Gallium Nitride Based Transistors for High-Efficiency Microwave Switch-Mode Amplifiers

177 Pages·2011·4.36 MB·English

by  Stephan Maroldt

Gallium Nitride Based Transistors
for High-Efficiency Microwave
Switch-Mode Amplifiers

Dissertation

zur Erlangung des Doktorgrades
an der Technischen Fakultät
der Albert-Ludwigs Universität Freiburg im Breisgau

vorgelegt von
Dipl.-Ing. Stephan Maroldt

Juni 2010
Dekan: Prof. Dr. Hans Zappe
Referent: Prof. Dr. rer. nat. Oliver Ambacher
Koreferent: PD Dr.-Ing. habil. Frank Schwierz
Datum der Prüfung: 11.11.2010

II
Abstract
Highly-efficient switch-mode power amplifiers form key elements in future fully-digital
base stations for mobile communication. This novel digital base station concept reduces
system energy consumption, complexity, size and costs, while the flexibility in terms of
multi-band operation and signal modulation improves. In this work, innovative core circuits
for digital high-efficiency class-D and class-S power amplifiers based on gallium nitride
(GaN) technology were developed for the application in digital base stations. A combination
of optimized GaN devices and improvements in circuit design allow a highly-efficient switch-
mode operation at mobile communication frequencies between 0.45 GHz and 2 GHz.
Transistor device modeling for switch-mode operation, the simulation environment, and a
broadband measurement system were established for the design and evaluation of digital
switch-mode power amplifiers. The design of broadband core circuits for switch-mode
amplifier concepts was analyzed for dual-stage amplifier circuits, using an initial GaN
technology with a gate length of 0.25 µm. A speed-enhanced driver stage improved the circuit
switching speed sufficiently above 1 GHz. Speed and efficiency of the amplifier core circuits
were studied related to transistor parameters like cut-off frequency or gate capacitance. A
reduced gate length was found to improve the switching speed, while a lower on-resistance
allows the reduction of the inherent static losses of the GaN-based switches. Apart from this,
the restriction to a 50 Ohm environment was found to be the major output power and
switching speed limitation, due to a poor drive capability at the input of the GaN circuit.
Finally, the optimized transistor and circuit design with an output gate width of 1.2 mm
were effectively implemented in the given environment for an operation up to 2 GHz with a
high drain efficiency of > 65% and a digital output power of 5 W. A maximum output power
of 9.7 W and a circuit efficiency of > 80% were achieved for an operation at 0.45 GHz when
adjusting the transistor size for lower operation frequencies.
A further decisive improvement of speed and circuit complexity was found by the
implementation of enhancement-mode GaN transistors based on a high-transconductance
gate-recess technology. Transistors with a threshold voltage of +1 V were demonstrated with
a high current drive capability and a maximum transconductance of up to 600 mS/mm. Their
reduced input voltage swing tremendously increases the compatibility of digital power
amplifier circuits based on GaN and external digital driver and modulation circuits based on
silicon technology.
Moreover, an innovative development, the series-diode GaN transistor, replaces an off-
chip hybrid diode in the class-S amplifier with an integrated solution. It reduces parasitic
switching losses and improves the total amplifier properties in terms of operation frequency,
efficiency, and circuit complexity. A differential switch-mode core chip featuring series-diode
transistors and additional on-chip filter elements enabled our partner EADS to realize the first
class-S amplifier at 2 GHz worldwide in a module.
III
Zusammenfassung
Schaltverstärker mit hoher Energieeffizienz und Ausgangsleistung sind Schlüsselelemente
für die neueste Generation von Mobilfunkbasisstationen. Diese neuen digitalen Basisstationen
verringern den Energieverbrauch, die Kosten und die Baugröße einer Basisstation, wobei
gleichzeitig die Möglichkeit eröffnet wird, mehrere Frequenzbänder und Mobilfunkstandards
mit einem System zu bedienen. Im Rahmen dieser Arbeit wurden unter Verwendung der
Galliumnitrid-Technologie (GaN-Technologie) neuartige integrierte Leistungsverstärker-
schaltungen entwickelt. Der Einsatz dieser Schaltungen in digitalen Klasse-D und Klasse-S
Schaltverstärker in einer digitalen Basisstation trägt erheblich zur Verbesserung der
Energieeffizienz bei. Durch eine Optimierung der GaN-Bauelemente und der gleichzeitigen
Weiterentwicklung im Schaltungsdesign konnte erstmals eine Anwendung dieser Schaltungen
im Mobilfunkfrequenzbereich zwischen 0.45 GHz und 2 GHz ermöglicht werden.
Dazu wurde zunächst ein Transistormodell für den Schaltbetrieb erstellt sowie eine
Simulationsumgebung und ein Breitbandmesssystem für die Entwicklung des digitalen
Schaltverstärkers eingerichtet. Der Entwurf der integrierten Schaltungen für den Einsatz in
breitbandigen Schaltverstärkerkonzepten wurde am Beispiel eines zweistufigen
Leistungsverstärkers untersucht, für den eine GaN-Technologie mit 0.25 µm Gatelänge
verwendet wurde. Hierbei konnte die Schaltfrequenz durch eine gezielte
Geschwindigkeitsoptimierung der Treiberschaltung auf über 1 GHz erhöht werden. Weiterhin
wurde der Einfluss der Transistoreigenschaften, wie zum Beispiel der Grenzfrequenz und
Gatekapazität, auf die Geschwindigkeit und die Effizienz der Basisschaltungen untersucht.
Dabei konnte gezeigt werden, dass eine Reduzierung der Gatelänge deutlich zur Erhöhung der
Schaltgeschwindigkeit beiträgt, während ein reduzierter Widerstand im eingeschalteten
Zustand des GaN-Transistors die statischen Verluste signifikant verringert. Neben diesen
Ergebnissen wurde die Beschränkung auf eine Umgebungsimpedanz von 50 Ω als ein
wichtiger geschwindigkeitslimitierender Faktor identifiziert. Durch die hohe Impedanz des
externen Treibers kann die Eingangsgatekapazität der GaN-Schaltung nur langsam geladen
und entladen werden, wodurch die Schaltgeschwindigkeit begrenzt wird.
Verbesserte Schaltungs- und Transistordesigns ermöglichten letztendlich die Entwicklung
einer schnellen Verstärkerbasisschaltung mit 1.2 mm Gateweite in der Ausgangsstufe. Diese
kann digitale Signale bis zu einer Frequenz von 2 GHz mit einer hohen Draineffizienz > 65%
schalten, wobei eine digitalen Ausgangsleistung von 5 W erreicht wurde. Durch eine
angepasste Gateweite konnte eine hohe Ausgangsleistung von 9.7 W mit einer
Gesamteffizienz von über 80% für eine Frequenz von 0.45 GHz erreicht werden.

