields, and hence the potential for device failure, can be mitigated by these strategies. A combination of the two approaches was shown to produce up to a ~67% reduction in peak fields. The reduced values were well below the threshold for breakdown in SiC material using biasing close to experimental reports. The field mitigation was shown to depend on the length of the metal overhang. Further, the calculations show that, upon field mitigation, the internal temperature rise would also be controlled. A maximum value of 980 K was obtained here for an 8 ns electrical pulse at a 20 kV external bias, which is well below the limits for generating local stress or cracks or defects.

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DOI

10.25777/ct96-vm56

ISBN

9781369285116

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