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California State University, Northridge a Fully CALIFORNIA STATE UNIVERSITY, NORTHRIDGE A FULLY ANALYTICAL BACK-GATE BIAS MODEL FOR n-CHANNEL SILICON CARBIDE MESFETs WITH BACK CHANNEL IMPLANT. A graduate project submitted in partial fulfillment of requirements For the degree of Master of Science in Electrical Engineering. By Sushma Malku May 2017 The graduate project of Sushma Malku is approved: _________________________ _________________ Prof. Benjamin F. Mallard Date __________________________ _________________ Dr. Banmali Rawat Date __________________________ _________________ Dr. Somnath Chattopadhyay, Chair Date California State University, Northridge ii Acknowledgement I successfully completed this project with generous support of prof. Dr. Somnath Chattopadhyay. Therefore, I wish to extend my sincere gratitude to him. He guided me in every step and provided me with more information regarding the project. This project would not be complete if it was not for their kind co-operation of my committee members Dr. Benjamin F. Mallard and Dr. Banmali Rawat for being as my committee and for providing valuable suggestions. I want to thank them for their co- operation and supervision. I would like to offer my special thanks to my family and god for their blessings, prayers and relentless support upon me and helping me in everything I do. iii Table of Contents Signature page ii Acknowledgement iii List of Figures vi List of Tables viii Abstract ix CHAPTER 1. INTRODUCTION 1 1.1 Different SiC polytypes 3 1.2 SiC MESFET performance 4 1.3 Characteristics of Reverse Recovery time of SiC MESFET 5 CHAPTER 2. SILICON CARBIDE (SiC) 7 2.1 Material information 7 2.2 Production of Silicon Carbide 7 2.3 Manufacturing of Silicon Carbide 9 2.3.1 Acheson process 9 2.4 SiC crystal structure 12 2.5 Properties of Silicon Carbide 14 2.6 Ion Implantation technology 14 2.6.1 Mathematical model of Ion Implantation 16 2.7 Schottky Barrier Diode 17 CHAPTER 3. PHYSICS OF MESFET 19 3.1 Introduction 19 3.2 Types of MESFET 19 iv 3.3 Structure and characteristics of MESFET 21 3.4 Operation of MESFET 22 3.5 SiC MESFET structure and characteristics 23 3.6 Applications of MESFET 25 3.7 Advantages and Disadvantages of MESFET 26 CHAPTER 4. NUMERICAL CALCULATIONS 27 CHAPTER 5. RESULTS AND DISCUSSION 34 CHAPTER 6. CONCLUSION 39 REFERENCES 40 v List of Figures Figure 1.1: Reverse Recovery waveform for ‘SiC’ SBD and Si FRD 5 Figure 1.2: Reverse Recovery waveform in forward current Dependency for SiC-SBD 6 Figure 2.1: Single Crystal Moissanite 7 Figure 2.2: 3 mm Diameter of SiC Crystals 8 Figure 2.3: Lely SiC Crystals 8 Figure 2.4: Simple design of Acheson furnace 9 Figure 2.5: Schematic diagram of Acheson process resistive furnace 10 Figure 2.6: Resistor furnace after cooling 11 Figure 2.7: The development of GUI in Acheson process 12 Figure 2.8: 3C-SiC cubic crystal structure 13 Figure 2.9: 4H-SiC crystal structure 13 Figure 2.10: 6H-SiC crystal structure 14 Figure 2.11: Schematic diagram of Ion implanter 15 Figure 2.12: Plot for implanted ion distribution before annealing 17 Figure 2.13: Schottky barrier formation between a metal and n-type Semiconductor 18 Figure 3.1: Transfer characteristics of MESFET 20 Figure 3.2: Basic MESFET structure 21 Figure 3.3: Operating region of MESFET at (a) Linear (b) cutoff and (c) Saturation regions 22 Figure 3.4: Schematic Diagram of MESFET 23 vi Figure 3.5: Simple design of Silicon carbide MESFET 24 Figure 3.6: Schematic diagram of 4H-SiC MESFET 24 Figure 5.1: Electrostatic potential versus channel length 34 Figure 5.2: Threshold voltage versus Back-Gate Voltage 35 Figure 5.3: Threshold voltage versus Back- Gate voltage 36 Figure 5.4: Electrical field in channel versus drain-to-source voltage 37 vii List of Tables Table 1: Different properties of SiC polytypes 3 Table 2: Differentiation between different properties of materials 25 viii Abstract A FULLY ANALYTICAL BACK-GATE BIAS MODEL FOR n-CHANNEL SILICON CARBIDE MESFETs WITH BACK CHANNEL IMPLANT. By Sushma Malku Masters of Science in Electrical Engineering. The main goal of this grad thesis is to develop an analytical model for n-channel MESFET device and understanding the device parameters incorporating the back-gate biasing effect. The device has been structured by n-channel using front and back doping processes. The physics based analytical model of SiC MESFET gave the clear picture of electrostatic potential distribution at any position of channel. The electric field distribution underneath the gate under drain source biasing shows an important properties of electric field distribution. The threshold voltage variations with back gate biasing for different substrate concentration and ion dose have been discussed to study the device properties for switching and frequency performance. The grad thesis incorporate the introduction of the thesis in chapter 1, silicon carbide material in chapter 2, MESFET physics in chapter 3, numerical calculations in chapter 4 and results and discussion in chapter 5. ix CHAPTER 1 INTRODUCTION Silicon