IV
Weiterhin wurden GaN-basierte selbstsperrende Transistoren mit sehr hoher Steilheit
mittels einer so genannten gate-recess Technologie entwickelt. Diese ermöglichen eine
zusätzliche Erhöhung der Schaltgeschwindigkeit sowie eine Vereinfachung der
Schaltungskomplexität. Es wurden Transistoren mit einer Schwellspannung von +1 V und mit
einer maximalen Steilheit bis zu 600 mS/mm realisiert, deren Ausgangsstrom und
Ausgangsleistung vergleichbar mit denen herkömmlicher selbstleitender Transistoren sind.
Durch ihre hohe Steilheit reduziert sich der benötigte Eingangsspannungshub zum An- und
Ausschalten des Transistors. Damit wird die Kompatibilität von GaN-basierten
Schaltverstärkerschaltungen zu externen digitalen Treiber- und Modulatorschaltungen, die in
Siliziumtechnologie hergestellt werden, erheblich verbessert und damit die Realisierbarkeit
der digitalen Basisstation vereinfacht.
Eine weitere innovative Entwicklung ist der Austausch einer hybriden Diode im
Modulaufbau des Class-S Verstärker durch eine integrierten Lösung on-chip, dem GaN
Transistor mit integrierter seriell geschalteter Diode. Dadurch werden parasitäre
Schaltverluste reduziert, die Schaltungskomplexität verringert und die maximale
Schaltfrequenz und Effizienz des gesamten Verstärkermoduls erhöht. Eine differentielle
Basisschaltung mit diesen Transistoren und zusätzlichen integrierten Filterelementen
ermöglichte dem Projektpartner EADS Deutschland die Realisierung des weltweit ersten
Class-S Verstärkermoduls bei 2 GHz.

