Modeling, Characterization and Design of Wide Bandgap MOSFETs for High

Temperature and Power Applications

UMCP: Neil Goldsman Gary Pennington(Ph.D)

Stephen Powell (Ph.D) Gabriel Lopez (Former Merit ->MS)

Steve Risner (Merit) Siddharth Potbhare (MS), Xiouhu Zhang(MS)

ARL: Skip Scozzie

Aivars Lelis (& UMCP Ph.D) Bruce Geil (& UMCP MS) Dan Habersat (& Former Merit)

ARO STAS: Barry Mclean & Jim McGarrity

Personnel Development: Contribution to ARL

• Gary Pennington: Will finish PhD 2003• Steve Powell: Will finish PhD 2003• Gabriel Lopez: Former MERIT (planned MS), employed

by Goldsman to help continue work at UMD and ARL• Aivars Lelis: ARL employee, PhD under Goldsman

(transferring our software to ARL for use and more development)

• Bruce Geil: ARL employee, MS under Goldsman (transferring our software to ARL for use and more development)

• Steve Risner: Very promising new MERIT student

Outline•Introduction:

-Benefits of Wide Bandgap Semiconductors-Difficulties to Overcome

•Atomic Level Analysis of Carrier Transport in 4H & 6H SiC: -Monte Carlo transport modeling: bulk and surface

• 4H & 6H SiC MOSFETS: -Developing new simulation methods to extract physic & propose how to improve performance.

-Effects of High Temperatures & High Voltage-4H & 6H MOSFET Comparison

•4H Schottky Diodes:- Modeling & Experiment vs. Temperature

Introduction: Benefits of Wide Bandgap Semiconductors (SiC)

• Extremely High Temperature Operation• Extremely High Voltage• Extremely High Power• Capable of Growing Oxide => MOSFETs• Potential for High Power and High Temperature

Control Logic• Power IC’s• High Temperature IC’s

Research Strategy

Device ModelingDrift-Diffusion

Material ModelingMonte CarloExperiment

SiC Device Research & Design

Atomic Level Investigation of Carrier Transport in SiC:

Monte Carlo Simulation

Developed surface SiC Monte Carlo to understand mobility degradation at the SiC-SiO2 interface.

Monte Carlo for SiC: Inversion Layer

1) Electronic subband energies, wavefunctions, the surface potential and the surface Fermi level.

2) Surface phonon, roughness, and charge scattering with detailed dependence on 1), screening effects, and the interface trap density of states.

Atomic scale physics of the SiC interface in the Monte Carlo:

Monte Carlo for SiC: Inversion Layer

MOSFET Inversion Layer is a Quantum Well.

Use Schrodinger and Poisson equations to find energy states and wavefunctions in the well self-consistently .

State & Band Diagram

Monte Carlo: Surface Mobility vs. Field

•Calculate Surface Mobility from basic physics.•Mobility vs. Surface Perpendicular Field agree with experiment:•decreasing mobility with increasing surface field.

216105.147.26 cmXFNm

212102.13/ cmXFN it

211104.5 cmXN f

315105.5 cmXN A

Determined charge densities

F

SiC Surface Monte Carlo: New Findings

•In 4H the lowest subband is shifted ~.1 eV above the bulk conduction band, leading to a large interface state density. The shift is not as dramatic in 6H.

•Electrons are farther from the interface in 4H compared to 6H leading to less scattering in 4H if the interface state density of 4H can be reduced.

•Lower interface state density in (1120) 4H-SiC gives 5X improvement in mobility over (0001) 4H-SiC.

•The (0001) orientation in 6H-SiC has the lowest interface state density but the (1120) and (0338) orientations allow the electrons to move further away from the SiC-SiO2 interface lowering the scattering rate (effects will be investigated).

