AlGaN/GaN HEMTs and HBTs Umesh K. Mishra
PART I AlGaN/GaN HEMTs
Materials Properties Comparison Material µ ε Eg BFOM JFM Tmax Ratio Ratio Si 1300 11.4 1.1 1.0 1.0 300 C GaAs 5000 13.1 1.4 9.6 3.5 300 C SiC 260 9.7 2.9 3.1 60 600 C GaN 1500 9.5 3.4 24.6 80 700 C BFOM = Baliga s figure of merit for power transistor performance [K*µ*Ec 3 ] JFM = Johnson s figure of merit for power transistor performance (Breakdown, electron velocity product) [Eb*Vbr/2π] U.S. Department of Defense
Advantages of WBG Devices Need Enabling Feature Performance Advantage High Power/Unit Width Wide Bandgap, High Field Compact, Ease of Matching High Voltage Operation High Breakdown Field Eliminate/Reduce Step Down High Linearity High Frequency HEMT Topology High Electron Velocity Optimum Band Allocation Bandwidth, µ-wave/mm-wave High Efficiency High Operating Voltage Power Saving, Reduced Cooling Low Noise High gain, high velocity High dynamic range receivers High Temperature Wide Bandgap Rugged, Reliable, Operation Reduced Cooling Thermal SiC Substrate High power devices with Management reduced cooling needs Technology Leverage Direct Bandgap: Enabler for Lighting Driving Force for Technology: Low Cost
Example of Advantage of WBG Devices 10-x power density ( > 10 W/mm) I N GaAs High Power Amplifier Module O U T 10-x reduction in power-combining Improved efficiency (> 60 %) Improved reliability Compact size Superior Performance at reduced cost I O N U T Equivalent High Power GaN Amplifier Module
Application Space 1000 100 Watts 10 1 0.1 Ship Radar Airborne Radar Military Commercial Base THAAD Navy Decoy Decoy Station Shipboard Radar,DD21 LONGBOW Satcom Satcom Base Station MMDS, UNII VSAT Driver WLL Hiperlan LMDS MVDS Amp 3G Digital Radio BAT P31 Missile Seekers CAR S C X Ku K Ka Q V W 2 GHz 10 GHz 30 GHz 60 GHz Frequency Band
Ball and Stick Diagram of the GaN Crystal
Polarization World
How does the electron gas form in AlGaN/GaN structures? -A Q, AlGaN π Q, AlGaN π Al x Ga 1-x N +++++++++++ GaN +++++++++++ Q, GaN π P( x) = ( Qπ, AlGaN ) + ( Qπ, GaN ) +++++++++++ Qπ, GaN Q π includes the contribution of spontaneous and piezo-electric contributions
How does the electron gas form in AlGaN/GaN structures? -C VAGaN l! EgGaN, + Ev= Maximum Dipole Moment AlGaN GaN Eg, GaN E v p s Q 2 π ( cm ) Q 2 π ( cm ) n s
How does the electron gas form in AlGaN/GaN structures? -D E DD AlGaN GaN E F Q ( cm ) N + 2 DD π Q π n s
Application of AlGaN/GaN 2DEG transport studies High density 2DEG mobility limited by alloy scattering. Introduce thin AlN interlayer remove alloy scattering by pushing 2DEG wavefunction out of the alloy region. Very useful result for attaining high conductivity 2DEG channels for HEMTs.
