An Analytical Two-Dimensional Model for Algan/Gan Hemt with Polarization Effects for High Power Applications

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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.

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

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.

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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

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

OF AlGaN/GaN HEMT 31

CHAPTER 5. RESULTS AND DISCUSSIONS 34

CONCLUSION 38

REFERENCES 39

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

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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

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LIST OF TABLES

TABLE 2.1: Electrical properties of Gallium Nitride (GaN) 6

ABSTRACT

AN ANALYTICAL TWO-DIMENSIONAL MODEL FOR ALGAN/GAN HEMT WITH POLARIZATION EFFECTS FOR HIGH POWER APPLICATIONS

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.

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.

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

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]. Gallium Nitrite FETs possess a dielectric with very low capacitance value and an electron velocity of 3 × 107cm/s. GaN material is widely used for the fabrication of many opto-electronic devices operating at high temperature and frequencies. In some cases, microwave transistors made of GaN materials are seen by admittance and scattering parameters due to their non-linear characteristics [12].

1.5 Factors that support GaN for High frequency Microwave applications:

Some of the military based applications have a great interest in efficient semiconductor material and high power devices based on GaN. The physical impacts and epitaxial layers are easier to decipher and acknowledge [12]. The structure of HEMT is much difficult than the MESFET designed using GaN. For various parameters of GaN, physics based analytical models are needed to use it in the semiconductor electronics which gives a new vision to computer aided digital design of GaN integrated circuits [13].

1.6 Objective:

This work mainly focusses on an analytical two dimensional model for investigating AlGaN/GaN high electron mobility transistor (HEMT) to obtain a peak cutoff frequency and a maximum sheet carrier density values to prove its efficiency and applicability for many radio frequency (RF) and microwave applications when polarization effects are included.

Chapter 2 explains about the Gallium Nitride evolution, various material properties, growth of GaN, Band energy structures and its defects in various circumstances. Chapter 3 reports on history of HEMT, Modes of HEMT, Types of HEMT, operation, principle and applications of HEMT. Chapter 4 focusses on AlGaN/GaN HEMT, polarization effects and 2DEG electron formation in AlGaN/GaN HEMT, and the corresponding equations for sheet carrier density, cutoff frequency and threshold voltage to obtain the peak values for efficient microwave applications.

CHAPTER 2

GALLIUM NITRIDE (GaN) MATERIAL

2.1 Evolution of Gallium Nitride:

Among all the semiconductors, group III nitride-based semiconductors are the main source of material which is used in producing LEDs, laser diodes, high power and temperature electronics in the recent times. One of the revolutions in LED technology, which is responsible for opening up of new markets is mainly because of the high brightness blue INGaN (indium gallium nitride). In 1990s, the historical development and rise of GaN material and its device automation in Japan is noted as the crucial developments in the solid-state devices till date [13]. Many key breakthroughs in material synthesis and fabrication is mainly due to the outstanding advancements in GaN based LED materials. Some of the reasons that slowed down the research of group III nitrides is, due to their weak crystal quality and the insufficient p-n junctions. As a result of these two reasons, the research have been slowed down for decades. In the recent times, the recognition of laser diodes has been taken more than 20 years from the first optical pumped stimulation emission, which can be seen in GaN crystals from where the first laser diodes have been fabricated. In this chapter, the status of the GaN materials and its evolution is summarized [30].

2.2 Material properties of Gallium Nitride (GaN): In n-based materials (AlGaIn), the range of the band gap varies from 1.91 eV in InN to 3.41 eV in GaN to 6.6 eV in AIN, while the band structure is called as the direct band gap across the whole range. Hence it can be concluded that, the group III nitride alloy system is said to be the whole visible range of wavelengths as covered [15]. The high quantum efficiency light emitters are fabricated through direct bandgap in GaN semiconductor material.

The below Table 2.1 displays the electrical properties of Gallium Nitride (GaN):

TABLE 2.1: Electrical properties of Gallium Nitride (GaN) [15]

2.3 Wide Bandgap of Gallium Nitride (GaN):

Room temperature operations can only be done for semiconductor detectors, which has band gap above 1.4eV. Therefore, the semiconductors which has band gap greater than 1.4eV are called as wide bandgap (WBG) [16]. Reduction in generated thermal electrons can be done by WBG, which can be used for high temperature applications. Comparison between the bandgap for different semiconductors can be seen in below Figure 2.1. GaN has the direct band gap in UV region and has an energy (퐸푔) of 3.2eV [17]. Hence it is the

wide band gap semiconductor. Although AIN has the widest band gap of 6.2eV, it has low electron and hole motilities. The lower critical field in SiC excludes it from GaN even both have same properties.

FIGURE 2.1: Energy bandgap for different semiconductors [16]

2.4 Structure of Gallium Nitride (GaN):

GaN combines anion (N) and cation (Ga) and it belongs to group 3-5. Each atom in GaN has a fully filled valence band. It has weak ionic bonding due to the shift of valence charge from N atom to Ga atom. The stable bulk GaN often exhibits a Wurtzite (WZ) structure, which can be cut in various orientations and planes. WZ has a stacking sequence of close-packed (111) planes of ABAB..., and it belongs to C6v space group []. GaN crystal has 3 planes, c-plane (polar plane), m-plane and a plane (non-polar) as shown below in Figure 2.2. Some of the fundamental limitations of convention c-plane can be overcome by non-polar GaN. Increased efficiency, reduced electrical resistance, elimination of color shifting are offered by Non-polar GaN by varying operation current, thereby reducing the device size.