V
Table of Contents

1 Introduction......................................................................................................................1
1.1 State of the Art: Switch-Mode Amplifiers for Mobile Communication...............3
2 Gallium Nitride Based Transistors for Microwave Switch-Mode Applications..............5
2.1 Semiconductor Technology for AlGaN/GaN Transistors.....................................5
2.1.1 Epitaxy of the Heterostructure........................................................................5
2.1.2 Processing of GaN Heterostructure Field Effect Transistors.........................7
2.2 Modeling of GaN Transistors for Microwave Broadband Switching...................10
2.2.1 Definition of Specific Transistor Properties...................................................12
2.3 Technological Realization of Enhancement-Mode GaN HFETs..........................14
2.3.1 Self-Aligned Gate-Recess Etching Technology.............................................16
2.3.2 Challenges and Further Requirements for Gate-Recessed HFETs.................21
3 Switch-Mode Power Amplifiers for Mobile Communication Using GaN HFETs..........23
3.1 Switch-Mode Concepts for Mobile Communication............................................23
3.1.1 Loss Mechanisms in Switch-Mode Amplifiers..............................................24
3.1.2 Loss Mechanisms: GaN Class-D Amplifier...................................................25
3.1.3 Current-Mode Class-S SMPA for Digital Amplifier Concepts......................28
3.1.4 3rd-Quadrant Issue of the HFET-Based Class-S Amplifiers...........................30
3.2 Characteristics of Digital-Switching Microwave Power Amplifiers....................33
3.2.1 Coexistence of Frequency and Time Domain for Digital Amplifiers............34
3.2.2 Broadband Power Switching Measurements in Time Domain.......................36
3.3 Digital Power Amplifier Using GaN HFET Technology......................................41
3.3.1 Simplified Large-Signal Switch-Mode Model...............................................41
3.3.2 Capacitance Charging Model for Digital Broadband Switching....................44
3.3.3 Absolute Switching Speed..............................................................................45
3.3.4 GaN Inverter Classes for Digital Driver Circuits...........................................49
3.4 Circuit Simulation, Design, and Specific Layout Aspects....................................55
3.4.1 General Simulation Considerations................................................................55
3.4.2 Major Circuit Design Parameters...................................................................56
3.4.3 Input Voltage Overdrive for Switching GaN HFETs.....................................56
3.4.4 Stability and Small-Signal Analysis...............................................................60
3.4.5 Layout Aspects...............................................................................................62
VI
4 Circuit Design for Digital Power Amplifiers...................................................................63
4.1 Conventional Dual-Stage Digital Amplifier.........................................................63
4.1.1 Design Approach............................................................................................64
4.1.2 Circuit Simulations.........................................................................................65
4.1.3 Measurement Results......................................................................................68
4.2 Switching Speed Improved Dual-Stage Digital Amplifier....................................77
4.2.1 Simulative Investigation of the Speed Improvement......................................78
4.2.2 Measurement Results......................................................................................83
4.3 Circuit Design Summary and Discussion of Loss Mechanisms...........................91
5 Impact of Device Properties on Circuit Switching Behavior...........................................95
5.1 Gate Length Scaling..............................................................................................96
5.1.1 Influence on Transistor Device Level.............................................................96
5.1.2 Digital Switching Performance.......................................................................98
5.2 Device On-Resistance Reduction..........................................................................102
5.2.1 Transistor Properties for Reduced Gate Contact Spacings.............................102
5.2.2 Impact of the On-Resistance on Circuit Level...............................................104
5.3 Gate Width Influence............................................................................................111
5.3.1 Impact on the Circuit Switching Speed..........................................................112
5.4 Circuits with Integrated Series Diodes..................................................................121
5.4.1 GaN HFETs with Integrated In Series Connected Schottky-Diode...............121
5.4.2 Digital Switch-Mode Amplifier Circuits Using SD-HFETs..........................124
5.5 Optimized Digital Switch-Mode Power Amplifiers..............................................130
5.6 Digital Amplifier Circuits Based on Gate-Recessed GaN HFETs........................139
5.6.1 GaN Inverters Using Enhancement-Mode HFETs.........................................139
5.6.2 Advantages of High-Transconductance HFETs for Digital Amplifiers.........140
5.6.3 Improvement of Switching Speed Due to Gate-Recessed HFETs.................141
6 Conclusions and Outlook..................................................................................................144
Appendix................................................................................................................................147
References..............................................................................................................................159
List of Publications.................................................................................................................165
Curriculum Vitae....................................................................................................................166
Acknowledgements................................................................................................................167
Eigenständigkeitserklärung....................................................................................................168
VII
List of Abbreviations
2DEG Two-dimensional electron gas
AlGaN Aluminum gallium nitride
BPDS Bandpass delta-sigma
BT Base transistor of an inverter circuit
CM Current-mode
CMCD Current-mode class-D
CPW Coplanar waveguide
CTC Current transfer characteristics
DC Direct current
DSA Driver-switching-ability
DTR SMPA driver with depletion transistor and resistive load
DTDR SMPA driver with depletion transistor and depletion FET/resistive load
DUT Device under test
E/D-mode Enhancement/Depletion-mode
FET Field effect transistor
GaAs Gallium arsenide
GaN Gallium nitride
Gbps Gigabit per second
HEMT High electron mobility transistor
HFET Heterostructure field effect transistor
LDMOS Laterally diffused metal oxide semiconductor
LT Load transistor of an inverter circuit
MISHFET Metal insulator semiconductor heterostructure field effect transistor
MESFET Metal semiconductor FET
MMIC Monolithic microwave integrated circuit
MSL Microstrip line
RF Radio frequency
RTC Resistance transfer characteristics
PA Power amplifier
PT Output power transistor of an integrated circuit
SD Series diode, Schottky-drain
SD-HFET Series-diode heterostructure field effect transistor
Si Silicon
SiC Silicon carbide
SiGe Silicon germanium
SiN Silicon nitride
SMPA Switch-mode power amplifier
VM Voltage-mode
VMCD Voltage-mode class-D

