Recent Progress in Understanding the Electrical Reliability of GaN High-Electron Mobility Transistors J. A. del Alamo Microsystems Technology Laboratories Massachusetts Institute of Technology RSAMD 2014 Golden, CO, Sept. 7-9, 2014 Acknowledgements: F. Gao, J. Jimenez, D. Jin, J. Joh, T. Palacios, C. V. Thompson, Y. Wu ARL (DARPA-WBGS program), NRO, ONR (DRIFT-MURI program),
Outline 1. Motivation 2. Electrical and structural degradation of GaN HEMTs 3. Hypotheses for GaN HEMT degradation mechanisms 4. Paths for mitigation of GaN HEMT degradation 2
Breakthrough RF-µw-mmw power in GaN HEMTs Micovic, MTT-S 2010 Micovic, Cornell Conf 2010 94-95 GHz MMIC PAs: P out >40 W/mm, over 10X GaAs! Wu, DRC 2006 3
GaN HEMTs in the field Counter-IED Systems 200 W GaN HEMT for (CREW) cellular base station Kawano, APMC 2005 100 mm GaN-on-SiC volume manufacturing Palmour, MTT-S 2010 4
GaN HEMT: Electrical reliability concerns High-power: – Not accessible to DC ON: stress experiments – Mostly benign – Device blows up instantly High-voltage OFF and semi-ON: – Degradation of I Dmax , R D , I Goff – V T shift – Electron trapping – Trap creation 5
Critical voltage for degradation in DC step-stress experiments 1.2 1.E+01 OFF-state, V GS =-10 V V DS 1.15 1.E+00 V GS =-10 V I Dmax /I Dmax (0), R/R(0) 1.1 1.E-01 R D |I Goff | (A/mm) G S D R S 1.05 1.E-02 AlGaN 1 1.E-03 2DEG 0.95 1.E-04 I Dmax I Goff 0.9 1.E-05 GaN V crit 0.85 1.E-06 Joh, EDL 2008 10 20 30 40 50 V DGstress (V) I Dmax : V DS =5 V, V GS =2 V I Goff : V DS =0.1 V, V GS =-5 V I D , R D , and I G start to degrade beyond critical voltage (V crit ) + increased trapping behavior – current collapse 6
Critical voltage: a universal phenomenon GaN HEMT on SiC GaN HEMT on SiC GaN HEMT on SiC Meneghini, IEDM 2011 Ivo, MR 2011 Liu, JVSTB 2011 GaN HEMT on Si GaN HEMT on Si GaN HEMT on sapphire Demirtas, ROCS 2009 Ma, Chin Phys B 2011 Marcon, IEDM 2010 7
Structural degradation: cross section - Small dimple in Gate early stages of I G degradation; - I D degradation AlGaN delayed GaN 1.2 1.E+02 V DS =0 stress #2 Device #1 I Dmax /I Dmax (0), R/R(0) 1.E+01 #3 I Goff 1 |I Goff | (mA/mm) #4 1.E+00 #5 0.8 1.E-01 1.E-02 I Dmax 0.6 1.E-03 #6: unstressed 0.4 1.E-04 10 20 30 40 0 10 20 30 V DGstress (V) Joh, MR 2010 8
Correlation between pit geometry and I Dmax degradation Joh, MR 2010 50 20 Permanent I Dmax Degradation (%) Pit 40 15 depth Current collapse (%) 30 10 20 5 10 0 0 0 2 4 6 8 0 2 4 6 8 Pit depth (nm) Pit depth (nm) Pit depth and I Dmax degradation correlate: both permanent degradation and current collapse (CC) 9
Structural degradation: planar view OFF-state step-stress, V GS =-7 V, T base =150 °C Makaram, APL 2010 200 nm V DG =15 V 200 nm V DG =19 V Unstressed 200 nm (V crit ) 200 nm V DG =42 V V DG =57 V 200 nm • Continuous groove appears for V stress <V crit • Deep pits formed along groove for V stress >V crit 10
Correlation between pit geometry and I Dmax degradation Makaram, APL 2010 Cross-sectional area averaged over 1 µm 12 12 Permanent I Dmax Degradation (%) Post-Stress Current Collapse (%) 10 10 8 8 6 6 4 4 2 2 0 0 0 50 100 150 0 50 100 150 Average Defect Area (nm 2 ) Average Defect Area (nm 2 ) I Dmax degradation and pit cross-sectional area correlate 11
Planar degradation: the role of time V DS =0, V GS =-40 V, T base =150 °C Joh, IWN 2010 10s 100s 10ks 1ks G A T E 0.5µm • Very fast groove formation (within 10 s) • Delayed pit formation • Pit density/size increase with time • Good correlation between I Dmax degradation and pit area 12
-4 -4 Time evolution of 0.25 0.25 10 10 Stress: V GS =-7 V and V DS =40 V Stress: V GS =-7 V and V DS =40 V 125 °C 125 °C 0.2 0.2 degradation for -5 -5 10 10 I Goff I Goff 0.15 0.15 |I Goff | (A) |I Goff | (A) | ∆ V T | (V) | ∆ V T | (V) constant V stress > V crit -6 -6 10 10 0.1 0.1 | Δ V T | | Δ V T | -7 -7 10 10 0.05 0.05 I Goff and V T degradation: -8 -8 • fast (<10 ms) 0 0 10 10 -4 -4 -2 -2 0 0 2 2 4 4 6 6 10 10 Initial Initial 10 10 10 10 10 10 10 10 10 10 saturate after 10 4 s • Stress time (s) Stress time (s) CC degradation: • slower • hint of saturation for long time Permanent I Dmax degradation: • much slower • does not saturate with time Joh, IRPS 2011 13
