CALIFORNIA STATE UNIVERSITY, NORTHRIDGE AN ANALYTICAL TWO-DIMENSIONAL MODEL FOR AlGaN/GaN HEMT WITH POLARIZATION EFFECTS FOR HIGH POWER APPLICATIONS A graduate project submitted in partial fulfillment of the requirements For the degree of Masters of Science In Electrical Engineering. By Swaroop Jallipeta AUGUST 2016 The graduate project of Swaroop Jallipeta is approved: ___________________________________ __________________ Dr. Jack Ou Date ___________________________________ ___________________ Prof. Benjamin Mallard Date ___________________________________ ___________________ Dr. Somnath Chattopadhyay, Chair Date California State University, Northridge ii ACKNOWLEDGEMENT First of all, I would like to extend my sincere thanks to my project chair Dr. Somnath Chattopadhyay for extending his support throughout the project and responsible for completion of the project. I am very thankful for all the motivation and support provided to me in completion of my project. I would like to show my appreciation and thanks to my committee member, Dr. Benjamin Mallard for his cooperation and helpful ideas during the course of my project. I extend my thanks also to the other committee member, Dr. Jack Ou for his support and guidance throughout the project. I am very appreciative for the Department of Electrical Engineering for providing me the infrastructure and all other necessary assistance for the successful completion of my project. Finally, I thank for my parents for all the support, love and encouragement throughout the time I worked on my project. iii TABLE OF CONTENTS SIGNATURE PAGE ii ACKNOWLEDGEMENT iii LIST OF FIGURES vii LIST OF TABLES ix ABSRACT x CHAPTER 1. INTRODUCTION 1 1.1 SEMICONDUCTOR DEVICVES 1 1.2 COMPARISON OF GaN OVER OTHER MATERIALS 1 1.3 ABOUT HEMT 2 1.4 RADIO FREQUENCY (RF) POWER IN GaN DEVICES 3 1.5 FACTORS THAT SUPPORT GAN FOR HIGH FREQUENCY MICROWAVE APPLICATION 3 1.6 OBJECTIVE 4 CHAPTER 2. GALLIUM NITRIDE (GaN) MATERIAL 5 2.1 EVOLUTION OF GALLIUM NITRIDE 5 2.2 MATERIAL PROPERTIES OF GALLIUM NITRIDE 5 2.3 WIDE BANDGAP OF GALLIUM NITRIDE (GaN) 6 2.4 STRUCTURE OF GALLIUM NITRIDE (GaN) 7 2.5 ENERGY BAND STRUCTURES OF GALLIUM NITRIE (GaN) 8 2.5.1 ENERGY BAND STRUCTURE OF ZINC BLENDE GALLIUM NITRIDE (GaN) 8 2.5.2 ENERGY BAND STRUCTURE OF WURTZITE GALLIUM NITRIDE (GaN) 9 iv 2.6 TYPES OF GALLIUM NITRIDE (GaN) 10 2.7 DEFECTS OF GALLIUM NITRIDE (GAN) 11 CHAPTER 3. HEMT 13 3.1 INTRODUCTION 13 3.2 HISTORY OF HEMT 14 3.3 MODES OF HEMT 15 3.3.1 ENHANCEMENT MODE HEMT 16 3.3.2 DEPLETION MODE HEMT 17 3.3.3 FLOURINE BASED PLASMA TECHNIQUE IN E/D MODE HEMT 18 3.4 TYPES OF HEMT’S 19 3.5 CONSTRUCTION AND PRINCIPLE OF HEMT 20 3.6 APPLICATIONS OF HEMT 21 CHAPTER 4. THEORY AND MODEL 22 4.1 AlGaN/GaN HEMT 22 4.2 CONSTRUCTION AND WORKING 22 4.3 POLARIZATION EFFECTS AND 2DEG FORMATION 23 4.3.1 POLARIZATION EFFECTS 23 4.3.2 TWO-DIMENSIONAL ELECTRON GAS (2DEG) FORMATION 25 4.4 DEVICE FABRICATION PROCESSING 27 4.5 THRESHOLD VOLTAGE 30 4.6 CURRENT AND FREQUENCY EQUATIONS v OF AlGaN/GaN HEMT 31 CHAPTER 5. RESULTS AND DISCUSSIONS 34 CONCLUSION 38 REFERENCES 39 vi LIST OF FIGURES FIGURE 1.1: Comparision of GaN over GaAs 2 FIGURE 2.1: Energy bandgap for different semiconductors 7 FIGURE 2.2: Wurtzite (WZ) structure of GaN 8 FIGURE 2.3: Band energy structure of Zinc blende GaN 9 FIGURE 2.4: Band energy structure of Wurtzite GaN 10 FIGURE 2.5: Defects of Gallium Nitride (GaN) 11 FIGURE 2.6 Energy formation of native defects in GaN vs Fermi level charges 12 FIGURE 3.1: AlGaAs/GaAs HEMT structure 13 FIGURE 3.2: AlGaAs/GaAs HEMT energy band diagram 14 FIGURE 3.3: Cross-sectional diagram of E-mode and D-mode HEMT 16 FIGURE 3.4: Schematic diagram for E-mode HEMT 17 FIGURE 3.5: Schematic diagram showing D-mode HEMT 18 FIGURE 3.6: Conduction band diagram for (a): D-mode AlGaN/GaN HEMT (b): E-mode AlGaN/GaN HEMT 19 FIGURE 3.7: Schematic structure of AlGaAs/GaAs HEMT representing 2DEG 21 FIGURE 4.1: Schematic diagram of AlGaN/GaN HEMT 23 FIGURE 4.2: Represantation of polarization charge contribution in the AlGaN/GaN HEMT 24 FIGURE 4.3: Representation of a) Inverse piezoelectric field b) Direct piezoelectric field 25 vii FIGURE 4.4: Diagram representing the increase in barrier thickness with corresponding trap energy states and 2DEG formation 26 FIGURE 4.5: Mesa stucture showing the RIE technique by Ti mask 28 FIGURE 4.6: Sample immersed in acetone to remove the photoresist 29 FIGURE 5.1: Variation of Sheet carrier density (푛푠푑) vs Gate to source voltage (Vgs) 34 FIGURE 5.2: Variation of Sheet carrier density versus AlGaN barrier layer and AlN layer thickness. 