FIGURE 2.2: Wurtzite (WZ) structure of GaN [18]

2.5 Energy band structures of Gallium Nitrie (GaN): In Gallium Nitride (GaN), the energy band structures can be described in two crystal stuctures namely:

 Zinc (Zn) blende crystal structure  Wurtzite crystal stucture

2.5.1 Energy band structure of zinc (Zn) blende GaN:

The below Figure 2.3 displays the band diagram structure of Zinc blende GaN and four various energies observed.

FIGURE 2.3: Band energy structure of Zinc blende GaN [19]

The bandgap energy (퐸푔) of Zn Blende structure in GaN is shown to be 3.2eV from the minimum conduction band and maximum valence band. From the figure above it is seen that, the bandgap ( 퐸푋) value is found to be 4.6eV in <100> orientation. The other bandgap energy (퐸퐿) is found to be around 5eV in <111> orientation. Another band energy called split-off band energy is found to be 0.02eV in valence band.

2.5.2 Energy band structure of Wurtzite Gallium Nitride (GaN):

The below Figure 2.4 displays the band diagram structure of Wurtzite Gallium Nitride (GaN) structure amd five types of energies are observed.

FIGURE 2.4: Band energy structure of Wurtzite GaN [19]

The bandgap energy (퐸푔) of Wurtzite structure in GaN is found to be 3.39eV from the minimum conduction band and maximum valence band. From the figure above it is seen that, the energy seperation bandgap (퐸퐴) value is found to be 4.7-5.5eV. The other seperation bandgap energy (퐸푀−퐿) at M-L valleys is found to be around 5eV. Another band energy called split-off band energy (퐸푠0) is found to be 0.008eV in valence band. The crystal band energy (퐸푐푟) is found to be 0.04eV.

2.6 Types of Gallium Nitride (GaN):

Different varieties of GaN can be various growth methods, that can be cataloged a unintentionally doped (UD), semi-insulating (SI) and highly-doped (HD) GaN. The background carrier concentration of 1016 푐푚−3 is typically found in GaN, which may be caused by nitrogen vacancies or oxygen impurities [20]. Due to the bulk leakage current under a large bias voltage, this value may become a little higher for device application. In order to obtain the semi-insulating GaN, a number of impurities like iron are chosen to compensate the shallow donors, which in turn migrates the relatively high-intrinsic concentration of free carriers in UD GaN [21]. High temperature annealing is required to activate donors and also as-growth and ion implementation is used to dope Fe atoms.

One of the problem is, deep level trapping centers will be formed which decreases the charge collection efficiency of the detector. This is due to the addition of the impurities [22]. To form ohmic contact highly doped GaN uses Silicon, which increases the carrier concentration. The reason why silicon is chosen because, it forms a fast shallow donor level which makes the Si allowing for complete donor activation at room temperature. One more reason for choosing silicon is due to the solubility of silicon in GaN is comparatively high. After high temperature activation it has a dopant redistribution of 1020 푐푚−3 [23]. 2.7 Defects of Gallium Nitride (GaN): There are various kinds of defects that deeply effect the physical properties of Nitride materials at certain circumstances. Some of the most common defects that arises during the growth of GaN material are point defects, pit defects, scratches and micro-pipes [24]. Some of the defects mentioned can be cleaned by fabrication process and by ultrasonic bath. The point defects which is caused by foreign particles can be removed by changing electrical properties and its concentration. These defects causes the early breakdown and effects the behavior of the device. The defects mentioned above is shown in Figure 2.5.

FIGURE 2.5: Defects of Gallium Nitride (GaN) [24]

The Nitride materials like GaN and AlN faces other forms of defects like native defects due to the lattice mismatch and disloctions with the susbstrate layers which causes a huge damage to the device performance as well as stucture [25]. These native defects are formed by the impurity doping which acts as the source of compensation and reduces the optical performance of the device. The process of annealing can also cause native defects. The energy formation of native defects in GaN with reference to the Fermi level charges are

shown below in Figure 2.6. From the Figure, it is distinctly seen that the condition of Ga is on the top and considered to be rich compared with other semiconductor materials.

FIGURE 2.6 Energy formation of native defects in GaN vs Fermi level charges [26]

CHAPTER 3

HEMT

3.1 Introduction

HEMT is abbreviated as High-electron-mobility transistor besides known as HFET or MODFET. HEMT’s are most widely used for low noise and high power applications at a millimeter range frequency [27]. HEMT’s provides high levels of performance at microwave based circuits. HEMT’s are not new to the semiconductor industry, but they are coming forward due to their performance in communication applications [32]. The acting techniques are particularly expected to proceed with the advancement of HEMT’s in the recent years [28].

In the first generation, the HEMT was designed based on AlGaAs/GaAs FET’s which is used to study widely on RF design and many microwave applications [3]. The basic structure of HEMT is the conduction band offset between the AlGaAs and GaAs layers. By introducing a two semiconductor materials with different bandgaps, a potential well is shaped at hetero-junction between barrier layer and channel layer, thereby forming two dimensional electron gas (2DEG) because of the higher conduction band in the barrier layer and the lower conduction band in the channel layer. The electrons in the 2DEG accumulates at the doped barrier layer, which reduces the scattering between the carrier acceptors and the ionized donors. The schematic structure of the AlGaAs/GaAs and its corresponding energy band diagram is shown below in Figure 3.1 and Figure 3.2 respectively.