The role of temperature in time evolution Incubation time • I G : weak T dependence • CC, I Dmax : T activated Joh, IRPS 2011 14
Temperature acceleration of incubation time 15 Permanent I Dmax degradation E a =1.12 eV 10 ln( τ inc ) (s) Current collapse 5 E a =0.59 eV 0 I Goff , E a =0.17 eV -5 28 30 32 34 36 1/kT (eV -1 ) • Different E a for I Goff , CC, I Dmax reveal different degradation physics • E a for permanent I Dmax degradation similar to life test data * * Saunier, DRC 2007; Meneghesso, IJMWT 2010 15
DC semi-ON stress experiments Stress conditions: AFM I D =100 mA/mm, V DS =40 or 50 V a <230 o C Step-T experiments: 50<T SEM Prominent pits and trenches under gate edge on drain side Wu, submitted to TED 16
Structural vs. electrical degradation 10 90 Gate Souce Drain 80 5 Trench/pit width, depth (nm) Trench width 70 Trench/pit width 0 Depth (nm) 60 -5 50 40 -10 Trench depth 30 -15 20 Trench/pit depth Δ I D =25.8% 10 -20 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 5 10 15 20 25 30 x ( µ m) Permanent I Dmax degradation (%) Trench/pit depth and width correlate with I Dmax degradation Wu, submitted to TED 17
Thermally activated degradation Source Drain Gate fingers 7.5 4.0 Source 3.5 7.0 Distance from center of gate finger Depth of damage (nm) 3.0 6.5 2.5 ln(1/|slope|) 6.0 2.0 E a =1.04 eV 5.5 1.5 5.0 1.0 4.5 0.5 Δ I D =21.6% Wu, ROCS 2014 4.0 0.0 0 20 40 60 80 100 120 140 160 180 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 -1 ) Distance from center of gate finger ( µ m) 1/kT channel (eV • Pit/trench depth increase towards center of gate finger self heating + thermally activated process • Permanent I Dmax degradation is thermally activated with E a ~1.0 eV 18
Sequential I G and I D degradation START 1.1 Stress conditions: I D =100 mA/mm, 1.0 V DS =40 or 50 V 0.9 a <230 o C Step-Temperature: 50<T 0.8 I Dmax /I Dmax (0) evolution of 0.7 stress experiment 0.6 0.5 0.4 0.3 Wu, ROCS 2014 1E-4 1E-3 0.01 0.1 1 10 |I Goff | (mA/mm) “Universal degradation” pattern: • I G degradation takes places first without I D degradation • I D degradation takes place next without further I G degradation 19
RF power degradation SEM AFM • RF power degradation pattern matches that of OFF-state DC stress • But not always… Joh, IEDM 2010 Joh, ROCS 2011 Joh, MR 2012 20
Summary of electrical and structural degradation 1. I G degradation • Fast • Electric-field driven • Little temperature sensitivity (E a ~0.2 eV) • Tends to saturate Correlates with appearance of shallow groove and small pits • On S and D side (bigger on D side) • Groove/small pits appear for V stress < V crit 21
Summary of electrical and structural degradation 2. Current-collapse degradation (trapping) • Slower • Enhanced by temperature, electric field • Tends to saturate for very long times Correlates with pit growth : • Pits randomly located on drain side • Pits grow with V stress , time and temperature • Pits eventually merge Dominant trap created by stress already present in virgin sample , E a =0.56 eV Joh, IRPS 2011 22
Summary of electrical and structural degradation 3. I Dmax , R D degradation • Much slower • Temperature activated (E a ~1 eV) • Electric-field driven • Does not saturate Correlates with geometry of pits and trench • Pits grow larger and merge into trench • Trench grows deeper 23
Initial hypothesis: Inverse Piezoelectric Effect Mechanism Strong piezoelectricity in AlGaN G S D |V DG | ↑ tensile stress ↑ AlGaN crystallographic defects beyond critical elastic energy 2DEG GaN G G Defects: S S D D Trap electrons AlGaN AlGaN ΔΦ bi ΔΦ bi n s ↓ → R D ↑, I D ↓ 2DEG 2DEG Strain relaxation GaN GaN I D ↓ E C E C Provide paths for I G defect defect Joh, IEDM 2006 state state Joh, IEDM 2007 I G ↑ E F E F Joh, MR 2010b AlGaN GaN AlGaN GaN 24
Model for critical voltage V GS =-5 V, V DS =33 V 16nm 28% AlGaN <Vertical Electric Field> <Elastic Energy Density> gate source gate drain source drain Elastic energy density (MJ/m 3 ) Vertical E-field E 3 (MV/cm) z z AlGaN AlGaN GaN x GaN x y x x y C 2 = C C e 2 33 W T = + − + − 13 13 33 T ( C C 2 ) S ( e ) E − + 1 2 C C 2 C C C 1 11 12 10 31 3 C C 11 33 13 12 33 33 33 ∝ + 2 ( T aE ) Mismatch stress Inverse piezoelectric 10 3 stress Joh, MR 2010 25
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