docRF-to-DC CHARACTERISTICS OF AlGaAs/GaAs HEMT SCHOTTKY
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RF-to-DC CHARACTERISTICS OF AlGaAs/GaAs HEMT SCHOTTKY
RF-to-DC CHARACTERISTICS OF AlGaAs/GaAs HEMT SCHOTTKY DIODE FOR RECTENNA APPLICATION KHAIRUL HUDA BINTI YUSOF UNIVERSITI TEKNOLOGI MALAYSIA PSZ 19:16 (Pind. 1/07) UNIVERSITI TEKNOLOGI MALAYSIA DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT Author’s full name : KHAIRUL HUDA BINTI YUSOF Date of birth : 26 AUGUST 1984 Title : RF-to-DC CHARACTERISTICS OF AlGaAs/GaAs HEMT SCHOTTKY DIODE FOR RECTENNA APPLICATION Academic Session: 2010/2011(2) I declare that this thesis is classified as : √ CONFIDENTIAL (Contains confidential information under the Official Secret Act 1972)* RESTRICTED (Contains restricted information as specified by the organisation where research was done)* OPEN ACCESS I agree that my thesis to be published as online open access (full text) I acknowledged that Universiti Teknologi Malaysia reserves the right as follows : 1. The thesis is the property of Universiti Teknologi Malaysia. 2. The Library of Universiti Teknologi Malaysia has the right to make copies for the purpose of research only. 3. The Library has the right to make copies of the thesis for academic exchange. Certified by : SIGNATURE 840826-14-6228 (NEW IC NO. /PASSPORT NO.) Date : 20th MAY 2011 NOTES : * SIGNATURE OF SUPERVISOR ASSOC. PROF. DR. ABDUL MANAF BIN HASHIM NAME OF SUPERVISOR Date : 20th MAY 2011 If the thesis is CONFIDENTIAL or RESTRICTED, please attach with the letter from the organisation with period and reasons for confidentiality or restriction. “I hereby declare that I have read this thesis and in my opinion this thesis is sufficient in terms of scope and quality for the award of the degree of Bachelor of Engineering (Electrical-Microelectronic)” Signature : .................................................... Name : ASSOC. PROF. DR. ABDUL MANAF BIN HASHIM Date : 20th MAY 2011 RF-to-DC CHARACTERISTICS OF AlGaAs/GaAs HEMT SCHOTTKY DIODE FOR RECTENNA APPLICATION KHAIRUL HUDA BINTI YUSOF A thesis submitted in fulfilment of the requirements for the award of the degree of Bachelor of Engineering (Electrical - Microelectronics) Faculty of Electrical Engineering Universiti Teknologi Malaysia MAY 2011 i I declare that this thesis entitled “RF-to-DC Characteristics Of AlGaAs/GaAs HEMT Schottky Diode for Rectenna Application “is the result of my own research except as cited in the references. The thesis has not been accepted for any degree and is not concurrently submitted in candidature of any other degree. Signature : ...................................... Name : KHAIRUL HUDA BINTI YUSOF Date : 20th MAY 2011 ii Especially dedicated to my beloved parents, brothers and sisters who have encouraged guide and inspired me throughout my journey in education. A warm thanks to all. iii ACKNOWLEDGEMENT First and foremost, grateful to the Allah SWT, with His permittance I succed to complete this project eventually. Unforgotten, this dedication is also credited to my supervisor, Prof. Dr. Abdul Manaf bin Hashim. He help me a lot to succeed in this project. He spent countless hours in advising my works. He is an extraordinary person. His energy and excitement in research never seem to end. This work would not be possibly successfully without his invaluable advices and guidance. I would like to express my special thanks to my mentor, Mrs. Farahiyah Mustafa for his guiding, patience and helpful discussions in this work, contributing me with ingenious ideas during discussion and providing information also advices to make the report to be more completed My deepest gratitude goes to my family especially to the most important people who always given their loves and support, ibu and ayah (Faridah Sulaiman dan Yusof Suboh), to my sisters Khairul Fazariah, Khairul Rodhiah, and Khairul Aimi and to my brothers Khairil Amri, Khairil Hilmi and Khairil Annas. Their endless love is the priceless treasure to give me light to overcome the darkest time. Last but not least, I want to express my great appreciation to all my friends especially Nur Affifah Omar, Khairul Nisaq, Farah Abd. Hamid, Nur Syazwani and Nur Ayuni, for their support and help directly or indirectly. Thanks for supporting me extremely all the time. iv ABSTRACT The Schottky diodes enjoined with coplanar waveguides are investigated for applications in on-chip rectenna device applications without insertion of a matching circuit. The design, fabrication, DC characteristics and RF-to-DC conversion of the AlGaAs/GaAs HEMT Schottky diode is presented. Current-voltage (I-V) measurements show good device rectification with a Schottky barrier height of 0.4349 eV for Ni/Au metallization. The differences of Schottky barrier height from theoretical value are due to the fabrication process and smaller contact area.The RF signals up to 1 GHz are well rectified by the fabricated Schottky diodes and stable DC output voltage is obtained. The conversion efficiency for measurement is up to 50 % and the conversion efficiency for calculation using closed-form equation is up to 60 % is obtained at 1 GHz with series connection between diode and load. The measured efficiency is smaller than calculation efficiency due to resistance blow up effect, effect of non-linear junction capacitance, effect of the finite forward voltage drop and the breakdown voltage. The fabricated n-AlGaAs/GaAs Schottky diode provide conduit for breakthrough designs for ultra-low power on-chip rectenna device technology to be integrated in nanosystems. v ABSTRAK Penyepaduan secara terus antara antena dwikutub dan diod Schottky diselidiki untuk aplikasi nanosistem melalui pandu gelombang sesatah (CPW) tanpa menggunakan sebarang litar penyesuaian. Fabrikasi, pencirian arus terus (DC), pertukaran pencirian radio frekuensi(RF) kepada pencirian arus terus(DC) dan juga perbezaan pertukaran kecekapan antara pengukuran dan pengiraan untuk AlGaAs/GaAs transistor-pergerakan-elektron-tinggi (HEMT) diod Schottky dibentangkan. Daripada pengukuran yang dilakukan terhadap Arus-Voltan (I-V), dengan nilai 1.37 kΩ perintang sesiri, nilai untuk penghalang kualiti Schottky ialah 0.4349 eV tetapi dengan menggunakan pengiraan, nilainya ialah 1.433 eV. Didapati nilai pengukuran lebih rendah berbanding dengan nilai pengiraan, ini mungkin disebabkan oleh proses fabrikasi and kawasan hubungan yang kecil. Isyarat RF naik sehingga 1GHz sepenuhnya directifikasikan oleh fabrikasi diod Schottky dan voltan keluaran DC yang stabil diperolehi. Pertukaran kecekapan pada frekuensi 1 GHz dengan penyambungan antara diod dan beban diperolehi, dimana bagi pengukuran nilainya meningkat sehingga 50 % dan bagi pengiraan yang menggunakan persamaan bentuk tertutup nilainya meningkat sehingga 60 %. Daripada pemerhatian didapati bahawa, pertukaran kecekapan bagi pengukuran lebih kecil berbanding pengiraan ini mungkin disebabkan oleh kesan daripada perintang terbaka, kesan tidak linear pemuat simpang, kesan voltan hadapan, dan kesan voltan gangguan. Manakala pertukaran kecekapan diperolehi bagi diode selari dengan beban dan diod sesiri dengan beban. Fabrikasi n-AlGaAs/GaAs diod Schottky menyediakan saluran terbaik untuk mereka teknologi rektena yang menggunakan pemberi kuasa terendah dalam aplikasi nanosistem. vi TABLE OF CONTENTS CHAPTER TITLE 1 2 PAGE DECLARATION i DEDICATION ii ACKNOWLEGEMENTS iii ABSTRACT iv ABSTRAK v TABLE OF CONTENTS vi LIST OF TABLES ix LIST OF FIGURES x LIST OF ABBREVIATIONS xii LIST OF SYMBOLS xiv INTRODUCTION 1.1 Research Background 1 1.2 Research Motivation 3 1.3 Research Objectives 4 1.4 Scopes of the Research 4 1.5 Research Activities 5 1.6 Overview of Thesis Organization 6 MATERIALS AND DEVICES FOR RECTENNA APPLICATION vii 2.1 Introduction 8 2.2 Properties of GaAs 8 2.3 Properties