Nanosecond Risetime Pulse Characterization of SiC p n ... · Nanosecond Risetime Pulse Characterization of SiC p + n Junction Diode Breakdown and Switching Properties Philip G. Neudeck

National Aeronautics andSpace AdministrationLewis Research Center

INSTRUMENTATION & CONTROLTECHNOLOGY DIVISION

PGN9/97

Nanosecond Risetime Pulse Characterization of SiC p+n Junction Diode Breakdown and Switching Properties

Philip G. Neudeck

NASA Lewis Research CenterCleveland, OH USA

International Conference on Silicon Carbide,III-Nitrides and Related Materials

September 1-5, 1997Stockholm, Sweden

Chris FaziU.S. Army Research Laboratory

Adelphi, MD USA



PGN9/97

Acknowledgments

NASA LewisDavid Larkin

J. Anthony PowellLuann Keys

Andrew TrunekJohn Heisler

Bruce ViergutzGene Schwarze

Jan NiedraGlenn Beheim

W. Dan Williams



PGN9/97

Outline

Pulse testing reveals very important SiC device behaviors not observedby conventional DC and RF testing.

Reverse bias diode pulse testingStable and unstable SiC reverse breakdown.

Forward bias diode pulse testing

These behaviors directly impact SiC power device performance & reliability.

Rectifier reverse recovery switching transients.

Perimeter-governed device minority carrier lifetimes.



PGN9/97

HVDCPowerSupply

1/2" SemirigidCoax TransmissionLine (~150 ft)

1 MΩHg GasSwitch

3.8X atten.

RG-58Coax(50 Ω) Digital

Oscilloscope

Channel 1V(t), Trigger

Channel 2I(t)

5X atten.

TektronixCT-2/P6041 CurrentProbeDevice

UnderTest

10 µF(1 kV)200 Ω

or 400 Ω

2X atten.

DCSupply

(250W 0 µH)

+

+

-

-

Pulse Test Circuit

Bias pulse is formed by discharge of semirigid coaxwhen Hg switch is momentarily triggered.



PGN9/97

V = 0 V V = 0 V

A Tale of Two Diodes(Part 1: DC Testing)

Wafer A* Wafer B**

Epitaxial Small-Area 4H-SiC p+n Diodes

VDC BKDN = 140 V VDC BKDN = 142 V

* NASA Lewis Run #1841J. Appl. Phys. 80, p. 1219

** NASA Lewis Run #1905IEEE EDL 18, p. 96



PGN9/97

A Tale of Two Diodes(Part 2: Reverse Bias Pulse Testing)

Wafer A(VDC BKDN = 140 V)

Wafer B(VDC BKDN = 142 V)

0

100

200

300

Vol

tage

(V

)

0

1

0 100 200 300 400 500Time (ns)

Cur

rent

(A

)

Shot #2

Input PulseAmplitude = 116 V

Experiment: Subject devices to single-shot reverse-bias pulses of increasing amplitude until catastrophic breakdown failure occurs.

0

50

100

Vol

tage

(V

)

0

1

-100 0 100 200 300 400 500 600

Cur

rent

(A

)


(b) Shot #2

Time (ns)



PGN9/97

A Tale of Two Diodes(Part 2: Reverse Bias Pulse Testing)

Wafer A(VDC BKDN = 140 V)

Wafer B(VDC BKDN = 142 V)

Time (ns)

Catastrophic Device Failure,Device Physically Destroyed!

0

1

-100 0 100 200 300 400 500 600

Cur

rent

(A

)

0

50

100

Vol

tage

(V

) Input PulseAmplitude = 94 V

(c) Shot #3

Device Still Good,Positive Temp. Coeff. Breakdown!

0

100

200

300

Vol

tage

(V

)0

1

2

3

4

0 100 200 300 400 500C

urre

nt (

A)

Time (ns)

Shot #22




PGN9/97

Pulse Breakdown Discussion

Behavior of devices on Wafer A is unacceptable for many power applications.• Extremely high reliability, immunity to “glitches” required for most

aerospace applications.

Differences between “unstable” Wafer A and “stable” Wafer B:

• Single epi-growth (Wafer B) vs. two-step epi growth (Wafer A).

• n-substrate (Wafer B) vs. p-substrate (Wafer A).

• SIMS revealed excess Al, N near Wafer A junction not present in Wafer B.

Exact physical mechanism still uncertain.

• Bulk failure mechanism - no evidence of surface breakdown.

Positive temperature coefficient breakdown observed only on very small-area (A < 1 x 10-4 cm2) Wafer B devices.

• Elementary (1c) screw dislocations affecting breakdown???



PGN9/97

PN Diode Reverse Recovery*

Idealized Test Circuit Diode Reverse Recovery Current Transient

(zero inductance)

ts = Storage Time

Minority carrier (hole) lifetime τp

related to storage time ts by:

* G. Neudeck, The PN Junction Diode, 2nd Ed., Addison-Wesley Publishing, p. 111.

ts = τ p erf– 1 1 + 1

IR / IF

2



PGN9/97

Reverse Recovery Current TransientsDevice Area = 8.1 x 10-3 cm2, Rs = 200 Ω

IF varied for approximately fixed IR IR varied for fixed IF

ts increases as IF increases. ts decreases as IR increases.