FIGURE 3.1: AlGaAs/GaAs HEMT structure

FIGURE 3.2: AlGaAs/GaAs HEMT energy band diagram [30]

3.2 History of HEMT:

The standard electronic component used for carrier transport in HEMT was first investigated in 1969, but did not come into light until March 1979 that was the first experimental device available for RF (Radio-frequency) design project [31].

Working individually, three scientists Ray Dingle, Daniel Delegebeaudeuf and Trong Linh Nuyen at Bell laboratories, NJ were the first to show the high mobilities in modulation doped heterojunctions in the year 1978. These materials continued to receive a name as hig speed transistors which has the switching times in the order of 10 PS in digital

electronic applications. Later the researchers at University of Illinois found these materials are capable of low noise operations at frequencies maximum up-to 60 GHz by 1985.

In view of the initial very high cost HEMT’s use are considerably limited. In the recent years, materials are available with their cost somewhat less, the use of HEMT’s are increasing. Due to the millimeter wave frequencies, HEMT’s are widely used in products such as radar equipment, mobile telecommunications and satellite television receivers. HEMT’s are also widely used in space technologies and many other RF design applications.

3.3 Modes of HEMT:

HEMT’s operate in two biasing modes namely:

1. Enhancement mode HEMT 2. Depletion mode HEMT

The improvement of Enhancement and Depletion mode HEMT’s on lattice matched sapphire substrate is of extensive hobby and hold guarantee for superfast and low power applications [33]. Circuits using such Enhancement or depletion mode HEMT technology offer point of interest employing only Depletion technology. The wide band gap of GaN suits for low power electronic applications but requires only Enhancement mode HEMT devices. Accompanying an E/D technology, the elimination of level changing stages is done in the circuit resulting in the low power consumption of the circuit. So the utilization of single power supply for circuits is acknowledged in E/D technology.

For the AlGaN HEMT, a thinner AlGaN layer has been used for Enhancement mode compared to the Depletion mode HEMT. The use of thinner barrier layer in some cases is used both for Enhancement and Depletion modes by altering the barrier layer doping. Enhancement mode HEMT’s are difficult to demonstrate compared to the Depletion mode HEMT’s, as E-mode induce a very smaller amount of charge carriers [34]. Also a continuous AlGaN etch stop layer is used in D-mode HEMT compared to the E-mode HEMT as shown in the Figure 3.3.

FIGURE 3.3: Cross-sectional diagram of E-mode and D-mode HEMT [34]

3.3.1 Enhancement mode HEMT:

The Enhancement mode HEMT is made available only when the positive voltage is supplied for circuit applications bringing about simplified circuits and lessened expenses. At zero gate bias, the transistor does not conduct current in the E-mode and is called to be “normally-off” mode. A thin AlGaN barrier layer is used in E-mode HEMT followed by an un-doped GaN layer and a sapphire or SiC substrate layer [35]. Because of the Schottky barrier between gate and AlGaN barrier layer, drain-source region can pinched off at zero bias due to the thin barrier layer supplied. To produce Enhancement mode with low resistance and to make the transistor to conduct current, a self-manufacturing process is needed in which the channel region bellow the electrodes should also maintain Enhancement mode. Due to the reduction of negative voltage supply in E-mode, the difficulty of the circuit and the price is reduced.

The sample Figure 3.4 for the Enhancement mode HEMT is shown below:

FIGURE 3.4: Schematic diagram for E-mode HEMT [35]

3.3.2 Depletion mode HEMT:

The Depletion mode is made available at the negative voltage supplies and usually the threshold voltage is noted as negative due to the high density 2-DEG induced by polarization effects. At Zero gate bias, the transistor is capable of conducting current in the Depletion mode and is called to be “normally-on” mode. Most of the power amplifiers using AlGaN/GaN HEMT features mostly in D-mode due to the negative threshold voltage [36]. In the D-mode HEMT, as the gate voltage diminishes from zero, the most extreme current is noted at this point. The AlGaN barrier layer is placed followed by unintentionally doped GaN channel layer and a semi-insulating GaN layer as shown in Figure 3.5. A 2DEG electron layer is formed between AlGaN layer and UID GaN layer as shown below. A sapphire layer or a SiC layer is placed below the semi-insulating GaN layer.

FIGURE 3.5: Schematic diagram showing D-mode HEMT [41]

3.3.3 Fluorine based plasma technique in E/D mode HEMT:

In the recent years, a fluorine based plasma technique is used for obtaining high performance in E-mode AlGaN/GaN HEMT. There is no need for changing of barrier layer thickness in obtaining E/D modes. Due to the energy band modulation by fluorine ions on the AlGaN/GaN hetero-structure layer, the threshold voltage can be maintained. As the fluorine ions are negatively charged, the potential created between the AlGaN barrier layer and the GaN channel layer is gradually raised, resulting a positive threshold voltage and thus Enhancement mode HEMT is fabricated [37].

According to our recent observations, the fluorine ions which are incorporated by CF4 plasma treatment on-to the barrier layer introduces a deep energy level state at the middle of the band-gap. Due to the formation of new energy state, the fluorine ions gives acceptor states which are negatively charged in the barrier layer resulting in the inclination of the conduction band on the AlGaN barrier layer as shown in the Figure 3.6 below. In the Enhancement mode operation, the negatively charged ions increase the height of conduction band promoting to the external barrier height [38].