of AlGaAs/GaAs HEMT Structure for 10 High Speed and High Frequency 2.4 Properties of Schottky Diode 2.4.1 Rectifier 12 2.4.2 Half-wave Rectifiers 13 2.4.3 Full-wave Rectifiers 14 2.4.4 Schottky Diode 15 2.4.5 Previous Work 21 2.5 Summary 3 12 23 DEVICE FABRICATION AND MEASUREMENT TECHNIQUE 4 3.1 Introduction 24 3.2 Schottky Diode 24 3.3 Sample Structure 25 3.4 Fabrication Process 26 3.5 Measurement Technique 33 3.6 Summary 34 MODELLING TECHNIQUE TO DEVELOP CLOSEDFORM EQUATION FOR RECTENNA APPLICATION 4.1 Development of Close Loop Equation for Conversion Efficiency (RL Series with Diode) 35 4.2 Development of Close Loop Equation for Conversion Efficiency (RL Parallel with Diode) 39 viii 5 5 RESULT AND DISCUSSION 5.1 Introduction 44 5.2 DC Current-Voltage (I-V) Measurement 44 5.3 Generated Input Voltage 46 5.4 Rectified Output Voltage 47 5.5 RF-DC Convention Efficiency 49 5.6 Summary 54 CONCLUSION 5.1 Contribution of Present Work 55 5.2 Directions of Future Work 56 5.3 Summary 57 REFERENCES 58 Appendices A 62 Appendices B 66 ix LIST OF TABLES TABLE NO. TITLE PAGE 2.1 Physical properties of GaAs at 300 0K 9 2.2 Work function of some elements 18 2.3 Electron affinity of some semiconductor 18 5.1 Diode parameters of the Schottky diode 49 x LIST OF FIGURES FIGURE NO. TITLE PAGE 1.1 Research flow of Schottky diode 5 2.1 AlGaAs/GaAs HEMT structure 11 2.2 Corresponding band diagram 11 2.3 Half-wave rectifiers 13 2.4 Full-wave rectifiers 14 2.5 (a) Symbol and (b) equivalent Circuit of 16 the Schottky diode 2.6 I-V characteristic for ohmic contact and 20 Schottky barrier 2.7 I-V characteristic 22 2.8 RF detection 22 3.1 Cross section of AlGaAs/GaAs HEMT 26 structure 3.2 Fabrication process of Schottky diode 27 3.3 Fabrication flows for mesa formation 28 3.4 Mesa formation for Schottky diode 29 3.5 Fabrication for ohmic contact formation 30 3.6 Schematic of ohmic contact 30 3.7 Fabrication flow for Schottky contact 3.8 formation 31 Schematic structure of Schottky contact 31 xi 3.9 Overall schematic structure of Schottky 32 diode 3.10 Schematic of RF-to-DC conversion 4.1 Equivalent circuit of Schottky diode series with load resistance 4.2 34 36 Equivalent circuit of Schottky diode parallel with load resistance 40 5.1 DC Current-Voltage (I-V) measurements 46 5.2 Generated input voltage 47 5.3 Rectified output voltages 48 5.4 RF-DC conversion efficiency 50 5.5 Conversion efficiency for the measurement and calculation (a) at frequency 50MHz and (b) at frequency 1GHz 5.6 51 Conversion efficiency for the RL series with diode and RL parallel with diode (a) at frequency 50MHz and (b) at frequency 1GHz 53 xii LIST OF ABBREVIATIONS 2DEG − Two-dimensional electron gas Al − Aluminum AlGaAs − Aluminium Gallium Arsenide As − Arsenide Au − Gold BDD − Binary diagram CMOS − Complementary metal–oxide–semiconductor CPW − Coplanar Waveguide Cr − Chromium dB − Logarithmatic Magnitude DC − Direct Current DI − Di-ionized EM − Electromagnetic FET − Field-effect-transistor Ga − Gallium GaAs − Gallium Arsenide Ge − Germanium G-S-G − Ground-Signal-Ground H2O − Water H2O2 − Hydrogen Peroxide H2SO4 − Sulfuric HCl − Hydrochloric HEMT − High-electron-mobility-transistor HF − High Frequency IC − Integrated Circuit IQ − Intelligent Quantum xiii LF − Lower Frequency MEK − Methyl-ethyl-ketone MESFET − Metal-semiconductor field effect transistor mV − Millivolt Ni − Nickel PR − Photoresist SBH − Schottky barrier height SHF − Super High Frequency SI − Semi-Insulating Si − Silicon xiv LIST OF SYMBOLS μ − Mobility of Electron μA − Microampere Å − Angstrong, 1 Å = 1 x 10-10 m A − Schottky contact area Cadd − Capacitor added Cj − Junction capacitance Cosc − Internal capacitance ddiode − Distances between Schottky-ohmic contacts EF − Fermi level f − Frequency fr − Resonant frequency k − Boltzmann’s Constant LCPW − CPW length Nd − Donor doping concentration ØB − Barrier height Øm − Metal work function Øs − Semiconductor work function q − Filling fraction Radd − Additional resistor Rj − Nonlinear junction resistance Rosc − Oscilloscope internal input resistance Rs − Series resistance T − Absolute temperature Vbi − Built-in potential Vt − Thermal voltage Vth − Threshold voltage xv εs − Permittivity of the semiconductor χ − Electron affinity 1 CHAPTER 1 INTRODUCTION 1.1 Research Background Recent revolutionary progress of the internet and wireless technologies has created a concept of the "Ubiquitous Network Society" for new centuries [1]. The evolution in these technologies is producing new smart chip known as an Intelligent Quantum (IQ) chip that was used in the wireless microwave power transmission [2]. The IQ chip was projected by the integrating quantum digital signal processors, memories, various sensors, transceivers etc on the chip. This type of chip is expecting to play important roles as tiny knowledge vehicles in the next generation ubiquitous network society. An IQ chip is an III- semiconductor chip with sizes of millimetre square proposed by Hasegawa et al. [3] or below where nanometre scale quantum processors and memories are integrated. One of the key elements to determine the concept and structure of IQ chips is the wireless power supply system. The system would be the groundwork for ubiquitous network society. The vision of ubiquitous network society is basically like a centre of sources where information can be shared and easily accessed by anyone at anytime and everywhere. The combination of new and the existing technologies make the vision becomes reality [4-6]. 2 The main component of the wireless power transfer is rectifying antenna which is also known as rectenna [4]. A rectenna is a combination of rectifying circuit and an antenna [5]. The antenna receives the electromagnetic power and the rectifying circuit converts it to electric power [4]. Schottky diode is considered as major rectifier due to its fast rectifying operation and suitability for on-chip integration. As a semiconductor material for Schottky diode, three-five (III-V) based compound materials have been considered as the most promising materials because of their stability, capability of making a good Schottky contact and well-developed fabrication process technology [7]. The III-V materials are the most promising for high-frequency devices because of the high electron mobility and other unique feature such as the formation of two-dimensional electron gas (2DEG) [8-16]. The devices switch faster as collisions are less frequent [2]. In this study, the investigation on the RF-to-DC characteristics of AlGaAs/GaAs HEMT Schottky diode via coplanar waveguide (CPW) transmission line without insertion of any matching circuit is carried out by using the following approaches; i. Fabricating the Schottky diodes on n-AlGaAs/GaAs high-electron-mobility transistor (HEMT) structure; ii. Measuring the characterization of the Schottky diode; and iii. Comparing the characterization of the Schottky diode between measurement and the calculation. 3 1.2 Research Motivation Generally, many portable devices are designed smaller than traditional version when the new invention had been applied to make its unique and very special. Similarly, the rectenna should manufacture in small dimensions, but does the rectenna will be work when the received power reduced to smaller amount? Realising on the limitation, wireless power transfer is chose as the best low-power application. The example that directly suitable for the behaviour of the antenna is IQ chip development. For this purpose the behaviour of the antenna and rectifying circuit have to be design at around the operating frequency [4-6]. As a by-product of our group’s investigations, design, fabrication and characterization of a Schottky diode on an AlGaAs/GaAs, HEMT structure for onchip RF power detection have been identified. The Current-Voltage(I-V) measurements showing that the devices have rectifying properties with a barrier height of 0.5289-0.5468 eV and the value of Vth is 1.1V. The fabricated Schottky diodes detected RF signals well and their cut-off frequencies up to 20 GHz were estimated in direct injection experiments [4-6]. The main requirement to improve the device performance is by improving the capability of the rectifier to detect low power RF. The value of threshold voltage must be lower than usual, and at the same time the efficiency level must be higher. This is to ensure that the devices capable to detect the higher frequency. However, further investigation on RF-DC conversion need to be implemented if we want to improve the device performance [4-6]. 