Reliability of a Semiconductor

Power Switch in a Power

Electronics Converter

Krishna Shenai, PhD

Senior Fellow Computation Institute, The University of Chicago, Chicago, IL (USA)

Adjunct Professor EECS Department, Northwestern University, Evanston, IL (USA)

July 25, 2018

IEEE PELS DL @ SFBAC

Today’s Topics for Discussion

• Current methodology used to design power converters

• Current approach used to assess semiconductor

power switch reliability in a power converter

• Case study: Lessons learned from extensive field-

reliability investigation of high-density power supplies

• Silicon vs. Wide Bandgap (WBG) power devices –

Future of WBG power devices

• Moving forward: How to design power converters with

“built-in” field-reliability

What is Field-Reliability?

Current reliability assessment methods are only good to

evaluate infant mortality

“Field-reliability” of a power

converter is among the least

understood topics today!

We are not able to design

power converters with

“built-in” field-reliability!

Information vs. Power Processor

MTBF > 100,000 hrs.

“End-of-Life” = 17 years

MTBF > 1,000,000 hrs.

“End-of-Life” = 125 years

Information Age Energy Age

Increased switching

speed at reduced

power consumption

Cost Increased energy

efficiency with smaller

profile

Cost

What is Needed?

Power

Converter

L

O

A

D

VSUP

iL(t) • Increased current

density, higher

switching frequency,

and higher Tjmax

• Higher system integration

• Increased power density

• Increased cooling density

Trends in Power Conversion

Smaller converter, lower cost and higher efficiency

Today’s Power Converter

Design Approach

K. Shenai et al, Proceedings of the IEEE, vol. 102, no. 1, pp. 35-52, Jan. 2014

Power Chip in an OEM – Tjmax to Ta

AMBIENT (Ta)

OEM

Power Converter

Power Module

Chip Tjmax

Miniaturization demands high-frequency

power conversion and system integration

Today’s Semiconductor Power Switch

Reliability Assessment Approach

• High-Temperature Reverse Bias (HTRB) Test

• High-Temperature Gate Bias (HTGB) Test

• Temperature Humidity Bias (THB) Test

• Thermal Cycling (-40°C to 125°C)

• Power Supply Operating Life Test

The HTRB Test

• Test performed to accelerate

failure mechanisms

• Typical stress conditions are:

- Ta = 125°C to 150°C

- Vdc ≥ VBR

• Test duration ~ 1000 hours

• Failure rate (λ) is estimated by

considering the dependence on

temperature (T), relative humidity (RH)

and electric field (E)

𝜆 = 𝐴 𝑒𝜙

𝑘𝑇 𝑒𝐵

𝑅𝐻 𝑒𝐶𝐸

EPC eGaN® FET Reliability - Example

R. Strittmatter et al, EPC eGaN® FETs Reliability Testing – Phase Six, www.epc-co.com, 2014

Reliability of High-End

Computer Server

Power Supplies –

a Case Study

Failures of Computer Supplies • IBM eServer 900

• 30% volume taken by PS

• 10% volume by cooling

Co

st

(¢/W

)

50

8

4

Frequency (kHz)

100 150

Eff

icie

ncy (

%)

12 90

85

80

Si

MT

BF

(a

.u.)

1

10

1

Power Density (a.u.)

2 3

Co

oli

ng

Co

st

(a.u

.) 100 4

2

1

Power loss in components and packaging!

ROAD BLOCK !

P. Singh et.al., IBM J. Res. & Dev., Nov. 2002

10 kW Power Supply

50 kHz to 75 kHz = 50% reduction in size

At UI-Chicago (1995-2004)

Founded and directed world’s first industry-

university-government consortium to improve

power supply reliability (1998-2004)

K. Shenai, IEEE Spectrum, vol. 37, No. 7, pp. 50-55, July 2000 (invited paper)

Power Supply Arcing

P. Singh et al, IEEE APEC Digest, pp. 225-229, 2001

Power Supply Arcing

P. Singh et al, IEEE APEC Digest, pp. 225-229, 2001

Partial Vacuum Test – identifies

location of failure

Power Supply Arcing

P. Singh et al, IEEE APEC Digest, pp. 225-229, 2001

Zinc Whisker Spray Test – identifies

minimum spacing between features

Safe Operating Area (SOA) Degradation

Zero voltage transition (ZVT) boost converter High turn-off dv/dt and avalanche stress on Q2

