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]
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
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
Issues With Mazimizing Al Mole Fraction in Alx 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
Issues With Increasing Mobility 30 INCREASING Al MOLE FRACTION DECREASES MOBILITY High Mobility AlGaN/GaN Structures Grown by RF Plasma Assisted MBE 7 10 4 N S, 10 12 cm -2 µ 300Κ [cm 2 / Vs] 25 20 15 10 5 0 x = 0.09 0 10 100 1000 1000/T, 1/K Mobility v. Al Fraction Plot 1500 1400 1300 1200 1100 1000 0 Relaxed 0.0 0.2 0.4 0.6 x Al 1.8 1.6 1.4 1.2 1.0 0.8 0.6 n s [10 13 cm -2 ] 6 10 4 5 10 4 4 10 4 3 10 4 2 10 4 1 10 4 µ, cm 2 /Vs µ 300Κ [cm2 / Vs] 1400 1200 1000 800 Al 0.3 Ga 0.7 N 600 0.1 0.2 0.3 0.4 0.5 0.6 R MS [nm]
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?
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
Schematic of Device Structure SiN Passivation SOURCE GATE DRAIN AlGaN 2DEG GaN Substrate: Typically Sapphire or SiC Nucleation Layer GaN, AlGaN or AlN
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? - B Q, InGaN π + Q, InGaN π In x Ga 1-x N +++++++++++ ( π, GaN ) ( π InGaN ) P = Q + Q ( x), Q, GaN π +++++++++++ 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
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 40 30 20 10 PAE (%) 3 2 1 0 Increasing V ds -5 0 5 10 15 20 25 P in (db) 0-10 -20-30
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)
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 UCSB CREE Cree UCSB HRL Cornell SiC Sapphire 60 Cree 50 Total power (W) 40 30 20 10 Total power SiC Sapphire Early Players Cornell UCSB 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
UCSB 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
UCSB 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
UCSB 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)
UCSB 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
UCSB SIMS: Mg by MOCVD Severe memory effect observed in non-interrupted MOCVD growth Slow decay tail into GaN:Si regrown on as-grown GaN:Mg layer Concentration (cm -3 ) 1E+21 1E+20 1E+19 1E+18 1E+17 1E+16 1E+15 Turnoff of Mg source Decay rate ~ 140 nm/decade Si Mg Low Mg background in clean reactor Growth direction 230 nm 400 nm 0 0.5 1 1.5 2 2.5 3 Depth (µm) Concentration (cm -3 ) 1.E+21 1.E+20 1.E+19 1.E+18 1.E+17 1.E+16 Regrown surface Decay rate ~ 115 nm/decade Si ~ 30 nm/decade Mg 1 1.5 2 2.5 3 3.5 Depth (µm) Memory effect Slow decay tail in regrowth
UCSB SIMS: regrowth with surface treatment Regrown in Mg-free reactor and all grown by MOCVD Presence of an excessive amount of Mg on the surface, which can be removed by acid etch Occurrence of Mg diffusion, ~ 40 nm/decade sharpness achieved Mg concentration (a.u.) 1E+23 1E+22 1E+21 1E+20 1E+19 1E+18 1E+17 1E+16 1E+15 C D E Regrowth surface 0 500 1000 1500 2000 Depth (nm) C: as grown, ~100 nm/decade D: etch in HFand HCl for 5 min, ~ 50 nm/decade E: etch in BHF (1:20) for 15 sec and HCl for 20 sec, ~40 nm/decade Detection limit
UCSB Selectively regrown n/p diodes Mask enhanced growth complicates the analysis Regrowth rate depends on the mask layout, diode size etc Bunny ear regrowth profile is often seen Only the emitter edge is active in device operation due to highly resistive base layer The junction quality depends on how the regrowth is initiated, e.g. Temp, P, flows, presence of Si and Al etc. Bunny ear regrowth profile of two different square diodes
UCSB Regrown n/p Diodes Characteristics Comparison of various structures regrown on 0.5 µm GaN:Mg Run # Layer structure Growth Parameter G.R. Temp/Press (nm/min) (C/Torr) 0005 10GF 400 nm GaN:Si (4e18 cm -3 ) ~30 1140/760 10 2 10 1 0007 14AE 0007 14AF 0105 13GC 250 nm Al 0.06 GaN:Si (1e18) x Al ~ 5% 250 nm AlGaN:Si (1e18) 75 nm GaN->AlGaN:Si (1e18) 450 nm GaN:Si (1e18) 30 nm GaN ~30 ~30 ~40 1100/300 1100/300 1100/300 I density (A/cm 2 ) 10 0 10-1 10-2 10-3 1x10-4 1x10-5 000510GF 000714AE 000714AF 010513GC 010517AA Year 2000 Year 2001 0105 17AA x Al ~ 5% 30 nm GaN:Si 30 nm AlGaN->GaN:Si 730 nm AlGaN:Si 60 nm GaN->AlGaN 60 nm GaN ~60 1100/300 10-6 -20-15 -10-5 0 5 V (Volts)
UCSB 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
UCSB 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
UCSB I-V Characteristics reasonable base contacts Improved B/E diodes Rectifying B/C diodes, Vbr > 300 V 0.12 I (µa) 50 40 30 20 10 0-10 -20-30 -40-50 -10-8 -6-4 -2 0 2 4 6 8 10 V (Volts) I (ma) 0.10 0.08 0.06 0.04 0.02 0.00-0.02-0.04-12-10-8 -6-4 -2 0 2 4 6 8 10 V BE (V) Base-emitter diode Base-collector diode
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)