4 1.3 Research Objectives The main objective of this research is to investigate the RF-to-DC characteristics of AlGaAs/GaAs HEMT Schottky diode for rectenna device application via coplanar waveguide (CPW) transmission line without insertion of any matching circuit. 1.4 Scopes of the Research The scopes of this research are as follows: i. Fabrication of an n-AlGaAs/GaAs HEMT Schottky diode; ii. Characterization (DC & RF) of an n-AlGaAs/GaAs HEMT Schottky diode; and iii. Comparison the value of measurement with the calculation. 5 1.5 Research Activities Figure 1.1 summarized the flow of this research. This study is focusing on the direct integration of Schottky diode without insertion of any matching circuit. Fabrication of Schottky diode using standard photolithography process Characterization of Schottky diode • Current-Voltage (I-V) Measurement • RF-to-DC Direct Power Measurement • RF-to-DC Conversion, Voltage and Current Measurement Compare characterization of Schottky diode between measurement and calculation Optimization of Schottky diode Figure 1.1: Research flow of Schottky diode 6 1.6 Overview of Thesis Organization This report consists 6 main chapters. Chapter 1 will consist of general idea of the research background, objectives, scopes and research activities. In Chapter 2, it will contain the basic concept, basic material and theory of Schottky diode as the devices for rectenna application. This chapter also discussed about the previous work result, in improving performance of the device for rectenna application. Chapter 3 generally focus on the device fabrication process and show how does the measurement technique is presented. On the whole of this chapter explains the fabrication process for the Schottky diode and integrated devices structure, and a measurement system. This study shows, the fabrication of Schottky diode and integrated devices is carried in clean room facilities. The major fabrications involved are photolithography, wet chemical etching, metal deposition and a standard lift-off technique. Therefore, the semiconductor material structure for the devices also discussed briefly. Chapter 4 consist of the modelling technique to development the close loop equation for conversion efficiency Schottky diode in rectenna application. Chapter 5 explains the DC and RF characteristics of Schottky diode on nAlGaAs/GaAs HEMT structure via CPW transmission line without insertion of any matching circuit for fast conversion of RF signals in rectenna application. 7 Chapter 6 is a summary and overall review of this project, where the contribution of present work and the direction of future work are describes. 8 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction This chapter describes the basic concept, basic material and theory of Schottky diode as the devices for rectenna application. This chapter also discussed about the previous work result, in improving performance of the device for rectenna application. 2.2 Properties of GaAs for rectenna application. In this new era, application especially computer and telecommunication use Silicon (Si) and Gallium Arsenide (GaAs) as a semiconductor for the high-speed device because its related III-V compounds and solid solutions [4-6].The high density and high speed characteristics make silicon one of the popular material for 9 very large scale integrated (VLSI) circuit devices. The III-V compounds have certain speed advantages over silicon in term of higher carrier mobility and effective carrier velocities [7]. Therefore GaAs is better compared to Si [4-6]. Table 2.1 shows the physical properties of GaAs. Table 2.1: Physical properties of GaAs at 300 0K [17] Property Parameter Crystal Structure Zincblend Lattice Constant 5.6419Ǻ Density 5.316 g/cm3 Melting Point 1238 0C Linear Expansion Coefficient 5.93x10-6 / deg Thermal Conductivity 0.455 W/cm.K Dielectric Constant 10.9 Refractive index 3.655 Bandgap at Room Temperature 1.428 eV Optical Transition Type direct Intrinsic Carrier Concentration 1.8x106 cm-3 Electron Mobility 8,500 cm2/V.sec Hole Mobility 420 cm2/V.sec Intrinsic Resistivity 3.8x108 Ω.cm In particular, GaAs has a direct band-gap that permits high-efficiency light emission as well as a conduction-band structure that leads to fast electron conduction. GaAs electronics find niches in small-sized circuits that require high speed or low power consumption. This characteristic makes GaAs-based, a very useful material and this material suitable in lasers manufacturing and LEDs application. As the dominant electronic semiconductor, Silicon does not have a direct band-gap nor does 10 it address light-emitting applications (except as part of the transmitting and receiving electronic functions) [18]. GaAs has higher saturated electron velocity and higher electron mobility, that allowing it to function at frequencies in excess of 250 GHz. Besides, GaAs devices generate less noise than Si devices when operated at high level of frequencies. GaAs devices can also be operated at higher power levels than the equivalent silicon device because it has higher breakdown voltages. These properties make GaAs circuitry applied in mobiles phones, satellite communications, microwave point-to-point links, and some radar systems [4-6]. Therefore, in order to achieve high speed devices at high frequency, GaAs-based material has been considered to be used in this research. 2.3 Properties of AlGaAs/GaAs HEMT Structure for High Speed and High Frequency for rectenna application. The High Electron Mobility Transistor (HEMT) is a hetero-structure field effect transistor. The term “HEMT” is applied to the device because the structure takes an advantage of superior transport properties of electrons in a potential well of lightly doped semiconductor material. The AlGaAs/GaAs HEMT structure is illustrated in Figure 2.1. As shown in the figure, a wide bandgap semiconductor material (doped AlGaAs) lies on a narrow band material (undoped GaAs) [19] and is also known as a hetero-junction of high purity GaAs. When the GaAs and AlGaAs are hetero-junctioned and Fermi levels become equals, electrons will be supplied to the GaAs and forming two-dimensional electron gas (2DEG) in the band through the interface because the electron affinity (χ = 4.07 eV) of AlGaAs is larger than that of GaAs [4-6]. 