Development of 4H-SiC devices and integrated circuits for robust operation in harsh environments

Abstract

Hazem Elgabra, “Development of 4H-SiC devices and digital logic circuits for robust operation at hostile environments", M.Sc. by research, Electrical and Computer Engineering Department, Khalifa University of Science, Technology and Research. This research is geared towards the increasing demand for smart power electronic systems that are suitable for harsh environment applications, such as high temperature and high power applications. Silicon carbide (SiC) integrated circuits (ICs) are appropriate for these applications since SiC, a wide bandgap semiconductor, is inherently capable of operating in such extreme conditions, making it a suitable candidate for harsh environment applications such as in automotive, avionic, nuclear, and defense. Bipolar Junction Transistors (BJTs) is used in the ICs that are preferred for these applications over Metal Oxide Semiconductor (MOS) devices because the gate dielectric present in MOS devices may be associated with stability issues at high temperatures. The aim of this research is to investigate, design, model, fabricate, and characterize 4H-SiC ICs capable of robust operation at high temperatures and high speeds. This work entails the design of different levels for the fastest 4H-SiC technology to date. A preliminary standard cell library of basic bipolar building blocks based on this technology is created to enable rapid implementation in application specific ICs. The work presented in this thesis is the first step into realizing complex control digital circuits (i.e. microcontrollers) in SiC designed to be part of a complete system working under extreme conditions. For this work, a 4H-SiC BJT is designed for high current gain and fast switching speeds using a 2D device simulation software, MEDICI. The designed BJT exhibits a current gain of 56 and a cut-off frequency of 1.4 GHz at room temperature. Circuit simulations are performed using SPICE. To simulate circuits in SPICE, an appropriate model for the BJT is created. Emitter-Coupled Logic (ECL) technology is chosen as the circuit technology for this work due to its wide temperature operation range and high speed characteristics. The circuits were designed and optimized using a fan-out of ten inverters and an 11-stage ring oscillator for worst-case noise margins and gate delays at different temperatures. The simulated gate delays and noise margins (tg/NMH/NML) exhibited by the optimized circuits are (2.8 ns/0.8 V/0.9 V), (3.3 ns/0.7 V/1 V), and (4.7 ns/0.6 V/0.8 V) at 27 oC, 250 oC, and 500 oC, respectively, showing excellent stability at all temperatures. These results compare well with Si based ECL circuits and are over three times faster than the fastest SiC IC till date (~9.8 ns), which is based on Transistor–Transistor Logic (TTL) technology. Additionally, for the first time, a complete comparative study of bipolar technologies in 4H-SiC is performed, where TTL and Schottky TTL circuits are also designed and evaluated. To wholly characterize the performance of the design, high level ICs such as adders, multiplexers, and flip-flops are also included for evaluation. The designed circuits operate over varying temperatures and power supply voltages, demonstrating the potential of robust high speed ICs in SiC for small-scale logic applications.

Date of Award	2015
Original language	American English
Supervisor	Shakti Singh (Supervisor)