carbide is the chemical element with carbon and silicon, having the chemical formula ‘SiC’. Originally, it was generated by an electro-chemical reaction at high temperatures. The reaction takes place between carbon and sand. SiC is extremely rough and has been made into grinding wheels and other byproducts for more than ten decades. SiC semiconductors have been known for many years, but its semiconducting properties have been sufficiently applied over the past twenty years. SiC is expeditiously becoming the semiconductor hand-picked for improved applications, where enhanced high temperature, an acrid environment, higher voltage, and huge power density execution is expected [1]. The properties of silicon semiconductor materials restrict workability of electronic devices to a confined range of applications. Power electronics dependent on Si transistor techniques have reached the limits of design on mass, diameter, and efficiency. A few of their properties are: Low denseness High stability Less thermal expansion coefficient Higher conductivity and hardness Higher radiation resistance. The huge thermal conductivity combines with thermal expansion, which gives strength to the material and provides better thermal shock resisting qualities. SiC is doped with n-type elements such as phosphorus and nitrogen, and with p- type elements such as aluminum and boron [1]. Heavy doping with boron, aluminum or nitrogen gives the metallic conductivity. Superconductivity can be detected at the same temperature of 1.5K in all the dopings of 3C-SiC with aluminum, 3C-SiC with boron, and 6H-SiC with boron. 1 Silicon carbide’s most dependable property is its wide bandgap (>2.3eV). Due to the requirement of energy being more for electrons to reach the conduction band which creates it, it is largely used for higher voltages and higher temperatures. Silicon material has an electric field breakdown of about 300kV/cm, and needs to be 10 times the size to hold the same voltage as silicon carbide, with its electric field breakdown at about 4MV/cm. This fact makes it more efficient and able to be used in high-speed devices, at much higher operational voltages [2]. This property has enabled silicon carbide devices to be used in switches for power grids, which improves the speed of switching, reducing power loss, and improving reliability. The various aspects including reliability and cooling are very important for the enhancement in power density that affects the performance of the power grid. Silicon carbide has various material properties that makes it a substantial material for use in various power applications. To name a few of those properties, they are: a higher electric field: 4 x 106 volts/cm a higher drift velocity: 2 x 107 cm/sec a higher thermal conductivity : 4.9 watts/cm-oK In contrast to silicon, SiC has higher thermal conductivity, bandgap and dielectric breakdown strength. Semiconductor materials, where both n and p-type regions are important for designing the device structures, can be processed in SiC. The most promising properties of SiC make the material popular, and so it is used to design power circuits which exceed the performance capability of silicon. SiC materials have high breakdown voltage, low resistivity and can perform at high temperatures. SiC ceramics maintain their strength to very high temperatures with insufficient or no gain boundary condition impurities, up to approximately 1600°C. There is no loss of strength in this process. This material is used as wafer trays, due to the properties it possesses, namely, purity in chemicals, retention strength at higher temperatures, and resistivity to chemical reactions at different temperatures. The properties of SiC make the metal-semiconductor field-effect transistors (MESFET) the most effective of the SiC device family. The exceptional thermal properties of SiC 2 makes it more popular for certain devices, because it gives more power compared to gallium arsenide, at any frequency level. The growth of SiC devices has been restrained because of the unknown scope of larger and higher quality SiC substrates. The improved substance and quality of the epitaxial film, better thermal management, and enhanced fabrication process have led to the remarkable performance of devices [2]. Due to these properties of SiC, devices made from it are used at high power and high temperature applications in the design of power amplifiers. Silicon dioxide (SiO2) has high dielectric stability of insulators and is the main product in building metal oxide semiconductor field effect transistors (abbreviated as MOSFET). It has an original oxide of SiC. It can grow thermally in oxygen, in dry conditions. Most of the other semiconductor devices, like gallium nitride, do not have this advantage. 1.1 DIFFERENT SiC POLYTYPES: In addition to all its useful properties, SiC performs polytypism. Polytypes, which are present in all device materials, are the same in two-dimensional closely-packed planes but different in the stack sequence, which is perpendicular to the plane.
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