Stress and Characterization Strategies to Assess Oxide Breakdown

in High-Voltage GaN Field-Effect TransistorsShireen Warnock and Jesús A. del Alamo

Microsystems Technology Laboratories (MTL)Massachusetts Institute of Technology (MIT)

Outline

• Motivation & Challenges• Time-Dependent Dielectric Breakdown (TDDB)

Experiments:‒ Current-Voltage‒ Capacitance-Voltage

• Conclusions

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Motivation• GaN Field-Effect Transistors (FETs) promising for high-voltage

power applications• Many challenges before transistors ready for deployment:

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Motivation

Inverse piezoelectric effectJ. A. del Alamo, MR 2009

• GaN Field-Effect Transistors (FETs) promising for high-voltage power applications

• Many challenges before transistors ready for deployment:

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Motivation• GaN Field-Effect Transistors (FETs) promising for high-voltage

power applications• Many challenges before transistors ready for deployment:

Inverse piezoelectric effectJ. A. del Alamo, MR 2009

Current collapseD. Jin, IEDM 2013

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Motivation

Inverse piezoelectric effectJ. A. del Alamo, MR 2009

Current collapseD. Jin, IEDM 2013

VT instabilityD. Johnson, TED 2013

• GaN Field-Effect Transistors (FETs) promising for high-voltage power applications

• Many challenges before transistors ready for deployment:

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Motivation

Inverse piezoelectric effectJ. A. del Alamo, MR 2009

Current collapseD. Jin, IEDM 2013

Oxide reliabilityVT instabilityD. Johnson, TED 2013

• GaN Field-Effect Transistors (FETs) promising for high-voltage power applications

• Many challenges before transistors ready for deployment:

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Time-Dependent Dielectric Breakdown• High gate bias → defect generation → catastrophic oxide

breakdown• Often dictates lifetime of chip

D. R. Wolters, Philips J. Res. 1985T. Kauerauf, EDL 2005

Typical TDDB experiments:Si high-k MOSFETs

Gate material melted after breakdown

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Challenges to study TDDB in GaN FETs• AlGaN/GaN metal-insulator-

semiconductor high electron mobility transistors (MIS-HEMTs)

• Gate stack has multiple layers & interfaces

→ Uncertain electric field distribution

→ Many trapping sites

• Complex dynamics involved→ Unstable and fast changing VT

P. Lagger, TED 2014

stress time ↑

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TDDB Experiments:Current-Voltage

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GaN MIS-HEMTs for TDDB study

GaN MIS-HEMTs from industry collaboration: depletion-mode

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Classic TDDB ExperimentConstant gate voltage stress experiment:

• Experiment gives time to breakdown and shows generation of stress-induced leakage current (SILC)

• Little other insight gained from measurement

trapping

SILC

Hard breakdown

tBD

IG

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Visualizing TDDB StatisticsTDDB uniqueness: Weibull distribution of time to breakdown

• As VGstress ↑, tBD ↓• Parallel distributions for different VGstress

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TDDB with Periodic Characterization Pause TDDB stress and sweep transfer characteristics at VDS=0.1 V

• Large VT shift → trapping in oxide or AlGaN• Immediate S degradation → interface state generation early in

experiment14

Validity of Characterization ApproachCompare statistics for standard and interrupted schemes

Same statistics for both schemes → characterization is benign15

Step-Stress TDDB• Step-stress to examine early stages of degradation• Step VGstress in 0.5 V increments until breakdown

• Low VGstress: IG ↓ ⇒ trapping• High VGstress: IG ↑ ⇒ SILC

VDS=0 V

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Step-Stress TDDBTransfer characteristics during Step-Stress TDDB

• S and VT degradation is progressive• At VGstress ~12.5 V, ΔVT < 0 (red lines)

‒ Sudden increase in S, appearance of SILC→ interface state generation

VDS=0.1 V

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TDDB Experiments:Capacitance-Voltage

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C-V Characterization

• At VGS>1 V, conduction band of GaN cap starts being populated

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C-V Characterization

TDDB characterization takes place here

• TDDB characterized in regime where GaN cap is populated with electrons

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Constant VGstress TDDB

• As stress time ↑→ CGG ↑→ Frequency dispersion ↑

• Consistent with trap creation and trapping ‒ In oxide and/or at MOS interface

CGG vs. stress time in 5 devices at 5 different frequencies:

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Step-Stress TDDB

• Moderate VGstress → CGG ↓ ⇒ trapping in AlGaN

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Step-Stress TDDB

• Moderate VGstress → CGG ↓ ⇒ trapping in AlGaN• High VGstress → CGG ↑ ⇒ trap generation in oxide

CGG changes shape

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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:‒ ΔVT > 0‒ IG ↓

• Beyond critical value of VGstress:‒ ΔVT < 0‒ Sudden ΔS ↑‒ Capacitance frequency dispersion ↑

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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:‒ ΔVT > 0‒ IG ↓

• Beyond critical value of VGstress:‒ ΔVT < 0‒ Sudden ΔS ↑‒ Capacitance frequency dispersion ↑

Consistent with electron trapping

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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:‒ ΔVT > 0‒ IG ↓

• Beyond critical value of VGstress:‒ ΔVT < 0‒ Sudden ΔS ↑‒ Capacitance frequency dispersion ↑

Consistent with electron trapping

Onset of trap generation in oxide/at MOS interface

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Acknowledgements

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Questions?

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