VIII
Symbols and Constants
Symbol Description Unit

BR Bitrate Gbps
BL Bit length ps
C Gate-source capacitance (normalized to W ) pF/mm
GS G
C Gate-drain capacitance (normalized to W ) pF/mm
GD G
C Drain-source capacitance (normalized to W ) pF/mm
DS G
d Barrier thickness of an AlGaN/GaN heterostructure nm
Bar
DE Drain efficiency %
DE(BR = 0) Extrapolated static drain efficiency (at a bitrate of zero) %
∆DE Slope of drain efficiency vs. bitrate pp/Gbps
ε Permittivity -
E Switching loss W/Gbps
sw
f Fundamental frequency GHz
0
f Maximum frequency of oscillation GHz
max
f Current-gain cut-off frequency GHz
T
gain Large-signal power gain dB
g Transconductance mS/mm
m
h Short-circuit current-gain (small-signal) dB
21
I Drain current (normalized to W ) mA/mm
D G
I Gate current (normalized to W ) mA/mm
G G
L Gate length nm
G
MAG Maximum available gain (small-signal) dB
MSG Maximum stable gain (small-signal) dB
PAE Power-added efficiency %
P RF input power W (dBm)
in
P RF output power W (dBm)
out
P (BR = 0) Extrapolated static RF output power (at bitrate of zero) W
out
P Static power dissipation/loss W
D_static
r Gate-width ratio between BT and PT in a dual-stage SMPA circuit -
BT-PT
r Inverter resistance transfer function -
drv
R Inverter output resistance Ω
drv
R Transistor on-resistance (normalized to W ) Ω (Ω·mm)
on G
τ Capacitance charging time constant ps
T, T Switching time constant ps
0
t On-/off-switching time, also fall/rise time ps
on/off
V Inverter voltage transfer function V
drv
V Drain-source voltage V
DS
V Gate-source voltage V
GS
V Threshold voltage V
th
W Gate width mm
G
x Aluminum content in the barrier of an AlGaN/GaN heterostructure %
Al
IX
1 Introduction

1 Introduction
Beside the World Wide Web, mobile communication is one of the most important
technical revolutions of our daily life in the last twenty years. Today’s internet and mobile
communication are merging. The permanent mobile accessibility of e-mail, social networks,
or media-sharing website services need very high data transmission rates of several
megabytes per second and user, increasing fast in the next years. A variety of communication
standards have been developed and established in recent years to follow the demand of our
society. A wide frequency range is used to generate the required bandwidth for the high data
rates, e.g., TETRA (terrestrial trunked radio) at 450 MHz, WiMAX (worldwide
interoperability for microwave access) at 2.45 and 3.5 GHz, 3rd generation mobile
communication (3G) like UMTS (universal mobile telecommunications system) at 0.9 and
2.1 GHz, and the upcoming 4th generation (4G) with LTE (long-term evolution) currently
between 0.7 and 2.6 GHz. All these techniques are using complex modulation schemes which
cause a low energy efficiency of the mobile communication base station systems, especially
due to the power amplifier. Most of the energy is wasted into heat instead of transmitting
information through an antenna. Today already more than 4 million operating base stations
worldwide [1] consume about five times more energy than the city of Berlin*. Taking into
account their still rapidly growing number especially in Asia [3], an ecological imbalance is
rising. The impact of global warming changes our daily life and a further expansion can
implicate worldwide dramatic consequences. New technical systems with impact on mass
market applications, like base stations for new mobile communication technologies, cannot be
developed without taking into account the social responsibility in terms of energy efficient
operation. To reduce the waste of energy the demand for the “green” base station intensifies.
Future advanced mixed-mode/digital concept
digital digital analog
digital driver
DSP digital
pre- + filter
output modulation
I/Q distortion SMPA
Integratedcircuitsbasedon silicon GaN

Fig. 1-1: Architecture of a future digital base station using a GaN-based switch-mode power amplifier.
To cope with this situation, efficiency-improving analog power amplifier concepts have
been suggested recently, e.g., envelope tracking [4] or Doherty amplifier [5]. Another leap in
efficiency and system architecture can be achieved due to a fully-digital base station (Fig. 1-1)
by using a highly efficient switch-mode power amplifier (SMPA). While today the complex,

* A typical base station with a power consumption of 2 kW has an energy consumption of 17.6 MWh per year. The city of
Berlin has an annual consumption of electrical energy of about 13000 GWh [2].
1