Stress and Characterization Strategies to Assess Oxide Breakdown in High-Voltage GaN Field-Effect Transistors Shireen Warnock and Jesús A. del Alamo Microsystems Technology Laboratories (MTL) Massachusetts Institute of Technology (MIT)
Outl tline • Motivation & Challenges • Time-Dependent Dielectric Breakdown (TDDB) Experiments: ‒ Current-Voltage ‒ Capacitance-Voltage • Conclusions 2
Motivation • GaN Field-Effect Transistors (FETs) promising for high-voltage power applications • Many challenges before transistors ready for deployment: 3
Motivation • GaN Field-Effect Transistors (FETs) promising for high-voltage power applications • Many challenges before transistors ready for deployment: Inverse piezoelectric effect J. A. del Alamo, MR 2009 4
Motivation • GaN Field-Effect Transistors (FETs) promising for high-voltage power applications • Many challenges before transistors ready for deployment: Inverse piezoelectric effect J. A. del Alamo, MR 2009 Current collapse D. Jin, IEDM 2013 5
Motivation • GaN Field-Effect Transistors (FETs) promising for high-voltage power applications • Many challenges before transistors ready for deployment: Inverse piezoelectric effect J. A. del Alamo, MR 2009 Current collapse D. Jin, IEDM 2013 V T instability D. Johnson, TED 2013 6
Motivation • GaN Field-Effect Transistors (FETs) promising for high-voltage power applications • Many challenges before transistors ready for deployment: Inverse piezoelectric effect J. A. del Alamo, MR 2009 Current collapse D. Jin, IEDM 2013 Oxide reliability V T instability D. Johnson, TED 2013 7
Time me-Dep epen enden ent D Dielectr tric ic B Brea eakdown • High gate bias → defect generation → catastrophic oxide breakdown • Often dictates lifetime of chip Gate material melted after Typical TDDB experiments: breakdown Si high-k MOSFETs T. Kauerauf, EDL 2005 D. R. Wolters, Philips J. Res. 1985 8
Challenges to study T TDDB in GaN F FET ETs • AlGaN/GaN metal-insulator- semiconductor high electron mobility transistors (MIS-HEMTs) • Gate stack has multiple layers & interfaces → Uncertain electric field distribution stress time ↑ → Many trapping sites • Complex dynamics involved → Unstable and fast changing V T P. Lagger, TED 2014 9
TDDB Ex Experi riments: Curr rrent-Vo Voltage 10
Ga GaN N MIS-HEMTs for T TDDB DB stud udy GaN MIS-HEMTs from industry collaboration: depletion-mode 11
Classic T TDDB Ex Experi riment Constant gate voltage stress experiment: Hard breakdown I G SILC trapping t BD • Experiment gives time to breakdown and shows generation of stress-induced leakage current (SILC) • Little other insight gained from measurement 12
Visualizi zing TDDB Statistics TDDB uniqueness: Weibull distribution of time to breakdown • As V Gstress ↑, t BD ↓ • Parallel distributions for different V Gstress 13
TDDB w with P Peri riodic Characteri rization Pause TDDB stress and sweep transfer characteristics at V DS =0.1 V • Large V T shift → trapping in oxide or AlGaN • Immediate S degradation → interface state generation early in experiment 14
Validity ty of Characteri rization Ap Approach Compare statistics for standard and interrupted schemes Same statistics for both schemes → characterization is benign 15
St Step ep-Stress ss T TDD DDB • Step-stress to examine early stages of degradation • Step V Gstress in 0.5 V increments until breakdown V DS =0 V • Low V Gstress : I G ↓ ⇒ trapping • High V Gstress : I G ↑ ⇒ SILC 16
St Step ep-Stress ss T TDD DDB Transfer characteristics during Step-Stress TDDB V DS =0.1 V • S and V T degradation is progressive • At V Gstress ~ 12.5 V, Δ V T < 0 (red lines) ‒ Sudden increase in S, appearance of SILC→ interface state generation 17
TDDB Ex Experi riments: Capacitance-Vo Voltage 18
C-V Characteri rization • At V GS >1 V, conduction band of GaN cap starts being populated 19
C-V Characteri rization TDDB characterization takes place here • TDDB characterized in regime where GaN cap is populated with electrons 20
Consta tant nt V Gstre ress TDDB C GG vs. stress time in 5 devices at 5 different frequencies: • As stress time ↑ → C GG ↑ → Frequency dispersion ↑ • Consistent with trap creation and trapping ‒ In oxide and/or at MOS interface 21
St Step ep-Stress ss T TDD DDB • Moderate V Gstress → C GG ↓ ⇒ trapping in AlGaN 22
St Step ep-Stress ss T TDD DDB C GG changes shape • Moderate V Gstress → C GG ↓ ⇒ trapping in AlGaN • High V Gstress → C GG ↑ ⇒ trap generation in oxide 23
Conclusions • Developed methodology to study TDDB in GaN MIS- HEMTs • TDDB behavior consistent with Si MOSFETs: ‒ Weibull distribution ‒ SILC before breakdown • For moderate gate voltage stress: ‒ Δ V T > 0 ‒ I G ↓ • Beyond critical value of V Gstress : ‒ Δ V T < 0 ‒ Sudden ΔS ↑ ‒ Capacitance frequency dispersion ↑ 24
Conclusions • Developed methodology to study TDDB in GaN MIS- HEMTs • TDDB behavior consistent with Si MOSFETs: ‒ Weibull distribution ‒ SILC before breakdown • For moderate gate voltage stress: ‒ Δ V T > 0 Consistent with electron trapping ‒ I G ↓ • Beyond critical value of V Gstress : ‒ Δ V T < 0 ‒ Sudden ΔS ↑ ‒ Capacitance frequency dispersion ↑ 25
Conclusions • Developed methodology to study TDDB in GaN MIS- HEMTs • TDDB behavior consistent with Si MOSFETs: ‒ Weibull distribution ‒ SILC before breakdown • For moderate gate voltage stress: ‒ Δ V T > 0 Consistent with electron trapping ‒ I G ↓ • Beyond critical value of V Gstress : ‒ Δ V T < 0 Onset of trap generation in oxide/at MOS ‒ Sudden ΔS ↑ interface ‒ Capacitance frequency dispersion ↑ 26
Acknowled edgem emen ents 27
Qu Ques estion ons? 28
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