RF Plasma-Assisted Molecular Beam Epitaxy of AlN/GaN Heterostructures ONR/MURI IMPACT Center AlN MBE GaN ~ 0.3 µm MOCVD GaN ~ 2-3 µm d: 24 50 Å MBE growth of AlN/GaN structures is performedon GaN templates thick GaN layers grown by MOCVD on (0001) sapphire (0001) Sapphire Templates of two types are employed: (a) unintentionally doped GaN (dislocation density: ~ 5x10 8 10 9 cm -2 ) (b) semi-insulating GaN (dislocation density: ~ 10 10 cm -2 ) Ga-stable growth (III/V flux ratio > 1) T S ~ 740 o C AFM image of type (a) GaN template U.S. Department of Defense
AlN/GaN Structures Grown on Semi- Insulating GaN Templates ONR/MURI IMPACT Center N S (10 13 cm -2 ) 4 3.5 3 2.5 2 1.5 300 K 77 K 1 20 25 30 35 40 45 50 AlN thickness (A) Mobility µ (cm 2 /Vs) 5000 4000 3000 2000 1000 300 K 77 K 0 20 25 30 35 40 45 50 AlN thickness (A) 2DEG sheet density reaches the value of 3.65x10 13 cm -2 in the AlN/GaN structure with a 49 Å barrier Both room-temperature and 77 K electron mobility decrease drastically as AlN barrier width increases
AlN/GaN Structures Grown on Unintentionally Doped GaN Templates ONR/MURI IMPACT Center N S (10 13 cm -2 ) 10 1 100 T (K) d ~ 35 A d ~ 45 A 10 10 4 µ (cm 2 /Vs) 1000 10.00 100.0 1000/T (K -1 ) AlN ~ 35 Å µ(300 K) = 1460 cm 2 /Vs N 2DEG ª N S (20 K) = 2.2x10 13 cm -2 AlN ~ 45 Å µ(300 K) = 1230 cm 2 /Vs N 2DEG ª N S (20 K) = 2.7x10 13 cm -2 Pessimistic estimate of the 2DEG sheet resistance at 300 K: R < 200 Ω/ AlN ~ 50 Å µ(300 K) = 330 cm 2 /Vs; N S (300 K) = 5.6x10 13 cm -2 µ(77 K) = 660 cm 2 /Vs; N S (77 K) = 3.6x10 13 cm -2
Atomic Force Microscopy of AlN/GaN Heterostructures ONR/MURI IMPACT Center 1 µm 3 nm AlN: 24 Å AlN: 49 Å 0 nm Cracks in AlN layer AlN: 37 Å Tensile strain relaxation process begins at d ~ 49 Å
Polarization doping concept and demonstration 3D electron slab by Polarization doping demonstrated. Carrier density verified by self consistent Schrodinger Poisson calculations. How do transport properties of the 3DES compare to the donor doped and 2DEG counterparts?
Comparison of transport properties
DC Device Level Issues (HEMTs) If it ain t good @ DC it ain t goin to be good @ RF I max S G Lg D ------------- AlGaN P SP + P PE + + + + + + + -------------------- GaN V V max P = 1V I 8 I= V n υ max max max S Maximize I Maximize n S, ν Maximize n S Maximize P SP, P PE Maximize Al mole fraction without strain relaxation Mazimize ν Minimize effective gate length Minimize Lg and gate length extension Maximize µ Minimize dislocations Smooth interface
AlGaN/AlN/GaN Heterostructure 25 nm Al 0.3 Ga 0.7 N 1 nm AlN UID GaN SiC Substrate + High charge + High mobility Hall Data:! Conventional Al 0.3 Ga 0.7 N/GaN HEMT n s = 1.2 10 13 cm -2 µ = 1200 cm 2 /V/s! Al 0.3 Ga 0.7 N/AlN/GaN HEMT n s = 1.65 10 13 cm -2 µ = 1716 cm 2 /V/s
Band Diagram of AlGaN/AlN/GaN HEMT 250 Å Al 0.3 GaN/GaN HEMT 250 Å Al 0.3 GaN/ 10 Å AlN/GaN HEMT 3 3 Thin AlN 2 2 Energy (ev) 1 Energy (ev) 1 Effective E C 0 0-1 -50 0 50 100 150 200 250 300 350 400 450 500 Thickness (A) -1-50 0 50 100 150 200 250 300 350 400 450 500 Thickness (A)! n s = 1.35 10 13 cm -2! Higher charge: n s = 1.56 10 13 cm -2 due to higher effective E C! Higher mobility due to the removal of alloy disorder scattering U.S. Department of Defense
