GaN Power Amplifiers for Next- Generation Wireless Communications

GaN Power Amplifiers for Next- Generation Wireless Communications Jennifer Kitchen Arizona State University Students: Ruhul Hasin, Mahdi Javid, Soroush Moallemi, Shishir Shukla, Rick Welker

Wireless Communications Circuits Lab - ASU - 19 students focused on PA, RF supply modulators, and RF transceiver circuits. - Over 1000 sq ft. laboratory space. RF Probe Station PA Load-Pull Station - RF measurement capability up to 40GHz. - PA characterization capability up to ~ 8GHz. 2

Outline Motivations & Objective for this Work GaN-on-Si Initial Explorations - Class AB PA - Switched-Mode Power Amplifiers: Transformer-Coupled Class D PA with SiGe Driver Other Switched-mode PA Topologies GaN-on-SiC Initial Explorations - High Power MMIC - Cubesat Applications - Doherty w/dynamic Load Modulation Conclusion 3

Motivation for Our Work Wireless Handset Transceiver s Power Consumption: PA + PM ~ 70-80% of Power Power Management DSP RF/AFE PA > 60% of Power Others Analog Base-Band Digital Base-Band & Memory RF [2005] [2014] Our Motivation: Higher Efficiency Lower Cost Higher Levels of Integration (Smaller Form Factor) Performance to meet future wireless standards 5G? 4

Applications to Industry & Technology Future-generation wireless devices: smartphones, pico/femto cell basestations, Cubesats, transceivers for IoT, beamforming MIMO systems with multiple transceivers, wearable electronics smaller sizes, lower power consumption, higher data rates, lower hardware cost 5

Main Objective To build modulation-agnostic (and multi-band) Linear Transmitters for handset and femtocell/picocell basestation applications. A single PA? Requirements: Maintain PA Efficiency over the entire output power range. Low Spurious Emissions & Good Signal Quality [ACLR, EVM]. Multi-Band Legacy RF Transmitters 6

How? We work with these architectures [2011] 7

Research Objectives - To explore GaN-on-SiC/GaN-on-Si as a potential process technology for implementing commercial PAs. - To innovate and implement wideband, high-efficiency GaN PAs with low cost and hardware overhead. - To improve the interface between signal processing in silicon and GaN power stages. 8

Our 2-Path Approach Modulation-Agnostic, High-Efficiency Amplifiers in GaN Digital Transmitters Employing Switched-mode PAs Efficiency-Enhancement & Linearization of Load-Modulated PAs RF Power DACs Digital RF &SM PAs Advanced Doherty Dynamic Load Modulation Envelope/Power Tracking PAs Low-loss switches High-efficiency drivers Low-jitter, high-speed switching Programmable/tunable load High-efficiency supply modulators Hardware overhead 9

GaN as an Enabler for Revolutionary PA Performance - Historically, low levels of integration. - Low yield. - Minimal reduction in knee voltage. - Requires high supply voltages for highest efficiencies. 10

GaN Processes We design amplifiers in both GaN-on-Silicon and GaN-on- Silicon Carbide (GaN-on-SiC) Each process has its own advantages/disadvantages: GaN-on-Si: Lower cost, lower frequencies. Commercial applications. GaN-on-SiC: Higher cost, higher frequencies, higher power. Basestation, space, and government applications. 11

GaN-on-Si Initial Explorations - Is the process capable of handling power/heat with low-cost assembly solutions? - What output power/efficiency are the devices capable of providing at low supply voltage? - Can we accurately predict performance for switched-mode PA applications? - Can we design high-efficiency switched-mode PAs for advanced communications standards (i.e. LTE, other)? 13

What Output Power/Efficiency are the Devices Capable of Providing? Build a distributed amplifier: BW = 0.4-4 GHz Psat = 3.5W 23% efficiency Build a class AB PA: low [10V] supply, low-thermally conductive die attach, small-form factor 14

Measured Class AB PA Performance Measured output power, gain and power added efficiency (PAE) 15

Comparison with Other Class AB PAs Reference Process Frequency (GHz) [2] SOI CMOS 0.13 µm Saturated Output Power (dbm) Peak PAE (%) Linearity 1.9 32.4 47 ACPR of -33 dbc (WCDMA) @ avg. Pout of 28.7 dbm [3] 65 nm 0.9 28.9 69.9 -- CMOS [4] GaN-on-Si 2.45 34.6 42.5 -- [5] GaAs HBT 0.824-0.849 28 36.76 ACPR of -54.61 dbc (CDMA) @ Pout of 28 dbm This Work GaN-on-Si 200 nm 0.82 0.90 30 55.44 Two tone IM3 > -40 dbc @ 6 db back off from P 1-dB GaN-on-Si process promises substantial improvements in PA performance when used in advanced architectures. 16