11 Figure 2.1: AlGaAs/GaAs HEMT structure [19] The most important point about the channel layer in the HEMT structured devices is the 2DEG layer that results from the band gaps difference between AlxGa1-xAs and GaAs [4-6]. The band diagram of correlated structure showing the 2DEG layer is illustrates in Figure 2.2, where the conduction-band energy relative to the Fermi energy of an N-AlGaAs-intrinsic GaAs hetero-junction in thermal equilibrium. Thermal equilibrium was achieved when electrons from the wide-bandgap AlGaAs flow into the GaAs and are confined to the potential well. However, the electrons are free to move parallel to the hetero-junction interface. In this structure, the majority carrier electrons in the potential well are now separated from the impurity dopant atoms in the AlGaAs; thus, impurity scattering tends to be minimized [20]. Figure 2.2: Corresponding band diagram [19] 12 By comparing the devices that employ bulk structure such as metal semiconductor-field-effect-transistor (MESFET), the characteristics of an HEMT is in that it is free from carrier freeze-out even at a very low temperature of 4.2 °K because it allows the concentration of donor impurities in n-AlGaAs layer to be sufficiently increased. And since electron mobility is much larger at low temperature than at room temperature, it plays an important role of as cryoelectronic devices [46]. This unique properties material (AlGaAs) is suitable for the development of the IQ chip which has been considered as the most promising chip structure for future ubiquitous network society use. 2.4 Properties of Schottky Diode for rectenna application. 2.4.1 Rectifier Rectification is a process whereby alternating current (AC) is converted into direct current (DC). Rectifiers are identified devices that perform this special task. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with just a single diode. Rectification is commonly performed by semiconductor diodes. Rectifiers can have various configurations and are chosen depending on applications [21]. Rectifiers can be divided by two types which are: Half-wave rectifiers. Full-wave rectifiers. 13 Half-wave rectifiers is working by rectify only the positive or negative half-cycles of the applied AC. But on the other hand, Full-wave rectifier rectifies both the halfcycles [22]. 2.4.2 Half-wave Rectifiers In a half-wave rectifier (Figure 2.3), either the positive or negative half of the AC wave is passed easily while the other half is blocked, depending on the polarity of the rectifier. This configuration is adopted because of simplicity of circuits [21]. Figure 2.3: Half-wave Rectifiers 14 2.4.3 Full-wave Rectifiers Full-wave rectification converts both polarities of the input waveform to DC, and is more efficient. However, more diodes are needed in this configuration. A full wave rectifier (Figure 2.4) converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform. The positive (negative) portions thus combine with the reversed negative (positive) portions to produce an entirely positive (negative) voltage/current waveform [21]. Figure 2.4: Full-wave Rectifiers 15 2.4.4 Schottky Diode A Schottky diode is a junction of a lightly doped n-type semiconductor with a metal electrode. The junction of a doped semiconductor is usually n-type with a special metal electrode [25]. Schottky diodes acts as rectifiers and have been used for over 25 years in the power supply industry. The primary advantages of this diodes are have very low forward voltage drop, where the value is typically 0.3 to 0.5 volt compare to the 0.6 to 0.8 found in silicon junction diode [25] (this lower voltage drop translates into higher system efficiency) and can produce fast switching speeds that approach zero time making them ideal for output stages of switching power supplies. This feature also stimulated their additional functioning in very supper frequency (330 GHz) applications for the communication links, generally referred to as microwave links [4-6]. This frequency suitable use for this research on the smart chip introduces the concept of a so-called IQ chips towards realization of the ubiquitous network society. This latter feature also including very low power involving signal and switching diode requirements of less than 100 picoseconds. These require small Schottky devices with low capacitance [21]. Schottky diode provides a high performance, cost-effective solution for today’s circuit designs for rectenna application [23]. The Schottky barrier diode is a majority carrier device. This means that there is no diffusion capacitance associated with a forward-biased Schottky diode. The elimination of the diffusion capacitance makes the Schottky diode a higherfrequency device. Also, when switching a Schottky diode from forward to reverse bias, there is no minority carrier stored charge to remove. Since there is no minority carrier storage time, so that the Schottky diode can be used in fast-switching application and also suitable used in this research [20]. 16 The Schottky rectifier properties described above are primarily determined by the metal energy barrier height of material deposited on the silicon by the manufacturer. A metal with a low energy barrier height will minimize forward voltage, but will also be restricted in its high temperature operating capability and have very high reverse leakage currents. A high barrier metal height selection will minimize temperature and leakage current sensitivity but will increase the forward voltage [21]. Figure 2.5 shows the symbol and equivalent circuit of the Schottky diode. The diode model consists of a series resistance RS, a nonlinear junction resistance Rj described by its dc IV characteristics, and nonlinear junction capacitance Cj. The junction resistance Rj is assumed to be zero for forward bias and infinite for reverse bias [24]. Figure 2.5: (a) Symbol and (b) Equivalent circuit of the Schottky diode [26] 17 The junction capacitance is defined as the following formula; Vbi Vbi V0 C j C j0 (2.1) Where C j 0 is the diode’s zero bias junction capacitance, and the Vbi is the diode’s built-in voltage in the forward bias region [24]. The diode’s zero bias junction capacitance is defined as; C j0 A q. S .N d kT 2Vbi q (2.2) A is the diode cross-sectional area, s is the permittivity of the semiconductor, N d is the donor doping concentration, Vbi is the built-in potential. The term kT / q often referred to as Vt is approximately 0.0259 V at room temperature whereas Vbi is approximately 1 V. The diode capacitance, C j has to be extracted from the RF measurement. The value of this capacitance was calculated by applying the theory of coplanar lines presented by Hoffman [4-6]. There are two type of characteristic of Metal-Semiconductor contact which are rectifying contact and non-rectifying contact. The rectifying contact preferentially allow the current flow in one direction (example: diode) and the nonrectifying, allow current flow in both direction (example: Ohmic contact). Table 2.2 shows the work function of some elements and the electron affinities of some semiconductors are given in Table 2.3. The parameter B 0 is the ideal barrier height of the semiconductor contact, where the potential barrier seen by electron in the 18 metal trying to move into the semiconductor. The barrier height is depend on the work functions (refer Table 2.2) of the metal and electron affinity (refer Table 2.3) of the semiconductor used for the contact. Table 2.2: Work function of some elements [20] Element Work function, m Ag, silver 4.26 Al, aluminium 4.28 Au, gold 5.1 Cr, chromium 4.6 Ni, nickel 5.15 Pd, palladium 5.12 Ti, titanium 4.33 Table 2.3: Electron affinity of some semiconductor [20] Element Electron affinity, χ Ge, germanium 4.13 Si, silicon 4.01 GaAs, gallium arsenide 4.07 AlAs, aluminium arsenide 3.5 The work function of the metal, m is the energy needed to remove an electron from the Fermi level of the metal, EF to the vacuum level. The vacuum level is the energy level of an electron just outside the metal with zero kinetic energy. The m has a volume contribution due to the periodic potential of the crystal lattice and a 19 surface contribution due to possible existence of a dipole layer at the interface. The work function, S of the semiconductor are defined similarly and variable quantity because of the doping concentration. An important surface parameter that does not depend on doping is the electron affinity where defined as the energy differences of an electron between the vacuum level and the lower edge of the conduction band [4-6]. This barrier is known as the Schottky diode and is given, ideally by [20]: B 0 (m ) (2.3) On the semiconductor side, Vbi is the built-in potential barrier and the buit-in potential barrier is given by [20]: Vbi B 0 n (2.4) The potential difference between EC and E F , n is define as [20]; n kT N C In q N d (2.5) Where kT / q is a thermal voltage ( Vt = 0.0259 volt at T = 300 K), N C is a effective density of states in conduction band and the N V is an effective density of states in conduction band. Contacts must be made between any semiconductor device, or integrated circuit and the outside world. These contacts are made via ohmic contacts. Ohmic contacts are low-resistance junction providing conduction in both direction between the metal and the semiconductor. Ideally, the current through the ohmic contact is a linear function of applied voltage, and the applied voltage should be very small [20] 20 (refer Figure 2.6). The ohmic contacts employed in these devices are relatively large in area and the resulting total contact resistance can be less than a few ohms. The metal may be a multilevel metal-alloyed system. Since the total thickness of the ohmic metallization is typically limited to 1000 to 2000 Å, ohmic metal is not generally used as a first level interconnect. Hence, another way to decrease the contact resistance is to place a layer of a narrow gap highly doped semiconductor material between the active region of the device and the contact metal [4-6]. This unique property of Schottky diode is suitable for the development of the IQ chip and appropriately to use in this research. Figure 2.6: I-V Characteristic for Ohmic contact and Schottky barrier 21 2.5 Previous Work The rectenna applications were investigated in many years. The issue that investigated in previous works is characterization of Schottky diode on an AlGaAs/GaAs HEMT structure, such as the I-V characteristic (cut-off frequencies and threshold voltage ) and RF detection (conversion efficiencies and barrier height), in order to give the good performance in the rectenna device application, towards the realization of a ubiquitous network society. In this previous work [6], the characterization of a Schottky diode on an AlGaAs/GaAs HEMT structure via coplanar waveguide (CPW) transmission line without insertion of any matching circuit was investigated. On the Current-Voltage (I-V) measurements showed that the devices had rectifying properties with a barrier height of 0.5289-0.5468 eV. The fabricated Schottky diodes detected RF signals well and their cut-off frequencies up to 20 GHz were estimated in direct injection experiments. In order to achieve a high cut-off frequency, a smaller Schottky contact area is required and to get a higher cut-off frequency can also be achieved by reducing the length of the coplanar waveguide transmission line. The result also shows as in Figure 2.5 and 2.6 below: 22 Figure 2.5: I-V Characteristic [4-6] Figure 2.6: RF Detection [4-6] Based on the previous work as in ref.[2], the work mainly discussed on the DC characteristics and RF-to-DC conversion of the AIGaAs/GaAs HEMT Schottky diode via coplanar waveguide (CPW) transmission line without insertion of any matching circuit. The measurement of I-V characteristic shows that devices have rectified properties with a barrier height of 0.5289 eV. This value is almost three times smaller than the ideal value of 1.443 eV. The reducing barrier height is 23 beneficial for improved RF response and rectification as it requires a lower tum-on voltage. For the RF-DC Conversion Measurement, the cut-off frequency of this device is estimated to be around 20 GHz, and through this result, the studies suggested that to achieve a high cut-off frequency, the rectifying metal-tosemiconductor contact area must be reduced. In reference [25], this work is actually discussed on the RF-to-DC conversion efficiency. As discussed in the research, the measured achieved conversion efficiencies are 84.4% at 2.45 GHz and 2.45% at 5.8 GHz. So, both frequencies have comparably low atmospheric loss, cheap components availability, and reported high conversion efficiency. 2.6 Summary The main discovery of this chapter is to show the properties of rectenna applications, besides the discussion of basic material. The AlGaAs/GaAs HEMT structure has been selected as the semiconductor material for devices because of the higher electron mobility that can be provided by the 2DEG layer. That is why the material is most popular material among the others in confining electron applications. The material structure is preferred for the IQ chip development, where it also has been considered as the most promising chip structure. The basic concept and theory of Schottky diode which applied for the rectenna also briefly discussed in this chapter. In order to come out better performance in the rectenna device application towards the realization of a ubiquitous network society, investigation also has been done on previous works. 24 CHAPTER 3 DEVICE FABRICATION AND MEASUREMENT TECHNIQUE 3.1 Introduction This chapter described the fabrication process for the Schottky diode and a measurement technique that has been identified to fulfil the objectives of the research. In this research, the fabrication of Schottky diode is carried out in clean room facilities and involving some major fabrication techniques such as photolithography, wet chemical etching, metal deposition and a standard lift-off technique. Besides, the semiconductor material structure for the devices also presented. 3.2 Schottky Diode The design, fabrication and measurement process of Schottky diode on nAlGaAs/GaAs HEMT structure are presented constructively in this part. 25 3.3 Sample of Structure An n-AlGaAs/GaAs HEMT structure is used to develop the Schottky diode because of the higher electron mobility that can be provided by 2DEG layer. The structure is actually grown by molecular beam epitaxy. The interfaces of n-doped AlGaAs layer and undoped GaAs layer define a 2DEG system, where the electron motion which perpendicularly to the layer is frozen out, thus producing highly mobile electrons. To develop the structure, specific thicknesses of layers has to be applied are as follows; 625 μm semi-insulated high-dielectric constant GaAs substrate, 500 nm GaAs buffer layer; 100 nm AlGaAs buffer layer; 20 nm undoped GaAs layer; 10 nm AlGaAs spacer layer; 50 nm n-doped AlGaAs (Si δ doping) barrier layer; 10 nm GaAs undoped cap layer. At the room temperature, the carrier mobility and the carrier sheet density obtained by Hall measurements are 6040 cm2/V-sec and 8.34 x 1011 cm-2, respectively [4-6]. The cross section of AlGaAs/GaAs HEMT structure is shown in Figure 3.1 below. 26 Figure 3.1: Cross section of AlGaAs/GaAs HEMT structure [26]. 3.4 Fabrication Process The fabrication process of Schottky diode is simplified by a flowchart in Figure 3.2. There are three formation steps used for the Schottky diode which are Mesa formation, Ohmic contact formation and Schottky contact formation. Those steps are suggested by F. Mustafa et al. in ref. [26] that will be explained in detail in this sub-chapter. 