AlGaN/GaN HEMTs and HBTs

Transcription

1 AlGaN/GaN HEMTs and HBTs Umesh K. Mishra

2 PART I AlGaN/GaN HEMTs

3 Materials Properties Comparison Material µ ε Eg BFOM JFM Tmax Ratio Ratio Si C GaAs C SiC C GaN 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]

4 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

5 DC Device Level Issues (HEMTs) If it ain t DC it ain t goin to be 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

6 Issues With Mazimizing Al Mole Fraction in Alx Ga 1-x N AlN G Eg Al x Ga 1-x N GaN 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 = nm 250 nm 250 nm relaxed DISLOCATIONS LEAD TO PREMATURE RELAXATION OF AlGaN AND A POTENTIAL RELIABILITY PROBLEM BECAUSE OF THE METALLIZED PITS

7 Issues With Increasing Mobility 30 INCREASING Al MOLE FRACTION DECREASES MOBILITY High Mobility AlGaN/GaN Structures Grown by RF Plasma Assisted MBE N S, cm -2 µ 300Κ [cm 2 / Vs] x = /T, 1/K Mobility v. Al Fraction Plot Relaxed x Al n s [10 13 cm -2 ] µ, cm 2 /Vs µ 300Κ [cm2 / Vs] Al 0.3 Ga 0.7 N R MS [nm]

8 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?

9 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

10 Application Space Watts 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

11 Schematic of Device Structure SiN Passivation SOURCE GATE DRAIN AlGaN 2DEG GaN Substrate: Typically Sapphire or SiC Nucleation Layer GaN, AlGaN or AlN

12 Ball and Stick Diagram of the GaN Crystal

13 Polarization World

14 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

15 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

16 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

17 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

18 Dispersion in AlGaN/Ga HEMTs DC I d (5 ma) Dispersion AC Load line V ds (V)

19 Source Lg GATE Electrons in Surface States AlGaN Drain GaN Depletion of 2DEG caused by occupied surface states

20 Performance of Passivated AlGaN/GaN HEMT on Sapphire

21 Performance of AlGaN/GaN HEMT on SiC (CLC) P out (db), Gain(dB), PAE (%) f=8ghz, Tuned for Power Pout Gain PAE Id P out = 10.3 W/mm PAE= 42% 41.6% I d (ma) P in (db)

22 Drain Bias Dependence of Rf Power (CLC) 10 PAE; P out f=8ghz P out (W/mm) PAE (%) Increasing V ds P in (db)

23 Flip-chip AlGaN/GaN HEMT for Thermal Management

24 I-V Curves from 8mm-wide HEMT V g start: +2V, Step: -2 V I d (A/divsion) V ds (V/divsion)

25 Low Flip-chip Wide Bandwidth Amplifier

26 Pulse Power Performance of mm-flipped Device Gain (db), DE(%), PAE(%) Gain DE PAE Pout P out (dbm) P in (dbm)

27 Power Performance vs. Year Cree 108W, CW Cree Power density (W/mm) 11 Power density Early Players Year Cornell UCSB CREE Cree UCSB HRL Cornell SiC Sapphire 60 Cree 50 Total power (W) Total power SiC Sapphire Early Players Cornell UCSB Year Includes ALL LEADING players in the field CREE CREE = Cree Lighting+ Cree-Durham Cree CREE HRL Cree NEC

28 Part II High Voltage Operation (> 330 V) of AlGaN/GaN HBTs

29 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)

30 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

31 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] Dislocated LEO 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

32 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 Depth (µm)

33 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 Mask p Base n - Collector n + Subcollector Sapphire Substrate n + Emitter p Base n - Collector n + Subcollector Sapphire Substrate Regrown emitter 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

34 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 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 Depth (µm) Memory effect Slow decay tail in regrowth

35 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 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

36 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

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38 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) GF 400 nm GaN:Si (4e18 cm -3 ) ~ / AE AF GC 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 ~ / / /300 I density (A/cm 2 ) x10-4 1x GF AE AF GC AA Year 2000 Year AA x Al ~ 5% 30 nm GaN:Si 30 nm AlGaN->GaN:Si 730 nm AlGaN:Si 60 nm GaN->AlGaN 60 nm GaN ~ / V (Volts)

39 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

40 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

41 UCSB I-V Characteristics reasonable base contacts Improved B/E diodes Rectifying B/C diodes, Vbr > 300 V 0.12 I (µa) V (Volts) I (ma) V BE (V) Base-emitter diode Base-collector diode

42 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)