Sheet charge density vs. Al mole fraction 3.00E+013 2.50E+013 250Å Al x Ga 1-x N/10Å AlN/GaN Experiment Simulation 2DEG Density (cm -2 ) 2.00E+013 1.50E+013 1.00E+013 0.25 0.30 0.35 0.40 0.45 Al mole fraction x! Strongly dependent on the Al mole fraction
Sheet charge density vs. AlGaN thickness 2.00E+013 Al 0.37 Ga 0.63 N/10Å AlN/GaN Experiment of AlGaN/AlN/GaN Simulation of AlGaN/AlN/GaN Simulation of AlGaN/GaN 2DEG Density (cm -2 ) 1.50E+013 1.00E+013 5.00E+012 Conventional AlGaN/GaN 0.00E+000 0 20 40 60 80 100 120 140 160 180 200 220 Thickness of AlGaN (Å)! Weak function of AlGaN thickness! Faster saturation than conventional AlGaN/GaN HEMT
Sheet charge density vs. AlN thickness Simulation 2.40E+013 250Å Al 0.37 Ga 0.63 N/ AlN /GaN 2DEG Density (cm -2 ) 2.20E+013 2.00E+013 1.80E+013 10 15 20 25 30 Thickness of AlN (Å)! 2DEG increases when AlN is thicker
Device Performance of AlGaN/AlN/GaN HEMT I D (ma/mm) 1000 800 600 400 200 V G = 2 V V G = 1 V g m = 200 ms/mm Pout (dbm), Gain (db) 35 Pout 8.47 W/mm Gain 30 PAE 25 20 15 10 40 35 30 25 20 15 10 PAE (%) 0 5 5-2 0 2 4 6 8 10 12 14 16 V DS (V)! I max = 950 ma/mm! g m = 200 ms/mm 0 0 0 5 10 15 20 25 30 P in (dbm)! 8.47 W/mm with a PAE of 28% @ 8GHz! Bias: class AB at 45 V 160 ma/mm! Gate dimension: 0.7 150 µm 2
Issues With Mazimizing Al Mole Fraction in Al x Ga 1-x N AlN G Eg Al x Ga 1-x N GaN 2.5 3 3.5 Lattice Constant InN GaN Pseudomorphic Relaxed with Misfit Morphology Mediated By Dislocaton x Al = 0.2 x Al = 0.4 x Al = 0.6 x HC = 0.3 250 nm 250 nm 250 nm relaxed DISLOCATIONS LEAD TO PREMATURE RELAXATION OF AlGaN AND A POTENTIAL RELIABILITY PROBLEM BECAUSE OF THE METALLIZED PITS U.S. Department of Defense
Minimizing Gate Length Extension ELECTRONS IN SURFACE STATES AND/OR BUFFER TRAPS DEPLETE THE CHANNEL CAUSING GATE LENGTH EXTENSION I d (5 ma) DC Dispersion AC Load line SEVERE CONSEQUENCE: DISPERSION BETWEEN SMALL SIGNAL AND LARGE SIGNAL BEHAVIOR BECAUSE OF THE LARGE TRAP TIME CONSTANTS V ds (V) WHY DO THESE TRAPS ARISE? U.S. Department of Defense
Schematic of Device Structure SiN Passivation SOURCE GATE DRAIN AlGaN 2DEG GaN Substrate: Typically Sapphire or SiC Nucleation Layer GaN, AlGaN or AlN
Dispersion in AlGaN/Ga HEMTs DC I d (5 ma) Dispersion AC Load line V ds (V)
Source Lg GATE Electrons in Surface States AlGaN Drain GaN Depletion of 2DEG caused by occupied surface states
Performance of Passivated AlGaN/GaN HEMT on Sapphire
Performance of AlGaN/GaN HEMT on SiC (CLC) P out (db), Gain(dB), PAE (%) 45 40 35 30 25 20 15 10 5 f=8ghz, Tuned for Power Pout Gain PAE Id P out = 10.3 W/mm PAE= 42% 41.6% 160 140 120 100 80 60 40 20 I d (ma) 0 0 0 5 10 15 20 25 30 P in (db)
Drain Bias Dependence of Rf Power (CLC) 10 PAE; P out f=8ghz 70 9 60 8 50 P out (W/mm) 7 6 5 4 3 2 1 Increasing V ds 40 30 20 10 0-10 -20 PAE (%) 0-5 0 5 10 15 20 25 P in (db) -30