FIGURE 3.6: Conduction band diagram for (a): D-mode AlGaN/GaN HEMT

(b): E-mode AlGaN/GaN HEMT [38]

3.4 Types of HEMT’S:

As per the lattice constant utilized in the semiconductor layers of HEMT, three types of HEMT’s are distinguished namely:

 Lattice-matched HEMT (LHEMT)  Pseudomorphic HEMT (PHEMT)  Metamorphic HEMT (MHEMT)

Latice-matched HEMT:

In the Lattice-matched HEMT, the same lattice constants of the semiconductor layers are employed on both sides of the heterostuctures. Some of the examples of lattice matched HEMT (LHEMT) are AlGaAs/GaAs, AlInAs/InGaAs/InP

Pseudomorphic HEMT:

In the Pseudomorphic HEMT, a slightly different lattice constants of the semiconductor layers are utilized on the two sides of heterostuctures, resulting in crystal defects. The PHEMT mainly focusses on epitaxial growth development. The PHEMT devices exhibit high performance in microwave applications. Due to the high range of applications, the microwave industries are developing PHEMT manufacturing to increase

demand for microwave products using PHEMT technology. Some of the examples of pseudomorphic HEMT (PHEMT) are AlInAs/InGaAs/GaAs, AlInSb/InSb [39].

Most of the PHEMT structures contain a super-lattice structure which is the grouping of undoped layer to form a thicker epitaxial layer to exhibit substrate conduction. In the AlGaN/GaN PHEMT the AlGaN layer has the larger bandgap compared to the channel GaN layer. A thin GaN buffer layer is grown on the top to lessen the thickness of the AlGaN layer, thus reduces the thickness and allows another AlGaN layer to be grown. This process is repeated for about 12 to 17 times to make sure a thick AlGaN buffer layer is formed.

Metamorphic HEMT:

In the Metamorphic HEMT, the lattice constants of the semiconductor layers utilized on the two sides of heterostuctures are different. So a buffer layer is developed in middle of the layers to reduce the complexity. This is an advancement to the PHEMT. In MHEMT devices, a proper lattice graded buffer layer is developed in middle of the substrate and the active layer to eliminate the strain caused by Indium layers on the channel (GaAs or GaN) layer. The buffer layer provides the ability to suit the lattice constant to the Indium layer and thus allowing to optimize transistors for higher frequency and millimeter wave applications [40]. The Metamorphic buffer layer serves to trap dislocations and to prevent from growing into the device channel. Some of the examples of metamorphic HEMT (MHEMT) are AlGaAs/InGaAs/GaAs, SiGe/Si

3.5 Construction and principle of HEMT:

The basic HEMT structure has a semi-insulating GaAs substrate on which a thin intrinsic layer of GaAs layer is placed, followed by an un-doped AlGaAs layer and a doped AlGaAs layer. This formation is proposed to ensure the separation of hetero junction interface from a doped AlGaAs substrate. Finally a thin GaAs layer is placed below. The transistor finally deposits aluminum to form a shottky contact in the middle which as a gate to the transistor, and the current flows from one ohmic contact to other ohmic contact which serves as source and drain.

In the HEMT, the heterojunction electrons are transferred from a semiconductor material having a higher conduction band energy to the semiconductor material having a

lower conduction band to maintain a low energy state. Then the electrons from donors move to the acceptors creating a potential well. Due to the potential created between the two semiconductor layers, a two-dimensional electron gas (2DEG) layer is formed above the AlGaAs layer as shown in the Figure 3.7 to increase the mobility of the conducting electrons and to reduce the coulomb scattering.

FIGURE 3.7: Schematic structure of AlGaAs/GaAs HEMT representing 2DEG [42]

3.6 Applications of HEMT:

 Attractive for integration into monolithic microwave devices (M.M.I.C)  Highly used in military equipment for both hybrid and monolithic products.  Used in constructing low noise amplifiers for frequencies up-to 100GHz  Useful in field of cellular telecommunications.  Major role in spacer applications.  High levels of performance in RF design and Microwave based circuits.  Helps in Direct Broadcast Receivers and radio astronomy

CHAPTER 4

THEORY AND MODEL

4.1 AlGaN/GaN HEMT:

The semiconductor based HEMT’s are fabricated using wide bandgap materials which came into view as outstanding applicants for high power and many microwave operations. In the recent years, AlGaN/GaN material system which has wide bandgap are largely used for RF applications and in Power amplifiers. The AlGAN/GaN semiconductor materials has peak characteristic values of breakdown field, saturation drift velocity and also has peak sheet carrier densities. Due to the small bandgap energy in the GaAs, GaN is the replacement to GaAs in power amplifiers in the recent years. The GaN HEMT is slightly similar to the typical MESFET’s that utilizes a gate terminal to control the flow of the current. Recently, AlGaN/GAN hetero-structures are grown on Sic substrate by metalorganic vapor channel deposition (MOVCD) technology.