27 Fabrication Process of the Schottky diode 1. Mesa Formation Pre-treatment Photolithography Process Etching Process 2. Ohmic Contact Pre-treatment Photolithography Process Metal Deposition (Ge/Au/Ni/Au) Lift-Off Process 3. Schottky Contact Pre-treatment Photolithography Process Metal Deposition (Ni/Au) Lift-Off Process Finish Figure 3.2: Fabrication process of Schottky diode A. Mesa Formation process Figure 3.3 illustrated the fabrication process’s flow of Mesa formation. The fabrication process is starts with Mesa formation, which is the first layer of the Schottky diode structure and the structure formed by a photolithography process. At 28 the beginning of the fabrication process, the AlGaAs/GaAs HEMT substrate is cut by using diamond cutter. After cutting the substrate, the sample is coated with SPR6810 photo-resist at spinning speed of 5000 rpm for 30 second and pre-bake at 90 oC for 5 minutes. The mesa patterning sample is exposed to UV light for 20 seconds through corresponding mask. Photo-resist development is performed by dipping in MFCD26 developer for 45 seconds and rinsing in DI water for 45 seconds. The sample should be dried by N2 blow and hard-bake at 90 0C for 5 minutes before proceeding with the etching process. Figure 3.3: Fabrication flows for Mesa formation [26] Next, the sample is kept at room temperature which is at 25 oC for about 18 second, and then it is etched by using Sulfuric Acid (H2SO4). The etchant is consists with H2SO4, Hydrogen Peroxide (H2O2) and DI water with ratio of 8, 1 and 1 respectively. As a result, about 500 nm of GaAs material removes from the etching process. Figure 3.4 shows the mesa strips, where the length and width are formed from this process are 327.5 µm and 40 µm. 29 Figure 3.4: Mesa formation for Schottky diode [26] Before being proceed to ohmic contact formation, the sample need to be cleaned using Hydrochloric Acid (HCl) and DI water with ratio of 1 and 50 respectively in order to remove native oxide on the sample surface. All the III-V semiconductors are readily oxidized by atmospheric oxygen (O2), so the surface that has been exposed to the air will be covered by a thin layer of native oxide. B. Ohmic Contact Formation process The second fabrication process is ohmic contact formation which is the second layer of the Schottky diode structure. The illustration of the ohmic contact process is shown in Figure 3.5 and Figure 3.6 shows the schematic of ohmic contact formation. The thicknesses of the sample of Ge/Au/Ni/Au are at ratio 50/100/25/30 and it’s deposited by electron beam evaporation. After the mesa formation process, the fabrication process continued with the ohmic contact process where the lift-off technique is used to pattern the deposited metal film using acetone or MEK. The ohmic metallization need to be alloyed using rapid thermal processing (RTP) in N2 ambient. Annealing process is performed at 450 °C for 5 minutes in inert N2 ambient. 30 Figure 3.5: Fabrication for ohmic contact formation [26] Figure 3.6: Schematic of Ohmic contact [26] C. Schottky Contact Formation process The third process is Schottky contact formation. The flow of the fabrication process to develop the Schottky contact formation is shown in Figure 3.1 and Figure 3.8 shows the schematic of Schottky contact formation. The thickness of the Ni/Au is 31 at ratio 50/50 nm and it is deposited by electron beam evaporation. After that, do the lift-off technique to pattern the deposited metal film by using the acetone or MEK. Figure 3.7: Fabrication flow for Schottky contact formation [26] Figure 3.8: Schematic structure of Schottky contact [26] 32 D. Overall Structure Figure 3.9 shows the overall schematic structure of Schottky Diode. In between of the Schottky and ohmic contact, there is Coplanar Waveguide (CPW) structure where has Ground-Signal-Ground pad structures, or known as G-S-G structure. The G-S-G pad structure can make direct integration without insertion of matching circuit and it also permits direct injection of RF signal through Cascade GS-G Infinity-150 micro-prober. Figure 3.9: Overall schematic structure of Schottky Diode [26] In this research, the area for Schottky contact is 20 µm x 20 µm, the length for the CPW is 20 µm and the different contacts between Schottky and ohmic is 40 µm. 33 3.5 Measurement Technique The RF-to-DC characteristics of AlGaAs/GaAs HEMT Schottky diode is obtained from measurements to know the performance of the device especially for the rectenna applications. 3.5.1 RF-to-DC Conversion, Voltage and Current Measurement Figure 3.10 shows a simple measurement setup of Schottky diode. From the figure, the coplanar waveguide (CPW) transmission line at the both sides of Schottky and ohmic contacts which possess ground-signal-ground (G-S-G) pad structures. By using Wheeler’s equation [30], the dimension of the gap, a is to be 60 μm and width, b is to be 90 μm respectively, in order to produce the characteristic impedance, Zo of 50 Ω. The device is measured by directly injecting RF power through the GSG CPW structure by using Cascade Infinity-150 microprobes. 34 Figure 3.10: Schematic of RF-to-DC Conversion 3.6 Summary This chapter is discussed the fabrication processes and the measurement techniques for the Schottky diode. In this research, the fabrication of Schottky diode is carried out in clean room facilities. Photolithography, wet chemical etching, metal deposition and a standard lift-off technique are the major techniques which are involved to this research. In addition, the structures for the semiconductor material for the devices also discussed in brief. 35 CHAPTER 4 MODELLING TECHNIQUE TO DEVELOP COLED-FORM EQUATION FOR RECTENNA APPLICATION 4.0 MODELLING TECHNIQUE 4.1 Development of Close Loop Equation for Conversion Efficiency ( Series with Diode) Figure 4.1 shows the equivalent circuit of the Schottky diode series with load resistance. The diode consists the series resistance ( RS ), nonlinear resistor ( R j ) and nonlinear capacitance ( C j ). Below the several assumptions to derive the conversion efficiency using closed form expressions, refer to T.W.Yoo et al. in ref. [31]: a) The current due to the junction capacitance is negligible when the diode is forward biased. b) The forward voltage drop across the intrinsic diode junction is constant during the forward-bias period. 36 Figure 4.1: Equivalent circuit of Schottky Diode series with load resistance The voltage waveform of V and Vd can be expressed as below due to several assumptions: V Vo V1 cost Vd Vdo Vd 1 cost , Vd V f (4.1) if diode is off if diode is on (4.2) V0 - the output dc voltage V1 - the peak voltage of an incident microwave V - the peak-peak voltage of an incident microwave Vd 0 and Vd 1 - the dc and the fundamental frequency components of diode junction voltage Vd respectively when the diode is off V f - the forward voltage drop of the diode when the diode is on 37 By using the Kirchoff’s voltage law the relationship between , and V are shows follows: RL V0 RL RS V (4.3) By taking the average, the Vd is derived as: Vd Vd 0 1 Vd 1 sin V f (4.4) is the phase angle where the diode is turned off and also the phase angle is defined as t . The switching occurs when Vd equal to the V f and and is calculated by: cos V f Vd 0 Vd 1 (4.5) When diode is off, the equation for current flowing through RS is written as follows: Rs RL d C jVd dt V Vd (4.6) The equation of C j is shown bellow: C j C0 C1 cost C2 cos2t 2 ...... (4.7) Substituting C j into (4.6) with a Fourier series of (4.7) and neglecting the terms higher than the second harmonic, the equation become: 38 Rs RL C1Vd 0 C0Vd1 sint Vd 0 V0 V1 cos Vd1 cost V1 sin sin(t ) (4.8) Since the above equation should hold during the off period of the diode, each term should separately zero: Vd 0 V V0 (4.9) Vd 1 V1 cos (4.10) V1 sin RS RL C0Vd1 C1Vd 0 C2Vd1 (4.11) The phase delay is obtained from equation above as follows: arctan RS RL C0 C1 cos C 2 1Vf arctan Ceff RL RS (4.12) (4.13) From the equation (4.3), (4.4), (4.5) and (4.9), the relationship between and r RS is derived as: RL V f V0 tan V V0 V f V V0 1 r V1 cos (4.14) is solved from a given V0 RS and RL the efficiency and the input impedance can be calculated from the time domain waveform V and Vd These waveforms are expressed as a function of , diode parameters, and V0 or V1 Pdc V02 RL 39 PLoss LOSSon, Rs LOSSon,diode LOSSoff , Rs LOSSoff ,diode LOSSon, RL LOSSoff , RL LOSSon, Rs 1 2 LOSSon,diode LOSSoff , Rs off 1 2 