Flip-chip AlGaN/GaN HEMT for Thermal Management
I-V Curves from 8mm-wide HEMT V g start: +2V, Step: -2 V I d (A/divsion) V ds (V/divsion)
Low Flip-chip Wide Bandwidth Amplifier
Pulse Power Performance of mm-flipped Device Gain (db), DE(%), PAE(%) 40 35 30 25 20 15 10 5 Gain DE PAE Pout 50 45 40 35 30 25 20 15 P out (dbm) 0 10 18 22 26 30 34 38 42 P in (dbm)
CREE, Inc. 20-Watt Broadband SiC MESFET Amplifier 16 14 44 42 Gain (db) 12 10 Balanced amplifier with Cree s 10-Watt commercial FETs, CRF22010 22 W at P 1dB across a 400 MHz band Advantage of wide bandgap transistors: power-bandwidth product greatly exceeding Si LDMOS 8 6 3GPP Test Model 1 with 16 DPCH 4 Gain P1dBm W-CDMA 32 2 30 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 Frequency (GHz) 40 38 36 34 P1dB (dbm)
CREE, Inc. 75-Watt SiC MESFET Amplifier 2 GHz test fixture for 60 W MESFET development 40 0 28 30 32 34 36 38 40 Input Power (dbm) 75 W CW, 11 db gain demonstrated from a single SiC MESFET Currently being optimized for a 60-Watt Class A MESFET product, targeted for release by the end of the year Output Power (dbm) 50 48 46 44 42 Power PAE Freq. = 2.0 GHz 50 40 30 20 10 PAE (%)
CREE, Inc. 10-Watt Broadband GaN HEMT Amplifier P1dB (dbm) 44 42 40 38 36 34 32 P1dB (dbm) SS Gain (db) 24 22 20 18 16 14 12 Small Signal Gain (db) 30 10 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 Frequency (GHz) 11 W at P 1dB across the 1.8-2.2 GHz band 17 db gain with only ±0.15 db ripple
Sponsored by Tom Jenkins, DUS&T/AFRL U.S. Department of Defense CREE, Inc. Demonstration of 100W CW GaN HEMT 103W @ 2.6 db compression 40 50 35 Freq. = 2 GHz V DS = 52V Peak Drain Eff.. = 54% 108 W achieved at P 3dB Output Power (dbm) 48 46 44 P out Gain 0 32 34 36 38 40 42 44 Input Power (dbm) 30 25 20 15 10 5 Gain(dB)
CREE, Inc. Summary of Technology Status Commercially-available SiC MESFETs 22 W broadband amplifier demonstrates powerbandwidth advantage of this technology 60 W Class A MESFET targeted for release by end of year GaN HEMTS on SiC Substrates excellent broadband and high power performance demonstrated emphasis of development is now on reproducibility and reliability SiC-based MMIC process developed for both technologies 3-inch SI substrates will enable cost-competitive manufacturing U.S. Department of Defense
Power Performance vs. Year Cree 108W, CW Cree Power density (W/mm) 11 Power density 10 9 8 7 6 5 4 3 2 1 Early Players 0 1996 1998 2000 2002 Year Cornell CREE Cree HRL Cornell SiC Sapphire 60 Cree 50 Total power (W) 40 30 20 10 Total power SiC Sapphire Early Players Cornell 0 1996 1998 2000 2002 Year Includes ALL LEADING players in the field CREE CREE = Cree Lighting+ Cree-Durham Cree CREE HRL Cree NEC
Part II High Voltage Operation (> 330 V) of AlGaN/GaN HBTs
Bipolar transistor key issues Injection γ "1 n " 1 [I=I 0 exp (qv/nkt)] Emitter Base Transport -- α " 1 Collector Subcollector Collection C bc " 0 Output Conductance v " v sat [2 x 10 7 cm/s] (Kolnik et. al.) I C / V CE " 0 V br " E crit W C [E crit ~ 2 MV/cm] (Bhapkar and Shur.) ( W B / V CE " 0)