4.2 Construction and working:

The HEMT structure consists of un-doped AlGaN fine layer, followed by a Si doped AlGaN layer, then followed by another un-doped AlGaN layer grown on a semi-insulating un-doped buffer layer of GaN, followed by an AlN nucleation layer. Finally a substrate layer of SiC is grown as shown below in figure 4.1. The spacing from source to gate is said to be ‘x’ along drain side and the respective channel potential is calculated at a distance ‘x’. The nucleation layer of AlN is grown to reduce the lattice discrepancy linking the un- doped GaN buffer layer and the SiC substrate layer. The semi-insulating GaN buffer layer is placed to avoid conduction and reducing the current leakage across the transistor. The transistor finally deposits Aluminum to form a schottky contact in the middle which carry out as a gate and the current runs across two ohmic contacts which serves as source and drain to the transistor.

In the GaN based HEMT’s, the semiconductor materials having different bandgaps like AlGaN and GaN materials are chosen which serves as a barrier layer and channel layer respectively. The heterojunction electrons are then transferred from AlGaN material having higher energy in the conduction band to the GaN having lower energy in the conduction

band, thereby creating a potential well at the hetero-interface between the two semiconductor materials [41]. Due to the potential created at the hetero-interface, a two- dimensional electron gas (2DEG) is formed as shown below in Figure 4.1. The two dimensional electron movement in the transistor was introduced mainly to improve the electron mobility.

FIGURE 4.1: Schematic diagram of AlGaN/GaN HEMT [30]

4.3 Polarization effects and 2deg formation:

4.3.1 Polarization effects:

Nitride based materials remains in Wurtzite crystal structure and Zinc (Zn) Blende crystal structure. Because of the peak thermal conductivity and high breakdown field effects in GaN, polarization plays an important role in GaN based HEMT’s.

The Wurtzite (WZ) crystal structure of GaN semiconductor material combined with the epitaxial growth is mainly accountable for the existence of piezoelectric field in the GaN HEMT. AlGaN and GaN semiconductor materials possess high piezoelectric fields. The asymmetric nature and non-zero dipole moment in the GaN material results in the spontaneous polarizations since they occur without applying any stress or electric field across the transistor [43]. Due to the stress occurred by the lattice discrepancy linking the AlGaN and GaN materials, a strain is developed creating piezoelectric polarizations near the AlGaN layer but not at the GaN layer due to its thickness. The +ve electrostatic charges and the –ve electrostatic charges are available at the AlGaN/GaN hetero-interface the

AlGaN layer respectively due to the above described polarization effects. This results in the addition of difference in the spontaneous polarization linking the two semiconductor materials as shown below in Figure 4.2

FIGURE 4.2: Represantation of polarization charge contribution in the

AlGaN/GaN HEMT [43]

The polarization of AlGaN material is higher than that of the polarization of GaN material because of the larger polarization constants. The resulting polarization difference gives out a sheet charge density at the meeting place which is positive. The electron compensation is done in the cooling process forming a 2DEG electron layer formation. The corresponding negative charge at the AlGaN layer gives out a -ve sheet charge density −훼 to make the structure neutral without applying any external electric field [44].

+훼 = 푃푠푝(퐴푙퐺푎푁) + 푃푝푧(퐴푙퐺푎푁) − 푃푠푝(퐺푎푁) (4.1)

The AlGaN and GaN materials are capable of exhibiting inverse/converse piezoelectric effect. The straight piezoelectric effect is observed when the semiconductor material experiences an electric charge as a mechanical stress applied to it. The Inverse/converse piezoelectric field is observed when the electric field is applied across the device by applied gate bias or drain bias. The direct and inverse piezoelectric fields are shown below in Figure 4.3

FIGURE 4.3: Representation of a) Inverse piezoelectric field b) Direct

piezoelectric field [44]

4.3.2 Two-dimensional electron gas (2DEG) formation:

Two dimensional electron gas (2DEG) is a set of electrons covered in a plain layer linking the two different semiconductor materials. The 2DEG structure is most important because it lays mostly in the molecular beam epitaxy (MBE) technology which is most suitable for an accuracy of the single layer. In the HEMT, the 2DEG region is of complex nature and is defined to the concept of quantum well. Anyways the 2DEG is formed across the transistor with no application of any exterior electric field [43].

In the AlGaN/GaN HEMT, the conduction band offset transfers the free electrons from the AlGaN barrier layer having higher conduction band energy to the GaN channel layer having lower conduction band energy, thereby creating layer of electrons known to be the two dimensional electron gas (2DEG) formation. The 2DEG formation also rely on the critical width of the buffer layer and the trap states arises due to the 2DEG formation [44]. The top side trap states are formed from the 2DEG are responsible for creating positive charge which compensates the induced -ve polarization charge at the top side of the AlGaN buffer layer.

FIGURE 4.4: Diagram representing the increase in barrier thickness with

corresponding trap energy states and 2DEG formation [45]

The energy traps also refers to the semiconductor materials energy bandgap and its dislocation in parts. From the Figure 4.4 it is shown that, the energy traps are below the fermi energy level having thicker AlGaN layer compared to the thinner AlGaN layer. So it is observed that the trap energy states are located deep in the AlGaN bandgap. At the time that the width of the AlGaN barrier layer increases to a certain extent, the electrons obtain sufficient energy to leave the traps. Then the free electrons are driven for the polarization induced barrier layer to the channel layer creating (2DEG) formation.