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-20 0 20 40 60 80 100

Cur

rent

(A

)

Time (ns)

0.7 A0.6 A0.5 A0.4 A0.3 A0.2 A

IF(t=0-)V

R = 30 V

-1.5

-1.0

-0.5

0.0

-20 0 20 40 60 80 100C

urre

nt (

A)

Time (ns)

VR = 40 V

VR = 30 V

IF(t=0 -) = 0.65 A



PGN9/97

Storage Time (ts) Dependence on IR/IF

10-9

10-8

10-7

10-6

0.1 1 10

Sto

rage

Tim

e t s

(sec

onds

)

IR/I

F

where τ p = 300 ns

ExperimentallyMeasured t

s Data

ts = τ p erf – 1 1 + 1

IR / IF

2

Experimentally measuredstorage time behaviorfollows predicted physicaltheory.

Effective minority carrier lifetime for this device is300 ns (A = 8.1 x 10-3 cm2)



PGN9/97

Storage Times (ts) of Larger vs. Smaller Devices

Effective minority carrier lifetime decrease with decreasing area suggestspresence of significant perimeter surface recombination effects.

10-9

10-8

10-7

1 10

A = 3.1 x 10-4 cm2

p = 80 nsτ

A = 8.1 x 10-3 cm2

p = 300 nsτ

t s Sto

rage

Tim

e (s

ec)

IR / I

F



PGN9/97

Device Hole Recombination =

Top View of Diode

n- Layer

Side View of Diode

Perimeter HoleRecombination

Bulk HoleRecombination

p+ Layer

p+n Diode Effective Lifetime

τp Eff. = τp extracted fromreverse recovery switchingmeasurement ts vs. IR/IF data.

Can estimate τp Bulk and sp Perim. from linear plot of 1/τp Eff. vs. P/A.

REff .A = RBulkA + RPerim.P∆pn

τ p Eff .A ≈ ∆pn

τ p BulkA + s p Perim.∆pnP

1τ p Eff .

≈ 1τ p Bulk

+ s p Perim.PA

y = b + mx



PGN9/97

The bulk minority carrierlifetime inherent to this SiCepilayer is much longerthan the average lifetimemeasured on asmall-area device. This is due to largeperimeter surfacerecombination.

0

5

10

15

20

0 50 100 150 200 250 300

1/ p

(µse

c-1)

Perimeter/Area (cm-1)

τ

133160200266400800∞

Device Diameter (µm)

τp Bulk ≈ 0.7 µs

Bulk Minority Carrier Lifetime Extraction

1τ p Eff .

≈ 1τ p Bulk

+ s p Perim.PA

y = b + mx

(4H-SiC, ND = 2 - 4 x 1016 cm-3)

τp Bulk ≈ 0.7 µs

1τ p Eff.

≈ 1τ p B u l k

+ sp Perim.PA



PGN9/97

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 50 100 150 200 250 300

1/t

s (

nsec

-1)

P/A (cm-1)

JF ≈ 1 kA/cm2

JR ≈ 2 kA/cm2

133160200266400800∞

Device Diameter (µm)

Storage Times at Constant Current Density

Indicates bulk Auger recombination insignificant compared to perimeter-governed SRH recombination.



PGN9/97

Discussion

This work demonstrates by example that perimeter surface recombination cansignificantly impact SiC bipolar device electrical characteristics viareduced effective minority carrier lifetimes.

• Greater impact on smaller (IC) devices than larger (power) devices.

• Lifetime reduction likely to be exacerbated by “multi-finger” or “multi-cell”geometries that increase effective perimeter-to-area ratio.

• Possible contributing factor to experimental observations of:

- Low current gains (< 20) in SiC BJT’s produced to date.

- SiC pn diode current densities below theoretical predictions.

- Fast switching response of SiC pn diodes and thyristors.

Development and optimization of appropriate SiC surface passivation andjunction termination technologies could reduce or eliminate lifetime-limitingrole of surface recombination in SiC bipolar devices.



PGN9/97

• Potential impact on n- or p-type 4H- and 6H-SiC at all doping densities (?).

• Effect present in ion implanted or heavily compensated SiC junctions?

Discussion (cont.)

Figure fromJanzen & Kordina,ICSCRM-95 p. 657.

τp Bulk = 0.7 µsLinkoping U.6H-SiCPL Decay Data

A = 8.1 x 10-3 cm2

A = 3.1 x 10-4 cm2

NASA4H-SiC



PGN9/97

Summary

Pulse testing reveals very important device behaviors not observed by conventional DC and RF testing.

• reliability

• switching speed

• current (density) rating

Observed behaviors directly impact SiC power device

Pulse testing should play an important role in SiC power device development and qualification.

Documents

Nanosecond Risetime Pulse Characterization of SiC p n ... · Nanosecond Risetime Pulse Characterization of SiC p + n Junction Diode Breakdown and Switching Properties Philip G. Neudeck