1 2 LOSSoff ,diode LOSSon, RL LOSSoff , RL 1 2 V0 V V f V0 V f 1 2 1 2 off V Vd V0 Vd d RS R L off V V 2 f RS off 2 off d V Vd V0 2 d Rs R L 2 off d RS R L off off 2 Rs R L 2 off Efficiency 4.2 f off off V V d V Vd 2 off RS d Pdc Ploss Pdc (4.15) Development of Close Loop Equation for Conversion Efficiency ( Parallel with Diode) Figure 4.2 shows the equivalent circuit of the Schottky diode parallel with load resistance. The diode consists the series resistance ( RS ), nonlinear resistor ( R j ) 40 and nonlinear capacitance ( C j ). Below the several assumptions to derive the conversion efficiency using closed form expressions, refer to T.W.Yoo et al. in ref. [31]: a) The current due to the junction capacitance is negligible when the diode is forward biased. b) The forward voltage drop across the intrinsic diode junction is constant during the forward-bias period. Figure 4.1: Equivalent circuit of Schottky Diode parallel with load resistance The voltage waveform of V and Vd can be expressed as below due to several assumptions: V Vo V1 cost (4.16) 41 Vdo Vd 1 cost , Vd V f if diode is off if diode is on (4.17) V0 - the output dc voltage V1 - the peak voltage of an incident microwave V - the peak-peak voltage of an incident microwave Vd 0 and Vd 1 - the dc and the fundamental frequency components of diode junction voltage Vd respectively when the diode is off V f - the forward voltage drop of the diode when the diode is on By using the Kirchoff’s voltage law the relationship between V0 , Vd and V are shows follows: RL V0 R L RS Vd (4.18) By taking the average, the Vd is derived as: V Vd Vd 0 1 d 1 sin V f (4.19) is the phase angle where the diode is turned off and also the phase angle is defined as t . The switching occurs when Vd equal to the V f and and is calculated by: cos V f Vd 0 Vd 1 (4.20) When diode is off, the equation for current flowing through RS is written as follows: R s d C j Vd dt V Vd (4.21) 42 The equation of C j is shown bellow: C j C0 C1 cost C2 cos2t 2 ...... (4.22) Substituting C j into (4.21) with a Fourier series of (4.22) and neglecting the terms higher than the second harmonic, the equation become: Rs C1Vd 0 C0Vd1 sint Vd 0 V0 V1 cos Vd1 cost V1 sin sin(t ) (4.23) Since the above equation should hold during the off period of the diode, each term should separately zero: V d 0 V0 (4.24) Vd 1 V1 cos (4.25) V1 sin RS RL C0Vd1 C1Vd 0 C2Vd1 (4.26) The phase delay is obtained from equation above as follows: arctan R S C 0 C1 cos C 2 1V f arctan C eff RS From the equation (4.18), (4.19), (4.20) and (4.24), the relationship between r RS is derived as: RL (4.27) (4.28) and 43 r tan Vf 1 V0 (4.29) is solved from a given V0 RS and RL the efficiency and the input impedance can be calculated from the time domain waveform V and Vd These waveforms are expressed as a function of , diode parameters, and V0 or V1 V02 RL Pdc PLoss LOSSon, Rs LOSSon,diode LOSSoff , Rs LOSSoff ,diode LOSSon, Rs 1 2 LOSSon,diode LOSSoff , Rs LOSSoff , diode 1 2 off R s V V V 1 2 off 2 off f R S off 2 off 1 2 Efficiency 2 f off off V V off d f d V Vd 2 d R s V Vd Vd d R S Pdc Ploss Pdc (4.30) 44 CHAPTER 5 RESULT AND DISCUSSION 5.1 Introduction This chapter describes the DC and RF characteristics of Schottky diode on nAlGaAs/GaAs HEMT structure via CPW transmission line without insertion of any matching circuit for fast conversion of RF signals in rectenna application. 5.1 DC Current-Voltage (I-V) Measurement Figure 5.1 shows the DC I-V characteristics with a 1.37 kΩ total series resistance, measured by directly injecting RF power through the GSG CPW structure using Cascade Infinity-150 microprober and parameter analyzer. From the measurement, the threshold voltage, Vth, for this device is estimated to be 1.1 V By applying the Richardson-Dushman equation (5.1) for the thermionic current, Schottky barrier heights (SBHs) has been used to calculate the measurement 45 of the reverse saturation current of the device [27]. 2 Is = AA * T exp b nVt (5.1) b - the barrier height in volts, Is - the reverse saturation current A* - the effective Richardson constant (8.16 Acm-2K-2) A - the area of the metal-semiconductor contact T - the absolute temperature Vt - the thermal voltage. The reverse leakage current from the measurement for the fabricated devices is 399 nA and the Schottky barrier height from the measurement is 0.4349 eV. Theoritical value of barrier height is 1.443eV. It shows that the experimental barrier height is lower than the theoretical value due to the fabrication process, i.e. annealing process, where it can result in the decrease in barrier height as suggested by Zhang et al. in ref. [28] and smaller contact area. To get the fine RF response, the barrier height should be reduced because smaller barrier height gives better RF rectification and the device can faster the operation due to lower turn on voltage [29-31]. 46 Figure 5.1: 5.3 DC Current-Voltage (I-V) Measurements Generated Input Voltage Figure 5.2 shows the generated input voltage, Vin peak where the input voltage is a function of the input power, Pin . Certain input power cannot increase the input voltage due to equipment capability and it must be ignore. So, to turn on the diode, input power must more than 5 dBm or 1.1 V, because turn-on voltage of the diode is 1.1V and at that level the device, can rectify well. 47 Figure 5.2: 5.4 Generated input voltage Rectified Output Voltage Figure 5.3 shows the rectified output voltage as a function of the input voltage at the different frequencies which are 10 MHz, 50 MHz and 1 GHz. The output voltages are only obtainable when the input voltages exceed the turn-on voltage of diodes which is 1.1 V. fc 1 RS C j (5.2) 48 C j - the junction capacitance RS - the series resistance of diode From the cut-off frequency equation (5.2) [4-6], the frequency of Schottky diode is proportional to 1 / Rs C j , the frequency is higher when the value of C j is smaller. Therefore, among of those three frequencies, the frequency at 1GHz is obtaining the largest and stable output voltage. Figure 5.3: Rectified output voltages 49 5.5 RF-DC Conversion Efficiency Figure 5.4 shows the measured RF-DC conversion efficiency at the different frequencies. The conversion efficiency,η of diode can be calculated using equation (5.3) below [25]: Pout 100 0 0 Pin (5.3) Pout - the DC power produced at the load RL - the load Pin - the injected power at input side of diode. Table 5.1: Diode parameters of the Schottky diode RL RS Cj Rj 56 1.37k 2.9 10 15 F 1 R L , load resistance; RS , series resistance; C j and R j , model the intrinsic junction for the diode. From the result in Figure 5.4, the conversion efficiency at frequency 1 GHz is up to 50%, at frequency 50 MHz the conversion efficiency is up to 40% and at frequency 10 MHz the conversion efficiency is up to 30%. All efficiencies are obtained from RF-DC conversion efficiency measurement. The conversion efficiency can be improved by reducing the value of contact resistance at several ohms and it can be achieved by removing the cap layer so that the ohmic contact can be formed directly on n-AlGaAs layer as suggested by T.W. Yoo et al. in ref. [31]. 