Hurdles with GaN bipolar transistors Lack of low damage etch to reveal base Leaky E/B junction Bad base contact No etch stop High R B Poor p-gan base contact Low p-gan base conductivity Deep acceptor (~160 mev) Hard to control junction placement in MOCVD due to memory effect of p-p dopant Mg Emitter Low minority carrier lifetime Surface leakage due to etch damage Base Collector Subcollector Dislocation causes leakage
Demonstration of dislocation enhanced leakage LEO used to investigate leakage of devices without dislocations. (Lee McCarthy et al.) AFM scan of wing vs Window on LEO GaN Leakage Current [A] 10 10-6 100 10-9 1 10-9 10 10-12 100 10-15 Dislocated LEO 1 10-15 -20-15 -10-5 0 5 10 15 20 Applied Bias [V] Leakage from Collector to Emitter, Wing vs Window Results: LEO device demonstrated Reduction in Leakage Stable operation past 20V Gain unchanged Devices on dislocated material also functional Regrowth Mask LEO GaN Regrowth Mask Standard template Explanation Thick substrate sufficiently reduces dislocations to prevent C/E short in window region Gain (τ e ) not currently limited by dislocation density U.S. Department of Defense
Strategy: Thick Collector Decent dislocation density High quality MOCVD templates achieved Dislocation density ~ 5e8 cm -2 Low background doping N D < 1e16 cm -3 (Assuming uniform doping N D and E critical = 2 MV/cm, requires 10 µm to achieve 1 KV breakdown voltage.) Doping vs. Depth (010704GA, 8 µm collector) 1.E+17 Doping (cm -3 ) 1.E+16 1.E+15 1.E+14 0 1 2 3 Depth (µm)
Emitter Regrowth Process Flow Selectively grow MOCVD emitter on base-collector structures. 1. Pattern regrowth mask 2. Regrow emitter layer by MOCVD 3. Remove mask and contact base and etch to collector 4. Contact collector, emitter 1. 2. Mask p Base n - Collector n + Subcollector Sapphire Substrate n + Emitter p Base n - Collector n + Subcollector Sapphire Substrate Regrown emitter 3. 4. n + Pd/Au Emitter p Base n - Collector Al/Au Al/Au n + Emitter p Base n - Collector n + Subcollector Sapphire Substrate n + Subcollector Sapphire Substrate
Device structure Utilization of uid GaN spacer and grading layer - HBTs with high emitter injection coefficiency Etch damage and current mask layout limits V br 4 nm GaN:Si (1e18 cm -3 ) contact 4 nm Al 0.05 GaN->GaN:Si (1e18 cm -3 ) grading 105 nm Al 0.05 GaN:Si (1e18 cm -3 ) emitter 8 nm GaN->Al 0.05 GaN (?3e18 cm -3 ) grading 8 nm uid GaN spacer 100 nm GaN:Mg (2e19 cm -3 ) base 8 µm uid GaN (4e15 cm -3 ) collector 2 µm GaN:Si (1e18 cm -3 ) subcollector Sapphire Effective collector thickness ~ 2-3 µm n + Subcollector n + Emitter 1000 Å p Base 8 µm n- collector Sapphire Substrate
HBT with 8 mm GaN collector Current gain (β) > 20 Common emitter operation > 300 V Non-passivated Base thickness 1000 Å Al 0.05 GaN emitter Vbr ~330 V
Summary Conclusion In selective emitter regrowth, a sharp Mg profile, ~ 40 nm/decade, enables the precise junction placement Improvement of regrown-emitter/base diodes Demonstration of high Vbr (> 300 V) with high β (DC common emitter operation up to 35)