Towards integrated AlGaN/GaN based X-band high-power amplifiers Citation for published version (APA): Jacobs, B. (2004). Towards integrated AlGaN/GaN based X-band high-power amplifiers. [Phd Thesis 1 (Research TU/e / Graduation TU/e), Electrical Engineering]. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR577275 DOI: 10.6100/IR577275 Document status and date: Published: 01/01/2004 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 14. Mar. 2023 Towards Integrated AlGaN/GaN Based X-Band High-Power Amplifiers PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 5 juli 2004 om 16.00 uur door Bart Jacobs geboren te Valkenswaard Dit proefschrift is goedgekeurd door de promotoren: prof.Dr.-Ing. L.M.F. Kaufmann en prof.Dr.-Ing. E. Kohn copromotor: dr. F. Karouta Druk: Universiteitsdrukkerij Technische Universiteit Eindhoven Ontwerp omslag: Paul Verspaget CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Jacobs, Bart Towards integrated AlGaN/GaN based X-band high-power amplifiers / by Bart Jacobs. - Eindhoven : Technische Universiteit Eindhoven, 2004. - Proefschrift. - ISBN 90-386-1593-0 NUR 959 Trefw.: galliumnitridehalfgeleiders / veldeffecttransistoren / passieve elektronische componenten / 3-5 verbindingen. Subject headings: wide band gap semiconductors / high electron mobility transistors / coplanar waveguides / III-V semiconductors. Aan mijn ouders Contents 1 Introduction 1 1.1 Advantages of the Gallium Nitride Material System . . . . . . . . . . . . . 1 1.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.3 Class A Amplifier Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.4 Objectives and Outline of this Thesis . . . . . . . . . . . . . . . . . . . . . 7 2 The Gallium Nitride Material System 11 2.1 Crystal Structure and Material Properties . . . . . . . . . . . . . . . . . . 11 2.2 Material Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2.1 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3 Polarization Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3.1 Undoped AlGaN/GaN HEMT Structures . . . . . . . . . . . . . . . 17 2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3 Reactive Ion Etching of GaN-Based Materials 21 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2 Reactive Ion Etching Setup and Working Principles . . . . . . . . . . . . . 21 3.3 Etching of GaN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.3.1 Description of Plasma Behavior Based on the DC Bias . . . . . . . 25 3.4 Etching of AlGaN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4 Metal-Semiconductor Contacts on AlGaN/GaN Heterostructures 31 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.2 Metal-Semiconductor Contacts . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.2.1 Fermi Level Pinning . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.2.2 Barrier Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.2.3 Metal-Induced Gap States . . . . . . . . . . . . . . . . . . . . . . . 36 4.2.4 Origin of the 2DEG in AlGaN/GaN Structures . . . . . . . . . . . 37 4.2.5 Experimental Approach . . . . . . . . . . . . . . . . . . . . . . . . 40 4.3 Ohmic Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.3.1 Metallization Scheme and Epitaxial Structure . . . . . . . . . . . . 41 4.3.2 The Transfer Length Method . . . . . . . . . . . . . . . . . . . . . 43 4.3.3 Optimization of the Ohmic Contact . . . . . . . . . . . . . . . . . . 46 4.3.4 Measurement Accuracy, Reproducibility, Line Definition and Mor- phology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 i ii CONTENTS 4.3.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.4 Schottky Contacts on AlGaN/GaN FET Structures . . . . . . . . . . . . . 50 4.4.1 Schottky Metallization . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.4.2 Wafer Description and Schottky Layout . . . . . . . . . . . . . . . . 51 4.4.3 Reference Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.4.4 Forward and Reverse Characteristics . . . . . . . . . . . . . . . . . 53 4.4.5 Optimization of Pre-Metallization Treatments . . . . . . . . . . . . 55 4.4.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5 AlGaN/GaN High Electron Mobility Transistors 61 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 5.2 Principles of Operation of the HEMT . . . . . . . . . . . . . . . . . . . . . 62 5.2.1 Breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5.2.2 High-Frequency Operation . . . . . . . . . . . . . . . . . . . . . . . 66 5.3 Dispersion Phenomena in AlGaN/GaN HEMTs . . . . . . . . . . . . . . . 68 5.3.1 Drain Lag and Buffer-Related Current Collapse . . . . . . . . . . . 68 5.3.2 Surface-Related Gate Lag and Transient Response. . . . . . . . . . 70 5.4 Optimization of AlGaN/GaN HEMT Processing . . . . . . . . . . . . . . . 73 5.4.1 Process Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.5 Submicron HEMTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 5.6 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 6 Passive Components on AlN 89 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 6.2 Matching Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 6.2.1 Quarter-Wavelength Transformer . . . . . . . . . . . . . . . . . . . 90 6.2.2 L-type Matching Networks . . . . . . . . . . . . . . . . . . . . . . . 90 6.3 Microstrip and Coplanar Waveguide Technology . . . . . . . . . . . . . . . 93 6.4 Substrate Material and Integration Technique . . . . . . . . . . . . . . . . 94 6.5 Constructing a CPW Library . . . . . . . . . . . . . . . . . . . . . . . . . 95 7 Coplanar Transmission Lines 99 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 7.2 Geometrical Quasi-TEM Range . . . . . . . . . . . . . . . . . . . . . . . . 100 7.2.1 Quasi-TEM Approximation . . . . . . . . . . . . . . . . . . . . . . 100 7.2.2 Non-CPW Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 7.3 Quasi-TEM Behavior of CPW Lines . . . . . . . . . . . . . . . . . . . . . . 102 7.3.1 Transmission Line Model . . . . . . . . . . . . . . . . . . . . . . . . 102 7.3.2 Conformal Mapping Results . . . . . . . . . . . . . . . . . . . . . . 103 7.4 Assessment of the Quasi-TEM Range . . . . . . . . . . . . . . . . . . . . . 