50 Conversion Efficiency [%] 60 50 f = 1 GHz 40 f = 50 MHz 30 f = 10 MHz 20 10 0 -10 0 10 20 30 Input Power, P [dBm] in Figure 5.4: RF-DC conversion efficiency The conversion efficiency which calculated using a closed-form equation was compared with the conversion efficiency measurement. The result in Figure 5.5 shows that the conversion efficiency measurement is up to 50 % and the conversion efficiency for calculation using closed-form equation is up to 60 %. These are obtained with series connection of diode and load. The measured efficiency is smaller than calculation efficiency due to resistance blow up effect, effect of nonlinear junction capacitance, effect of the finite forward voltage drop and the breakdown voltage as suggested by T.W. Yoo et al. in ref. [31]. Conversion Efficiency [%] 51 60 f= 10MHz 50 Calculation 40 Measurement 30 20 10 0 -10 0 10 20 30 Input Power, Pin [dBm] Conversion Efficiency [%] (a) 60 f= 50MHz 50 Calculation 40 Measurement 30 20 10 0 -10 0 10 20 Input Power, Pin [dBm] (b) 30 52 Conversion Efficiency [%] 70 f=1GHz 60 Calculation 50 Measurement 40 30 20 10 0 -10 0 10 20 30 Input Power, P [dBm] in (c) Figure 5.5 Conversion efficiency for the measurement and calculation at (a) 10 MHz and (b) 50 MHz (c) 1 GHz Comparison between the parallel resistance and series resistance are shown in Figure 5.6. From the graph, it can be seen that the conversion efficiency for RL parallel with diode is up to 80 % and the conversion efficiency for RL series with diode is up to 60 % is obtained. So, it can be concluded that the connection (RL parallel with diode) is better than connection (RL series with diode) because the conversion efficiency of that connection is higher. 53 100 Conversion Efficiency [%] f=50MHz 80 Parallel 60 40 Series 20 0 -10 0 10 20 30 Input Power, P [dBm] in (a) 100 Conversion Efficiency [%] f=1GHz 80 Parallel 60 40 Series 20 0 -10 0 10 20 30 Input Power, P [dBm] in (b) Figure 5.6: Conversion Efficiency for the RL series with diode and RL parallel with diode (a) at frequency 50MHz and (b) at frequency 1GHz 54 5.7 Summary The DC-RF characteristic of the Schottky diode on n-AlGaAs/GaAs HEMT structure for rectenna application is presented. From the Current-Voltage (I-V) measurement result, a better device rectification is obtained where a Schottky barrier height is 0.5468 eV for Ni/Au metallization. The differences of Schottky barrier height from theoretical and measurement value are due to the fabrication process and smaller contact area. Besides, from the generated input voltage measurement result, input power must more than 5 dBm or 1.1 V to turn on the diode, because the diode turn-on voltage is 1.1V. The fine rectification device can be obtained when the value of threshold voltage, for the measurement were reduced, so the device can operate earlier. At the high frequency, output voltage is larger compared to the other frequencies and it is obtained from the output voltage measurement. Based on the result for conversion efficiency from measurement the value is up to 50 % and the conversion efficiency from calculation (using closed-form equation) is up to 60 %. Both results are obtained is series connection of diode and load. 55 CHAPTER 6 CONCLUSION 6.1 Contribution of Present Work The main objective of this research is to investigate the RF-to-DC characteristics of AlGaAs/GaAs HEMT Schottky diode for rectenna device application via coplanar waveguide (CPW) transmission line without insertion of any matching circuit. The research is divided into several parts to achieve the objective of this research, which are designing and fabricating the Schottky diode on nAlGaAs/GaAs HEMT structure, characterize the Schottky diode by directly injecting RF power through the GSG CPW structure using Cascade Infinity-150 microprober and by parameter analyzer. From the Current-Voltage (I-V) measurements result, it shows that good device rectification is obtained where a Schottky barrier height is 0.5468 eV for Ni/Au metallization. The differences of Schottky barrier height between theoretical and measurement values are due to the fabrication process and smaller contact area. The input power must more than 5 dBm or 1.1 V to turn on the diode, because from the generated input voltage measurement graph, the response activity for the diode to 56 turn on is when the input voltage reached 1.1 V, so the turn-on voltage for the diode is stated in the start of the response. The fine rectification device can be obtained when the value of threshold voltage, for the measurement is reduced, so the device can operate earlier. At the high frequency, output voltage is larger compare to the other frequency and it obtained from the output voltage measurement. From the RF-DC conversion efficiency analysis, the conversion efficiency for measurement is up to 50 % and the conversion efficiency for calculation using closed-form equation is up to 60 % is obtained with series connection of diode and load. The measured efficiency is smaller than calculation efficiency due to resistance blow up effect, effect of non-linear junction capacitance, effect of the finite forward voltage drop and the breakdown voltage. 6.2 Directions of Future Work The directions of future work are as the follows: (1) In this research, the value of threshold voltage, better rectification, the value of is around 1.1V. To get the must be reduce, so the device can operated earlier. 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IEEE Transactions on Microwave Theory and Techniques.40: 1259-1266. 62 APPENDIX A CALCULATION CONVERSION EFFICIENCY USING CLOSED-FORM EQUATION (RL SERIES WITH DIODE) 63 RL SERIES WITH DIODE LOSSon, Rs off 1 2 RS RL off 1 2 RS RL V 2 1 V V V0 d 2 f off cos 2 4V0V1 cos 4V f V1 cos off V1 cos 4V0 4V f V0 4V f d 2 2 1 2 2 V1 cos sin V1 4V0V1 sin 4V f V1 2 RS RL 2 1 1 2 2 2 sin 8V0 8V0V f 8V f V1 cos sin 4V0V1 sin 2 4V f V1 sin V1 sin V1 sin LOSSon,diode 2 RS RL 1 2 RS RL 1 V V 1 f V0 V f d f cos 2V0V f 2V f d f sin 4V0V f 4V f V f V1 sin 2 off V V 1 2 off 1 2 off V 1 2 2 RS RL 2 RS RL V V 2 RS RL off off 1 LOSSoff , Rs off 1 2 V V V0 d 2 d off cos 2 4V0V1 cos 4Vd 0V1 cos off 4V1Vd1 cos cos 4V0 8V0Vd 0 8V0Vd1 cos 4Vd 0 8Vd 0Vd1 cos 2 2 64 4Vd1 cos 2 d 2 1 2 V1 cos sin Vd 1V1 sin 2 2 RS RL 2 1 1 2 2 V1 cos sin 4Vd 1 cos sin 16Vd1V0 sin 2 4Vd 0V1 sin 4V0V1 sin 4Vd1V1 cos 4Vd1 4Vd1V1 cos 2 16Vd1Vd 0 sin 8V0 8Vd 0 V1 V1 8V0 8Vd 0 2 4Vd1 16Vd 0V0 2 LOSSoff ,diode 2 2 off 1 2 RS RL 2 RS RL 2 2 2 2 off 1 2 V V 1 d0 V V d V0 Vd d off cos V1Vd 1 cos cos 2V0Vd 0 2V0Vd 1 off cos 2Vdo 2Vd 0Vd1 cos 2Vd 0Vd1 cos 2Vd1 cos 2 d 2 2 1 1 2 2Vd 1 Vd 1V1 sin Vd 1V1 sin cos cos 2 RS RL 4 2 1 1 Vd 0V1 sin Vd 1V1 sin 2 Vd 1V1 cos V1Vd 0 sin cos 4 1 2 2 V1Vd 0 cos sin Vd 1V1 sin cos 4Vd 0V0 4Vd 0 2 off 1 V V f 2 RS off LOSSon, RL 1 2RS off V cos 2 1 off 2 d 2 2V0V1 cos 4V f V1 cos V0 4V0 2 65 V f 4V f d 2 1 2RS 1 2 2 2 V1 cos sin V1 2V0V1 sin 4V f V1 1 2 2 2 sin 2V0 8V0V f 8V f V1 cos sin 2V0V1 sin 2 4V f V1 LOSSoff , RL 1 2RS 1 2RS 2 off V V d 2 d off 2 off V cos 2 2 1 2V0V1 cos 4Vd 0V1 cos 4Vd 1V1 off cos cos V0 4Vd 0V0 4Vd1V0 cos 4Vd 0 8Vd1Vd 0 cos 2 2 4Vd1 cos d 2 2 1 4V1Vd1 cos( ) 4V1Vd1 cos( ) 2V0V1 sin( ) 4Vd 0V1 sin( ) 2RS 1 2 8V0Vd 1 sin( ) 16Vd 0Vd 1 sin( ) 2V0V 1sin( ) 4Vd 0V1 sin( ) V1 2 cos( ) sin( ) V1Vd1 sin(2 ) 8V0Vd 0 2V0 8Vd 0 4Vd1 2 2 1 2 2 2 V1 4Vd 1 cos( ) sin( ) V1 cos( ) sin( ) V1Vd 1 sin(2 ) 2 8V0Vd 0 V1 2V0 8Vd 0 4Vd1 2 2 2 2 66 APPENDIX B CALCULATION CONVERSION EFFICIENCY USING CLOSED-FORM EQUATION (RL PARALLEL WITH DIODE) 67 RL PARALLEL WITH DIODE LOSSon, RS 1 2R S off V off V V 1 2R S 2 f d off 2V0V1 cos 2V0V f V1 cos 2 2 2 0 off 2V1V f cos V f d 2 1 2RS 1 2 2 2V0 2V0V1 sin 4V0V f 2 V1 cos sin V1 2V f V1 sin 2V f 2V0V1 sin 2 2 1 2 V1 cos sin 2V f V1 sin 2 1 LOSSon, diode 2R S 1 2R S off V V 0 V V V f f d off V1V f cos V f d 2 f off 1 2 2V0V f V1V f sin 2V f V1V f sin 2R S LOSSoff ,RS off 1 2R S 1 2RS 2 off V V d 2 d off 2 off 2V V 0 1 cos 2V0Vd 1 cos V1 cos 2 2 off 2V1Vd1 cos cos Vd1 cos 2 V0 d 2 2 68 1 2RS 1 2 2 2V0V1 sin 4V0Vd 1 sin 2 V1 cos sin V1 1 2 2 2 2V1Vd 1 cos V1Vd 1 sin 2 Vd 1 cos sin Vd1 2V0 2 1 2 2 2V0V1 sin V1 cos sin V1 2V1Vd1 cos 2 1 2 2 V1Vd 1 sin 2 Vd 1 2V0 2 1 LOSSoff , diode 2R S 1 2R S 1 2RS 2 off V 2 0 2 off V V V d d d off V0V1 cos V1Vd 1 cos cos Vd 1 cos 2 d 2 off 1 2 2 V0V1 sin 4 V1Vd 1 2 Vd 1 2V0 V0V1 sin cos 1 1 V0V1 cos sin V1Vd1 cos sin cos V1Vd1 sin 2 2 1 1 1 2 2 2 cos V1Vd1 sin cos V1Vd 1 sin cos V1Vd1 sin 4 4 4 V1Vd1 cos
