Towards integrated AlGaN/GaN based X-band high-power amplifiers

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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
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Published: 01/01/2004
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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