Degradation of transfer curve of Q2 with time Degradation of SOA of Q2 with time

N. Keskar, M. Trivedi and K. Shenai, IEEE IAS Digest, pp. 1098-1102, 1999

Silicon MOSFET Failures due to Dynamic Avalanching

N. Keskar, M. Trivedi and K. Shenai, IEEE IAS Digest, pp. 1639-1645, 1999

Accelerated HTRB Stress Test

“Good’ MOSFET

“Bad’ MOSFET

K. Shenai, 12th Annual Automotive Reliability Workshop,

Nashville, TN, May 2007

SEB Failure Rates of 1000V Silicon Power MOSFETs

Power MOSFET Failure: Effect of Die Size

Actual measured data down to 83% of rated voltage

No data for lower stresses due to very low failure rates

Low-level

leakage results

in significant

device de-rating.

K. Shenai, 12th Annual Automotive Reliability Workshop, Nashville, TN, May 2007

Field-Failures of Power

MOSFETs in Power Supplies

Residual material defects in silicon caused field-failures of

power MOSFETs in high-end server power supplies

K. Shenai, IEEE NAECON 2010, Dayton, OH, July 2010

Silicon IGBT Failures During

Short-Circuit and Inductive

Switching Conditions –

Simultaneous High-Voltage

and High-Current Situation

Short Circuit Failure in Silicon IGBTs

600V/50A

punch-through IGBT

Failure after 18 μs

M. Trivedi and K. Shenai, IEEE Trans. Power Electronics, vol. 14, no. 1, pp. 108-116, Jan. 1999

Silicon IGBT Failure Under Clamped Inductive Stress

M. Trivedi and K. Shenai, IEEE Trans. Power Electronics, vol. 14, no. 1, pp. 108-116, Jan. 1999

Physics of “Hot Spot” Formation Poynting vector S = E x H

δ𝑢

δ𝑡= −𝑔𝑟𝑎𝑑 𝑺 − 𝑱 . 𝑬

𝑢 = 1

2 (E.D + B.H)

Energy conservation law

u = electromagnetic

energy density

Dipole radiation pattern

Electric field strength (color)

Poynting vector (arrows)

K. Shenai et al, IEEE Proceedings, vol. 102, no. 1, Jan. 2014, pp. 35-52

Si vs. WBG Power Devices –

Future of WBG Power Devices

System-Level Benefits of WBG Power Devices

Shenai’s Figure of Merit -

2400x improvement

QF2

AEM

Shenai’s Figure of Merit -

2400x improvement

QF2

AEM

Silicon Power Switch

WBG Power Switch

50,000 cm3

18 kg

4,500 cm3

0.2 kg

Shenai et al, IEEE TED, pp. 1811-1823, 1989

At the system level, the objective should be to increase power and cooling densities

WBG Power Switch

for

• Increased energy savings

• Reduced system cost

• Robust & reliable system

Smaller Chip : Lower Cost

Si vs. WBG Material Properties

W

H

Y

W

B

G

?

Increased Energy Efficiency

Smaller Converter Profile

Replace Si with WBG

Increased switching frequency and

system integration

Higher Junction Temperature Improved package and thermal

management

Wide bandgap (WBG) semiconductors, such as SiC and GaN devices, offer

superior electrical and thermal performances compared to silicon

K. Shenai et al, "Optimum Semiconductors for High-Power Electronics," IEEE Trans. Electron

Devices, vol. 36, no. 9, pp. 1811-1823, September 1989.

Status of Commercial WBG

Power Devices • Vertical SiC JBS Power Diodes

( 300 V < VBR < 1700 V)

• Lateral Low-Voltage ( VBR < 650 V) GaN

Power Transistor

• Vertical High-Voltage ( 900 V < VBR < 1700 V)

SiC Power MOSFET

Cost of WBG device is 2-3X higher than that of Si device

Circuit design complexities – Gate driver issues

System-level benefits of WBG devices are minimal

WBG device field-reliability is unknown

SiC Wafers – Current & Future

Parameter Silicon SiC

Growth

Temperature

< 1000°C > 2000°C

Method Czochralski PVT

Defect Density < 1/cm2 Very High

Cost Low Very High

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

2000 2002 2004 2006 2008 2010 2012 2014

Dislocation Density (cm-2)