The sheet carrier density (2DEG) for AlGaN/GaN HEMT is given as:

£(푚) 푛푠푑(푚, 푥) = (푣푔푠 − 푉푇퐻 − 푣푐푝(푥)) (4.2) 푄(푑푎+푑푏+ώ푑)

Where,

£(푚) = Dielectric constant of AlGaN/GaN HEMT

푛푠푑(푚, 푥) = Sheet carrier density

푄 = Electronic charge

푑푎 = Width of the doped AlGaN layer

푑푏 = Width of the undoped AlGaN layer

ώ푑 = Thickness of the 2DEG

푚 = Aluminum mole fraction

푥 = source-gate spacing

푣푔푠 = Gate-source voltage

푣푡ℎ = Threshold voltage

푣푐푝(푥) = Channel potential at a distance x

4.4 Device fabrication processing:

The AlGaN/GaN HEMT fabrication process is carried out in many steps as described:

 Mesa Insulation  Metallization  Ohmic contacts  Schottky contacts  Passivation layer

Mesa insulation:

Mesa insulation is the process of creating an island of active layer on the device structure to interrupt the conductive 2DEG electron layer which provides the electrical insulation between the two neighbor structures. This process take place with respect to the position of 2DEG layer. The etching process finds difficult in the GaN material compared to the other semiconductor material because of the robust bond energies existing in the nitride materials. The beginning of the etching process is performed by choosing the resist mask to be placed on top of the semiconductor material. Due to the low etch rate of the GaN, the

wet etching is not applicable in finding an appropriate mask. So the dry etching process have begun to be the supreme technique for GaN material [46].

The Reactive ion etching (RIE) and Inductively coupled plasma (ICP) are commonly used techniques for dry etching process. The major variation linking the two techniques is that, RIE uses a single RF plasma tool to manage both energy of the ion and its density whereas ICP uses a separate plasma tool to manage ion energy and its density. Different gases like C푙2/C퐻4 /Ar are used as plasma to the techniques. Different combinations of gases are used to obtain the high etch rate. The RIE technique with Ti metal is used as mask, we get spikes with uneven etch profile as shown below in Figure 4.5.

FIGURE 4.5: Mesa stucture showing the RIE technique by Ti mask [46]

Metallization:

Metallization is also an important step in Device fabrication processing. The metallization process is carried out in different ways as Electron beam evaporation, sputtering and electron plating. Once the lithography process is carried out, the sample is placed in metallization machine where electron beam evaporation takes place. Once this process is finished, the sample is placed in a beaker of acetone to remove the resist layer and leaves only the metal which is directly evaporated in this process as shown below in Figure 4.6.

FIGURE 4.6: Sample immersed in acetone to remove the photoresist

Ohmic contacts:

Good ohmic contacts plays a crucial role in HEMT’s device high performance. Due to the high current and small voltage seen in HEMT, it is important to create an ohmic contact with low resistance. Due to the high bandgap energy in AlGaN material on the surface, it is arduous to create sturdy ohmic contact with the 2DEG layer. In the HEMT structure, the current flows across the two ohmic contacts which serves as sorce and drain to the transistor. The multilayer metals like Ti/Al/Ni/Au used as ohmic contacts shows good contact properties for AlGaN and GaN semiconductor materials. The upper Au layer creates contact with the outside world. The lower two layers (Ti/Al) creates contact with the metal and the semiconductor layer. The Ni layer creates a separation layer and prevents Au layer for penetration to contact inside.

Schottky contacts:

Schottky contact is a controlled formation for obtaining low leakage current and forming high barrier height across the gate. Schottky contacts with low gate lengths have to be fabricated under lithography process to achieve higher reproducibility. To obtain a large schottky barrier height, the metals with large work functions like Pt,Ni and Au are used as metal Schottky contacts. Pt settled down on the GaN material shows perfect Schottky performance of 1.01eV barrier height whereas Ni gives a barrier height of 0.6- 0.99eV. Thermal stability is also an important factor for AlGaN/GaN HEMT and for actual device working. The thermal stability limits for the commonly used metals like Pt/Au/Ni as Schottky contacts is in between 300-700°C.

Passivation layer:

The Passivation layer is used to reduce the parasitic effects leading to current collapse as well as to enlarge the device presentation. The device has been settled down by plasma enhancement with chemical vapor deposition (PECVD). Gases like Si푂2, SiN are used as plasma in this device. Accurate deposition process has to be done and should be smaller to achieve high performance of the device. Finally, the sample should be covered for passivation after the deposition.

4.5 Threshold voltage:

To modify the threshold voltage in the AlGaN/GaN HEMT, gate recess process is used which is to lower the width of the barrier layer placed below the gate metal. By this technique, the threshold voltage can be moved from negative to positive that is changing the device from D-mode to E-mode. The polarization charges also come into account in calculating the threshold voltage for the conventional AlGaN/GaN HEMT.

The threshold voltage including the Polarization charge effects can be calculated by:

휓푏 푑ⱷ ¥퐸푐 퐸푓푐 푄 푑 푥 푄푑푛푆푡 푄푛퐵푡 푉푇퐻 = − − + − ( ) ∫ 푑푥 ∫ 푛푆𝑖(푥) 푑푥 − − (4.3) 푄 £ 푄 푄 £ 0 0 £ 푐퐵푐

Where,

휓푏 = Schottky barrier height of the metal-semiconductor

ⱷ = (Spontaneous + piezoelectric) polarization charge

¥퐸푐 = Conduction band discontinuity

퐸푓푐 = Difference of the fermi level and the conduction band of GaN

푑 = AlGaN layer thickness

푛푆𝑖(푥) = Silicon doping concentration

푛푆푡 = Total charge surface traps per unit area

푛퐵푡 = Total charge buffer traps per unit area

푐퐵푐 = Buffer-channel capacitance per unit area.