106 7.5 De-embedding Algorithm for CPW Lines . . . . . . . . . . . . . . . . . . . 109 7.5.1 The Modified TRL Approach . . . . . . . . . . . . . . . . . . . . . 109 7.5.2 Summary of the Algorithm. . . . . . . . . . . . . . . . . . . . . . . 112 7.5.3 Comparison with Algorithm Based on Power Waves . . . . . . . . . 113 7.6 Quasi-TEM Coplanar Lines on AlN . . . . . . . . . . . . . . . . . . . . . . 114 CONTENTS iii 7.6.1 Scalable Model for the Line Capacitance . . . . . . . . . . . . . . . 115 7.6.2 Scalable Model for the Propagation Constant . . . . . . . . . . . . 116 7.6.3 Influence of the Measurement Setup. . . . . . . . . . . . . . . . . . 118 7.6.4 Scalable Model for the Adaptor . . . . . . . . . . . . . . . . . . . . 119 7.6.5 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 119 7.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 8 Orthogonal Elements 123 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 8.2 Coplanar Bend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 8.2.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 8.2.2 Development of a Scalable Model . . . . . . . . . . . . . . . . . . . 126 8.2.3 Properties of Coplanar Bends . . . . . . . . . . . . . . . . . . . . . 127 8.3 Coplanar T-junctions and Crosses . . . . . . . . . . . . . . . . . . . . . . . 128 8.3.1 Measurements on Multi-Ports . . . . . . . . . . . . . . . . . . . . . 128 8.3.2 Development of a Scalable Model . . . . . . . . . . . . . . . . . . . 130 8.3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 9 Capacitors and Resistors 135 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 9.2 Tapers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 9.3 Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 9.3.1 Breakdown Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 142 9.4 Resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 9.4.1 Series Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 9.4.2 Resistor to Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 9.4.3 Model Accuracy and Discussion . . . . . . . . . . . . . . . . . . . . 147 9.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 10 Application Examples: Matching Networks 149 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 10.2 Stub-Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 10.3 35-50 Ohm Matching Network . . . . . . . . . . . . . . . . . . . . . . . . . 153 10.4 2-to-1 Combiner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 10.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 11 Conclusions 159 11.1 HEMTs on AlGaN/GaN Heterostructures . . . . . . . . . . . . . . . . . . 159 11.2 Passive Components on AlN . . . . . . . . . . . . . . . . . . . . . . . . . . 161 11.3 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 A Waveguide Circuit Theory 163 A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 A.2 Waveguide Voltage, Current and Characteristic Impedance . . . . . . . . . 164 A.3 The Scattering Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 iv CONTENTS A.3.1 Travelling Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 A.3.2 Power Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 A.4 The Transmission Line Model . . . . . . . . . . . . . . . . . . . . . . . . . 168 B Processing of CPW Elements on AlN 171 B.1 Properties of Ceramic AlN . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 B.2 Process Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 C Scalable Models 175 C.1 Functions for the Adaptor . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 C.2 Functions for the Bend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 C.3 Functions for the Taper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 C.4 Functions for the Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . 175 C.5 Functions for the Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 C.6 Finding a Suitable Multi-Variable Function . . . . . . . . . . . . . . . . . . 177 C.6.1 Functions for T-junctions and Crosses . . . . . . . . . . . . . . . . 177 List of Symbols 181 List of Acronyms 187 Summary 191 Samenvatting 193 Dankwoord 195 Curriculum Vitae 196 Chapter 1 Introduction 1.1 Advantages of the Gallium Nitride Material Sys- tem For a new semiconductor material system to become successful, it must have clear ad- vantages over current solutions. Often the new material system must outperform other materials, enable new applications, or promise significant cost reductions. Table 1.1 shows some important properties for the most common semiconductors to- day (Si, gallium arsenide (GaAs) and indium phosphide (InP)) and the recently emerging wide bandgap semiconductors (WBGS) silicon carbide (SiC) and gallium nitride (GaN). For electronic applications, the added value of WBGS can be found in the combination of a high breakdown voltage with a high electron velocity. This promises the realization of high-frequency, high-power applications that cannot be realized in the other material systems. Another advantage is related to the bandgap itself. A wide bandgap implicates strong bindings in the material making it less susceptible to chemicals and temperature variations. Devices made using these materials can therefore be used in harsh environ- ments. In addition, WBGS can be used to fabricate light emitting devices in the blue to ultraviolet range. Clearly WBGS have advantages, but applications must be found that commercially justify the development of these materials into mature technologies. Some of these appli- cations will be discussed in the next section. 1.2 Applications Almost all of the early research done on WBGS was directed towards optoelectronic applications. This was due to the fact that blue was the only color missing on the commercial light emitting diode (LED) market. Before GaN became available, SiC was usedbutitsindirectbandgapresultedinratherpoorefficiencies. Usingtheindiumgallium aluminumnitride(InGaAlN)alloysystem, highlyefficientLEDswithwavelengthsranging from ultraviolet to blue/green can be realized. GaN-based LEDs can be used in conjunction with yellow and orange LEDs, made using the aluminum gallium indium phosphide (AlGaInP) material system, to realize full- 1