Legacy vs. Digital Tx Architecture Ongoing Work Presented Work 18

Digital Tx Design Challenges - How do we efficiently amplify digital waveforms? - Process technologies: GaN-on-Si. - High-efficiency switched-mode PA architectures. - Novel coding schemes: RF PWM/PPM, RF M. - How do we predict transmitter and PA performance? - Transient device models. - Performance verification in a system-level context. 19

GaN-on-Si Switching Power Stage ParBERT generates 2.25Gb/s switching signals with adequate power for driving 1200 m devices. High impedance probes for measuring output power at 1.125GHz. Power level is adjusted via the RF input signal pulsewidth (PWM). 20

GaN-on-Si Switching Power Stage 21

Coding Efficiency x PA Efficiency Digital Transmitter Efficiency transmitter = coding * PA V rms 2 Coding efficiency directly affects transmitter efficiency. 22

Coding Efficiency for Various Digital Processing Schemes M on FPGA (Systolic Array) Coding Efficiency 1.0 Bit 4:1 RZ 18.6% 1.5 Bit 4:1 RZ 37.6% Continuous Time M 1 Bit 4:1 NRZ 35.9% Asynchronous M 1.5 Bit 43.8% Pulse-Width Modulation 1 Bit 4:1 RZ 46.8% 23

GaN Class-D PA w/sige Driver 24

SiGe Driver Chip Specifications Specification Value Capacitive Load Range 1.25pF - 8pF Rise/Fall Time at Output <40ps On-Chip Input Termination Output Swing Supply Voltage 50 4.0 V single-ended 5.0 V Power Consumption 0.5-1.6W Temperature Range -40C to 125C 25

Power Amplifier Die-to-Die Assembly 26

Power Amplifier Board Assembly Differential Output Balun Die Differential Input 27

Power Amplifier Measurements Single-Carrier WCDMA: 6dB PAR 28

Comparison of This Work to Other Switched-Mode PAs Reference Freq (GHz) Class Pout (W) Gp (db) PAE (%) (%) Device Modulation 1 2.14 E 20 13 70 73 GaN Non Const Envelope 2 2 F 16.5 13 85.5 91 GaN Const Envelope 3 0.9 CMCD 20.7/ 51.1 75 GaN Const Envelope 4 2 E 11.4 12.6 74 GaN Const Envelope 5 2 E 4 57 62 GaN Const Envelope 6 2.14 F 10.5 12.2 70.9 75 GaN Const Envelope 7 2.14 CMCD 50 14.3 60.3 63 GaN Const Envelope 8 0.9 CMCD 870m 71 GaAs Const Envelope 9 0.337/ 1.125 VMCD 1.82 29.5 GaN Non Const Envelope 29

GaN-on-SiC Initial Explorations - What are the frequency and power limitations for GaN as a MMIC? - How do we integrate multiple MMICs at a module-level and manage thermal requirements? - How do we design for high-reliability and long-term field deployment? 31

High-Power MMIC - 40 Watt single-mmic Output Stage: 4-6GHz (currently in fab). - Thermal analysis shows good heat dissipation. Junctions remain within safe operating temperatures. - The ability to simulate, analyze, and measure PA temperature hot spots is instrumental to increasing efficiency and power. 32

+30GHz PA for Cubesat Application Build revolutionary power amplifiers for +30GHz aerospace and satellite applications. Requirements: Robust, Radiation Tolerant, Wide Temperature Range +30GHz operation with >30W output power (Psat) >50% PA Efficiency Small form factor integrated package solutions Specification Frequency Range Value 35.5-36.5GHz Saturated Output Power (0.5dB 32 W Compression) PA MMIC PAE ~25 % Gain Linearity ~25 db IM3@35dBm/tone: -31dBc IM5@35dBm/tone: -37dBc TID Hardness [krad] 3,000 Temperature Range -55C to +125C Fabrication Ka-Band CubeSat PA PA MMIC/ Integrated Module 33 PA MMIC Process Power Supply Qorvo GaN 0.15um 22 V

High Power Density PA MMIC & Module Single PA MMIC Integrated Module High Power Density Small Size Advanced GaN 34

Dynamic Load Modulation (DLM) Doherty Design DLM No additional λ/4 transformer required - CREE GaN-on-SiC devices. - Doherty Architecture w/dlm in the Main PA. - High efficiency at high power backoff. - Smaller form factor by removing the /4 at output. 35

Simulated Performance Summary 11 db back-off Gain (db) vs Output Power (dbm) and Drain Efficiency vs Output Power (dbm) at 1.9 GHz 36

Simulated Performance Summary State of the art performance, highest reported efficiency at 10dB power backoff, and wide bandwidth of operation 37

Summary We have explored GaN-on-Si and GaN-on-SiC for various applications: Commercial cellular Next generation digital architectures Cubesat applications GaN-based amplifiers show promise for: Medium power commercial applications. High frequency Cubesat, Satcom, or government applications. Is the cost worth the performance? 39