BPD

TSD

TED

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

2000 2002 2004 2006 2008 2010 2012 2014

BPD Density (cm-2)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

2000 2002 2004 2006 2008 2010 2012 2014

TSD Density (cm-2)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

2000 2002 2004 2006 2008 2010 2012 2014

TED Density (cm-2)

Defect Engineering of 4H-SiC Wafers

Micropipe

s

(a) (b) (c) High resolution synchrotron monochromatic X-ray topographs recorded at Argonne’s

Advanced Photon Source (APS) facility. (a) Back-reflection X-ray topograph (g = 0004)

images of close-core threading screw dislocations (TSDs) and basal plane

dislocations (BPDs) in a (0001) 4H SiC wafer; (b) Grazing incidence X-ray topograph

(g = 11-28) of 4H-SiC substrate showing TSDs (right and left handed) and TEDs; (c)

Transmission X-ray topograph showing the images of BPDs.

Defects in State-of-the-Art

Commercial 4H-SiC Wafers

Formidable Material Challenge

SiC seed

Vertical (c-axis) 4H-SiC boule

growth proceeds from top surface

of large-area seed via hundreds to

thousands of threading screw

dislocations (TSDs).

Crystal grown at T > 2200 °C --> High thermal gradient & stress --> More dislocations

Growth in the c-axis direction, enabled by screw-dislocations providing steps!

After Ohtani et al. J. Cryst. Growth 210 p. 613.

Threading screw dislocation growth spirals

(THE sources of steps for c-axis growth)

found at top of grown 4H-SiC boule.

Contention: Elimination of screw dislocations from power devices not possible while

maintaining commercially viable crystal quality and growth rate and via this approach.

c-a

xis

Role of Crystal Defects on the

Electrical Characteristics of PiN Diode

Avalanche Testing of Power Devices

First proposed by Shenai C. S. Korman et al, in Dig. Int. High-Frequency Power Conversion, pp. 128-139, 1988

Measured Avalanche Energy (EAVL) of Power Diodes

600V/8A 4H-SiC JBS diode 600V/6A silicon MPS diode

Most WBG data sheets do not list EAVL

First demonstrated by Shenai K. Shenai et al, Proc. IEEE, Feb. 2014

Why EAVL of SiC Power Diodes < EAVL of Si Power Diodes?

Measured dv/dt of SiC Power Diodes

dv/dt (V/nsec.)

Tc = 25°C

0

1.5

3

4.5

6

0 15 30 45 60

SW2

PD2

Peak D

iode C

urr

ent

(A)

Failure Instant

SW2: 600V/6A 4H-SiC JBS diode

PD2: 600V/8A silicon MPS diode

SD1: 300V/10A 4H-SiC JBS diode

SW1: 200V/12A silicon MPS diode

Most WBG data sheets do not provide dv/dt ratings

First demonstrated by Shenai’s group K. Acharya and K. Shenai, Power Electronics Technology, pp. 672-677, Oct. 2002

Why dv/dt Capability of SiC Power Diodes < dv/dt Capability of Si Power Diodes?

Safe Operating Area (SOA) of Power MOSFETs

Why SiC SOA is

smaller than

silicon?

K. Shenai et al, Proc. IEEE, Feb. 2014

IXFB30N120P C2M0080120D

4H-SiC Material Defects and

MOS Gate Oxide Reliability

K. Yamamoto et al, “Influence of threading dislocations on lifetime of gate thermal oxide, ”Mat. Sci. Forum, vols.

717-720 (2012), pp. 477-480.

Commercial SiC JBS Diodes

Measured @ 25ºC

Punch-through Design Measured @ 25ºC

Silicon SiC

Avalanche breakdown Punch-through (leaky)

Minimum on-resistance Not optimized

Too much fat left in SiC diodes

K. Shenai and A. Chattopadhyay, IEEE Trans. Electron Devices, vol. 62, no. 2, pp. 359-365, Feb. 2015.

Field-Induced Lattice Deformation in 600V 4H-SiC JBS Diode

Defect delineation study

performed using hard X-rays

at Argonne’s Advanced Photon

Source (APS).

At 900V reverse bias, TSDs in

the vicinity of the metal-

semiconductor junction were

excited and acted as charge

generation centers that led to

diode breakdown.