4.6 Current and frequency equations of AlGaN/GaN HEMT:

The channel potential 푣푐푝(푥) is calculated as:

푣푐푝(푥) At x=0 퐼푑−푠(푚)푟푠푝

푣푐푝(푥) At x=l 푣푑−푠 − 퐼푑−푠(푚)(푟푠푝 + 푟푑푝)

Where,

퐼푑−푠(푚) = Drain-source current equation for the linear region.

The drain to source current equation for the linear region is calculated as:

2 −푧2+ √(푧2) −4푧1푧3 퐼푑−푠(푚) = (4.4) 2푧1

Where,

2 (2푟푠푝+푟푑푝) 퐶0((푟푑푝) +2푟푠푝푟푑푝) 푧1 = − 퐷1 2

푣푑−푠 푧2 = 퐶0((푣푑−푠(푟푠푝 + 푟푑푝) − 푣푔푠́ (2푟푠푝 + 푟푑푝)) − 푙 − ) 퐷1

(푣 )2 푧 = 퐶 (푣 ́ 푣 − 푑−푠 ) 3 0 푔푠 푑−푠 2

퐸푐푓푣푠 퐷1 = ⱷ0퐸푐푓−푣푠

푤ⱷ0휀(푚) 퐶0 = (푑푑+푑푖+훥푑)

푘 푇 푣 ́ = 푣 − 푣 − 푏 푔푠 푔푠 푡ℎ 푞

푟푠푝 = Parasitic source resistance

푟푑푝 = Parasitic drain resistance

푤 = Channel width

푙 = Channel length

푣푑−푠 = Applied drain to source voltage

퐸푐푓 = Critical electric field

ⱷ0 = Low field mobility

The Drain to source current equation for the saturation region is calculated as:

퐶0퐸푐 퐼퐷푠푎푡 = (푣푔푠 −́푉퐷푠푎푡) (4.5) (푑푎+푑푏+ώ푑)

Where,

퐼퐷푠푎푡 = Drain saturation current

푉퐷푠푎푡 = Drain saturation voltage

The Drain to source voltage equation for the saturation region can be calculated as:

2 −푦2+ √(푦2) −4푦1푦3 푉퐷푠푎푡 = (4.6) 2푦1

Where,

2 2 (퐶0퐸푐푓) 3 퐸푐푓 2 ⱷ0퐸푐푓−푣푠 1 푦1 = (2푟푠푝 + 푟푑푝) − (퐶0) ( ) (푟푑푝 + 2푟푠푝푟푑푝) + 퐶0 ( − ) − 퐷1 2 푣푠 2 2 퐶0 퐸푐푓(푟푑푝 + 푟푠푝)

2 2 ⱷ0퐸푐푓푣푔푠́ (퐶0퐸푐푓) 3 퐸푐푓 2 푦2 = 퐶0 ( + 퐸푐푓푙) − 2푣푔푠́ ( (2푟푠푝 + 푟푑푝) − (퐶0) ( ) (푟푑푝 + 푣푠 퐷1 2

2 ́ 2푟푠푝푟푑푝)) + 퐶0 퐸푐푓푣푔푠(3푟푠푝 + 2푟푑푝)

2 2 (퐶0퐸푐푓) 3 퐸푐푓 2 2 푦3 = (2푟푠푝 + 푟푑푝) − (퐶0) ( ) (푟푑푝 + 2푟푠푝푟푑푝) ∗ 푣푔푠́ − 퐶0푣푔푠́ 푙퐸푐푓 − 퐷1 2 2 (퐶0푣푔푠́ ) 퐸푐푓(2푟푠푝 + 푟푑푝)

As the Aluminum content increases in the AlGaN layer, piezoelectric charges increases, evolving in the increase of trans-conductance and frequency.

The frequency (푓ℎ) equation of AlGaN/GaN HEMT can be calculated as:

푔푎ⱷ0 푓ℎ = (4.7) 2휋푙퐶0

Where,

푔푎 = Transconductance of AlGaN/GaN HEMT

푑퐼푑푠 푔푎 = (4.8) 푑푣푔푠

By differentiating the above equation at constant 푣푑푠

We have,

1 1 푔푎 = (−퐶0(2푟푠 + 푟푑) + 2 (2푧1(−퐶0(2푟푠 + 푟푑)) − 4퐶0푧2푣푑푠)) (4.9) 2푧2 2√푧1 −4푧2푧3

Where,

The values of 푧1, 푧2, 푧3, 퐶0, 푟푠 푎푛푑 푟푑 are explained above at the linear drain current equation.