35 FIGURE 5.3: variation of frequency (푓ℎ) vs channel length (l) 36 viii LIST OF TABLES TABLE 2.1: Electrical properties of Gallium Nitride (GaN) 6 ix ABSTRACT AN ANALYTICAL TWO-DIMENSIONAL MODEL FOR ALGAN/GAN HEMT WITH POLARIZATION EFFECTS FOR HIGH POWER APPLICATIONS By Swaroop Jallipeta Master of Science in Electrical Engineering The main objective of this graduate project is to develop an analytical model of AlGaN/GaN high electron mobility transistor (HEMT) device for studying the sheet carrier density in the quantum well and cut-off frequency. This analytical model has been developed by using Matlab. The sheet carrier density in the triangular quantum well has been evaluated by the influence of layer thickness of the doped AlGaN and AlN spacer layer as well as the gate-source biasing to understand the quality of heterojunction and carrier transport. The cut-off frequency has been computed to study the effect of channel length on RF performance of the device. The graduate project constitutes the introduction of the project in Chapter 1, Gallium Nitride material in Chapter 2, HEMT material in Chapter 3, Theory and model in Chapter 4 and results and discussions in Chapter 5. x Chapter 1 Introduction 1.1 Semiconductor Devices: Since the 19th century, the most popular semiconductor is silicon. Silicon has many advantages like low cost, reliability, when compared to selenium or germanium, which are available earlier [1]. After silicon, the next important semiconductor material came into picture is Gallium Nitride (GaN). After the invention of the metal-semiconductor devices like MOSFET and MESFET, the semiconductor industry for electronics has been dominated by GaN material [2]. Since then, the transistor brought an advantage to the present day life by its usage in various automation fields. The various present day demands opened to the discovery of many kinds of field effect transistors, which are now obtained in the market. Due to high operative power at high temperatures and frequencies, GaN is widely used for many aerospace and military applications [3]. GaN is also used in various electronic and opto-electronic device applications. Most of the high frequency and high power devices use GaN for its application in the future generation. GaN has a wide bandgap energy of ~3.4 eV compared to Si having 1.12eV at room temperature (300K). 1.2 Comparison of GaN over other materials: When compared with silicon, GaAs based solid state devices, GaN gives three times bandgap, ten time’s higher electrical breakdown qualities, and a great carrier mobility [4]. The five qualities in GaN that made available for microwave and other high range applications are switching and conduction efficiency, cost, size and breakdown voltage [5]. The below Figure 1.1 shows some of the properties and advantages of GaN over GaAs. 1 FIGURE 1.1: Comparision of GaN over GaAs [4] 1.3 About HEMT Due to Gallium nitride high power performance, it is used for fabricating High electron mobility transistor (HEMT). HEMT’s are also known as hetero-structure (HFET) or modulated doped FET (MODFET) [6]. The piezoelectric effect and natural polarization effect accumulates a two dimensional electron gas (2DEG) layer in HEMT. This is utilized by GaN and makes the low on-state resistance as one of its transistor characteristics. It shows an extreme performance as a power device because of the wide bandgap and the high breakdown voltages. Most of the GaN based HEMT’s are best for the solid state power amplifiers at a frequency above 30 GHz. Regardless of the great performance of the GaN HEMT’s in the past, there are still several important issues that to be solved at the millimeter wave 2 frequencies(30-300GHz). GaN HEMT provides a current density of 850 mA/mm and a peak transconductance of 300 mS/mm. It also provides a cut-off frequency of 160GHz [7]. AlGaN/GaN HEMT’s with an 8 GHz cut-off frequency and 9.8 W/mm are fabricated by some of the researches [8]. One of the researches tells that AlGaN/GaN HEMT device with a Silicon Carbide substrate layer attained a cut-off frequency of 25 GHz with DC transconductance of 150 mS/mm and a 50 GHz maximum frequency with a 950 mA/mm drain saturation current from S parameter values done on a 100um HEMT [9]. 1.4 Radio frequency (RF) power in GaN devices: The GaN devices are very important in addressing the high power in microwave devices. It has a velocity of 1.4cm/s which is very optimal and also have a 1500cm2/Vs flat field [10]. For the high power applications, a material having a flat around 1500 cm2/Vs is suitable. Recent day growth and development led the GaN high electron mobility transistors to an endearing force yielding approximately 9.6 W/mm at 8 GHz [11].