Collaborators:

Stony Brook University

Brookhaven National Labs

K. Shenai – unpublished work, 2014

Lessons from the Past:

Higher chip cost and thermal

limitations rendered GaAs

chip technology always a

technology of the future

Why Power Electronics

Converters Fail in the Field?

Why Power Supplies Failed in the Field?

Output capacitor leakage; reactive charge dumping from transformer

leakage inductance; power supply arcing caused by zinc whiskers

K. Shenai, IEEE Spectrum, vol. 37, No. 7, pp. 50-55, July 2000 (invited paper)

Failures in Electronic Systems

What is the

Junction Temperature Tj ?

Industry Response

Question: Please indicate which components you consider most important to be

addressed by future research to improve the reliability of power electronics converter systems?

J. Falck al, IEEE Industrial Electronics Magazine, vol. 12, no. 2, June 2018, pp. 24-35

Industry Response

Question: Please rank the following options for achieving high

reliability for power electronics systems?

J. Falck al, IEEE Industrial Electronics Magazine, vol. 12, no. 2, June 2018, pp. 24-35

Physics of Failures

What is the role of material defects

on cost, performance and

reliability of a semiconductor

power switch?

Power Semiconductor Switch

L

O

A

D

A

B C

vSUP

iL(t) , diL(t)/dt

vAB(t), dvAB(t)/dt

on

off

• Low on-state resistance (RDS(ON))

- to reduce conduction power loss

(I2 RDS(ON))

• Low capacitances

- to reduce switching power losses

(CV2f)

• Good reliability

• Low chip cost

Load is

inductive

Losses in a Power MOSFET

Power converters are designed by considering mainly PON

Source (S)

n- epi

Drain (D)

p-body RW

CDS

CGD

CGS

RD

Gate (G)

RG

Drift-Region Design

n buffer

n+ substrate

n+ source

On-State Power Dissipation

Δ𝑇 = 𝑇𝑗 − 𝑇𝑐 = 𝐼𝑂𝑁2 𝑅𝑂𝑁 𝑅𝑗𝑐

Tjmax = 150°C is “industry standard”

> 200°C is desired

Thermal Management of

WBG Power Devices

Silicon Substrate (~ 200 microns)

2DEG

GaN (< 1 micron)

AlGaN Buffer

(a few microns)

S D G Heat Source

Lateral GaN Power Transistor

SiC Substrate (~ 350 microns)

SiC (few microns)

S G

Vertical SiC Power Transistor

D

S

Moving Forward:

How to design power converters

with “built-in” field-reliability?

Industry-Driven Consortium

Major OEMs

• EV

• Grid

• Aerospace

• Computer/Telecom

OEMs

Research

Labs Academia

Inverter &

Converter

Manufacturer

Component

Suppliers

Need to Account for

Material – Device – System Interactions

Top-down

systems-driven

reliability

engineering

approach

OEMs Motor Control, Utility Grid, EVs,

Power Supplies, etc.

Converter

Suppliers

Inverters & DC-DC Converters

Switch & Module

Manufacturers

Power Semiconductor

Chips & Modules

Material

Suppliers

SiC & GaN Wafers Defect Density

(Growth Rate & Wafer Size)

Current Density

(Tjmax & VBD)

Power Density

(Cooling Density)

Cost

(Ta, MTBF)

Systems-Driven Reliability

Engineering

QUESTIONS?

Thank You

OPTIONAL SLIDES

Reliability =

measure of continuous service accomplishment (or time to failure)

Metrics

Mean Time To Failure (MTTF) measures reliability

Failures In Time (FIT) = 1/MTTF, the rate of failures

Traditionally reported as failures per 109 hours of operation

Ex. MTTF = 1,000,000 hours FIT = 109/106 = 1000

Mean Time To Repair (MTTR) measures Service Interruption

Mean Time Between Failures (MTBF) = MTTF+MTTR

What is Reliability?

The Famous “Bathtub” Curve

Accelerated HTRB Stress Test

K. Shenai, 12th Annual Automotive Reliability Workshop, Nashville, TN, May 2007

Weibull Probability Model

Acceleration Parameters

Predicted Failure Rate = Acceleration Factor (AF) x Acceleration Test Failure Rate

MOSFET “Field-Reliability” Model

SiC MOSFET Gate Oxide Failures

Role of Bulk Material Defects on SiC

MOSFET Gate Oxide Reliability