Bias Temperature Stressing (BTS) of Commercial Si and SiC Power

MOSFETs

Krishna Shenai and John N. Hryn Argonne National Laboratory, Argonne, IL

ARL SiC MOS Program Review

Friday, Aug. 15, 2014, 9:20 am – 10 am

W H Y

S i C ?

Increased Energy Efficiency

Smaller Converter Profile

Replace Si with SiC

Increased switching frequency and system integration

Higher Junction Temperature Improved package and thermal

management

Need to benchmark SiC MOSFET with Si MOSFET and IGBT

Silicon MOSFET Structures

CoolMOS has lower RON compared to DMOSFET

IPW90R120C3 – 900V/36A, 0.12 Ω

SiC Power MOSFET Stuctures

CPMF-1200-S080B 1200V/50A, 80 mΩ

S2306 1200V/22A, 160 mΩ

4

MOSFET Parameter Extraction and Datasheet Anomalies

Cree 1200V, 25 mΩ chips packaged in TO-254 packages were used for this evaluation

On-Resistance, RDS(on)

Large spread in on-resistance! What value of RDSon we should use?

CPM2-1200-0025B chip in TO-254 package

Trans-conductance, gfs

What value of trans-conductance should be quoted since SiC MOSFETs hardly attain channel saturation?

Remember it is a power switch!

Datasheet value: gfs = 23.6 S for VDS = 20V and IDS = 50A

CPM2-1200-0025B chip in TO-254 package

25°C

●

MOS Gate Threshold Voltage, VGS(th)

D. K. Schroder, Semiconductor Material and Device Characterization, 3rd Edition, New York: Wiley, 2006, ch. 4, pp. 222-225.

CPM2-1200-0025B chip in TO-254 package

Silicon MOSFETs use VDS = VGS, IDS = 1mA What criteria should be used ?

Breakdown Voltage, V(BR)DSS

Unlike Si power MOSFETs, SiC power MOSFETs are rated for punch-through breakdown and not for avalanche breakdown!

CPM2-1200-0025B chip in TO-254 package

Body Diode On-State Characteristics

Excessive Gate Oxide Charge in SiC power MOSFETs

Commercial 1200V/30A Si power MOSFET

HTGB Stress Testing of Cree and Rohm SiC power MOSFETs and

Infineon CoolMOS

Need to comapre SiC power MOSFETs with Si power MOSFETs

HTGB Test Matrix TO

-25

4

TO-2

54

TO

-24

7

No Heat Sink Used

Case exposed to room ambient

Stressed up to 1000 hrs.

Static measurements

taken at 0, 250, 500 and 1000 hrs.

• VTH extracted from trans-conductance using the linear extrapolation method • No change in VTH for CoolMOS • For SiC MOSFETs, VTH increased with +ve gate bias stress, and decreased with -ve gate bias stress

• RON changed with both types of gate bias stresses for all devices • RON changes are not consistent with VTH changes

• For Si CoolMOS, body diode on-state characteristics drifted with +ve HTGB stress • For SiC power MOSFETs, negligible changes were observed

Effect of +ve HTGB Stress on CoolMOS Body Diode

• For Si CoolMOS, no change was observed with -ve HTGB stress • For SiC power MOSFETs, body diode on-state characteristics shifted with –ve HTGB stress only when measured at VGS = -5V and not at -20V!

Effect of -ve HTGB Stress on SiC MOSFET Body Diode

Gate Pulse Stress

TO-254

VGS(on) = +20V; VGS(off) = -5V; RG = 10Ω; D = 50%

Tcase = 25°C and 150°C; No heat sink is used

Case exposed to room temperature ambient

Stressed up to 1,000 hrs.

Static measurements taken at 0, 500 and 1,000 hrs.

TO-254

TO-247

Threshold Voltage Stability During Gate Pulse Stress

• No change in gate MOS threshold voltage was measured for Si CoolMOS. • For SiC power MOSFETs: - Cree device showed both an increase (with stress duration) and drift (with switching frequency) in gate MOS threshold voltage, - Rohm device showed relatively stable (with stress) but drifting (with switching frequency) gate MOS threshold voltage.

On-Resistance vs. Gate Pulse Stress

• Both Si CoolMOS and Cree SiC MOSFET showed an increase in the on-resistance with gate pulse stress and also frequency-dependent behavior. • Rohm SiC MOSFET showed relatively stable on-state resistance.

MOSFET Body Diode Stress Testing

Stress Conditions:

VGS = -5V IF = 20A

Tcase = 150°C

MOSFET and body diode static parameters were measured at stress intervals of 0, 1, 10 and 100 minutes.

Body Diode On-State Voltage vs. Forward Current Stress Duration

Negligible change in on-state voltage with body diode forward current stress for all devices

Need to increase the stress level

VGS = -5V, IF = 20A stress at Tcase = 150°C

Body Diode Series Resistance vs. Forward Current Stress Duration

Diode series resistance increases with body diode forward current stress for all devices.

The increase in on-resistance is predominant for SiC power MOSFETs.

VGS = -5V, IF = 20A stress at Tcase = 150°C

Summary and Recommendations

• SiC MOSFET data sheets need to be consistent with Si MOSFET data sheets for static parameters reported. • The relevance of MOSFET static parameters on circuit application must be evaluated. • The main problem in SiC MOSFETs appears to be too much MOS charge. • Si CoolMOS device shows robust gate threshold voltage,

but showed effect on the body diode characteristics and on-resistance under various stress conditions.

• Need to benchmark SiC power MOSFETs with Si MOSFETs.

Please submit papers

and/or attend