CHAPTER 5

RESULTS AND DISCUSSIONS

FIGURE 5.1: Variation of Sheet carrier density versus Gate to source voltage

FIGURE 5.1: Variation of Sheet carrier density (풏풔풅) vs Gate to source voltage (Vgs)

The Figure 5.1 shown above plot is the Sheet carrier density (푛푠푑) versus gate to source voltage (Vgs) for various drain to source voltages of 2v, 4v, 6v with gate length −4 −4 (Lg) of 0.12 × 10 푐푚 , gate width (W) of 100 × 10 푐푚, a saturation velocity (푉푠푎푡) of 6 18 −3 9 × 10 푐푚/푠, doping concentration (푁푑 of 5 × 10 푐푚 and a threshold voltage of -3v. The sheet carrier density linearly increases with the gate-source biasing range of -3v to

+3v. From the above graph it is clearly seen that maximum sheet carrier density of 1.7 × 1013푐푚−2, 1.3 × 1013푐푚−2, 1.1 × 1013푐푚−2 at maximum Vgs have been observed for drain-source voltage (푉푑푠) of 6v, 4v and 2v respectively. The graph above is simulated and computed through MATLAB represented from the equations (4.2).

FIGURE 5.2: Variation of Sheet carrier density versus AlGaN barrier layer

and AlN layer thickness.

FIGURE 5.2: Variation of Sheet carrier density versus AlGaN barrier layer

and AlN layer thickness.

The Figure 5.2 shown above plot is the Sheet carrier density (푛푠푑) versus AlGaN barrier layer and AlN layer thickness (d) with gate length (Lg) of 0.12 × 10−4푐푚 , gate width (W) of 100 × 10−4푐푚, a saturation velocity (vsat) of 9 × 106푐푚/푠, a doping concentration

18 −3 (Nd) of 5 × 10 푐푚 , rain-source voltage (푉푑푠) and threshold voltage (푉푇퐻) of -3v. The sheet carrier density in the quantum well exponentially increases with doped AlGaN layer and AlN spacer layer in the range from 1 × 10−8푐푚 to 8 × 10−8푐푚. The maximum sheet carrier density of 1.79 × 1013푐푚−2 in the quantum well has been observed for layer thickness of doped AlGaN and AlN spacer layer of 8 × 10−8푐푚. The graph above is simulated and computed through MATLAB represented from the equations (4.2).

FIGURE 5.3: variation of frequency versus channel length.

FIGURE 5.3: variation of frequency (풇풉) vs channel length (l).

The Figure 5.3 shown above represents the plot of Frequency (푓ℎ) versus channel length

(l) for different layer thickness of doped AlGaN and AlN spacer (d=푑푎 + 푑푏) values of 5 × 10−8푐푚, 10 × 10−8푐푚, 15 × 10−8푐푚 with gate length (Lg) of 0.12 × 10−4푐푚, gate

−4 6 width (W) of 100 × 10 푐푚, saturation velocity (푉푠푎푡) of 9 × 10 푐푚/푠, doping 18 −3 concentration (푁푑) of 5 × 10 푐푚 and 푉푑푠 of 8v. From the plot it is clearly seen that maximum Frequencies value of 210퐺퐻푧, 150퐺퐻푧, 80퐺퐻푧 has been observed for different layer thickness of doped AlGaN and AlN spacer layer (d) values of 5 × 10−8푐푚, 10 × 10−8푐푚, 15 × 10−8푐푚. The cut-off frequency linearly decreases with increase of the channel length from 0.4 × 10−4푐푚 푡표 1.6 × 10−4푐푚. The graph above is simulated and computed through MATLAB represented from the equation (4.7).

CONCLUSION

An analytical model of AlGaN/GaN high electron mobility transistor (HEMT) device has been developed for studying the sheet carrier density in the quantum well and cut-off frequency. The optimization of gate-source biasing and layer thickness of doped AlGaN and AlN spacer layer have been conducted to obtain maximum value of sheet carrier charge density in the quantum well. In order to achieve the maximum cut-off frequency, the cut- off frequency has been evaluated by variation of channel length and the resulted cut-off frequency is obtained in order of 210GHz for the channel length of 0.5µm. This research work is an exploration of high frequency applications in microwave industry using AlGaN/GaN HEMT with AlN nucleation layer and SiC substrate layer. Therefore, this research work is having great potential scope for future valuable research on polarization and piezo electric charge with the combination of sheet carrier density in the quantum well.

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[38]. X. Hu, G. Simin, J. Yang, M. A. Khan, R. Gaska, and M. S. Shur, “Enhancement mode AlGaN/GaN HFET with selectively grown pn junction gate,” Electron. Lett., vol. 36, no. 8, pp. 753-754, 2000.

[39]. HEMt’s and PHEMT’s by Laucoin.

[40]. Metamorphic HEMT technology for microwave millimeter wave and submillimeter wave applications by James.J. Komiak, Phillip, M.smith.

[41]. M. A. Khan, J. M. Van Hove, J. N. Kuznia, and D. T. Olsen, “High electron mobility GaN/AlxGa1-xN heterostructures grown by low-pressure metalorganic chemical vapor deposition,” Appl. Phys. Lett., vol. 58, no. 21, pp. 2408-2410, May 1991.

[42]. Improved performance of AlGaN/GaN HEMT by 푁2표 plasma treatment by Mimin Han, Zhang Kai, Zhao Sheng-Lei.

[43]. Comparision of AlGaN/GaN and AlGaAs/GaAs based HEMT device under doping consideration by Sana Firoz, Ric Chauhan.

[44]. Two Dimensional electron gas (2DEG) systems by Mohamad Mir, Babak Laghighi.

[45]. Design and fabrication of AlGaN/GaN HEMT’s with high breakdown voltages by University of Glasgow.

[46]. Fabrication and characterization of AlGaN/GaN high electron mobility transistors by Vorgelegt Von.