Physics of 6H SiC MOSFETs:Device Modeling

p+ substrate

p-type epilayer

Gate metal

source drain

Surface MobilityElectron

Surface Phonon

Surface Roughness

Interface Trap

Fixed Charge

/1

invsat

llS

SS

1sr

1sp

1CitB

BS

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ll,

Oxide

Bulk

Electron Flow

TTTTTT HFsrspCitBS 111111

q

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GRqJt

pq

GRqJt

nq

NNpnq

Bpnpn

ppp

nnn

p

n

AD

)()(

)(

)(

Drift-Diffusion Equations

•Poisson •Electron Continuity• Hole Continuity

)(

)(

GREEJH

HTt

Tc

g

v

Temperature Eqns

SiC MOSFET Modeling:

MOSFET Doping Profile

Start with Doping Profile & Mobility as InputSolve semiconductor equations numericallyObtain internal and terminal device characteristics

Current-Voltage Characteristics & Trapped Charge

Agreement with Experiment at Room Temperature

Ids vs Vds Characteristics at 300 K W = 200 m, L= 4 m

Results show charged interfaces increase with gatevoltage and peak near source and drain junctions & isthe dominant mechanism for mobility degradation.

Nit(x) vs Vgs at 300 K: W = 200 m, L= 4 m

Modeling predicts reduction of surface roughness alone shows minimal effect

IDS vs VGS (linear scale) IDS vs VGS (log scale)

Curves show 10-fold reduction of surface roughness scattering alone: small effect at large gate voltage

Combined Effect of Interface and Surface Roughness Scattering

IDS vs VDSIDS vs VGS

Reducing surface roughness scattering only improves mobility after interface trap density is significantly reduced!

Temperature Dependence

IDS vs VGS for T=27, 100, 200 C (Subthreshold Region)

Experiment and Device Model Agree:Increasing Temperature Increases Terminal Current in Subthreshold as well.

Modeling Interface Traps vs. Temperature

700K

500K

300K

Calculations show interface trapping decreases at higher temperature, increasing mobility, which explains the rise in current with temperature.

Charged Trap Density vs. Channel Position at 3 different Temperatures

Predict Effect: Reduced Interface Traps v.Temperature

Calculations predict reducing interface states by 100 timesreverses trend in I-V characteristics. Current reduces at highertemperatures due to reduced effect of interface states.

Drain current versus drain voltage at 5 temperatures

700K

300K

500K

Simulation of n-type 4H-SiC MOSFETs

Comparison between 6H and 4H SiC

Interface States: 4H vs. 6H

Vthresh for 4H Calculated for Different Temperatures

Comparing 4H and 6H IV Curves at Different Voltages and T = 300oC

Comparing 4H and 6H IV Curves at Different Temperatures

4H High Voltage OperationVd = 100V Vg = 5V T = 300oC

Source Drain

Potential vs. Position in 4H MOSFET

4H MOSFET IV Curves at High Voltages

Vds = 10V-100V Vgs = 5V T = 300oC

X10-3

Impact Ionization Coefficient vs. Field for Electrons

Impact Ionization Coefficient vs. Field for Holes

x 106x 106

Electric Fields in 4H MOSFET: High Voltage

Vgs = 5V Vds = 100V T = 300 oC

E_field in x direction E_field in y direction

Source Drain Source Drain

Impact Ionization Rate in 4H MOSFETVgs = 5V Vds = 100V T = 300 oC

x 1024

x 1024

SourceDrain

Negligible until very large bias voltages

MOSFET SUMMARY•Detailed theoretical and experimental characterization of 6H MOSFETs performed.

•Agreement with experiment obtained.

•Role of interface states quantified as limiting factor in mobility.

•Transconductance increase with increasing temperature explained and quantified in linear region.

•Simulations indicate saturation velocity in channel depends on interface states as well as surface phonons.

•Simulator adapted to model 4H MOSFETs

•6H MOSFET characteristics appear to be better than 4H

•Difference appears to be due to interface state and quantum confinement effects.

•4H high voltage behavior modeled

•Impact ionization negligible until applied drain voltages approach 100V.

Experiment and Simulation of n-type 4H-SiC Schottky diodes

Moving from 6H to 4H SiC•Improved Bulk Mobility

•Higher Breakdown Voltage

Schottky: Introduction

• Experiment: -IV measurements under

different temperatures were performed in ITS8000 testing system.

• Simulation:-Using a simulator based on the drift-diffusion model

Ohmic contact

Metal/Ti

Schottky contact

Epitaxial Drift Layer (4μm)

Fig.1. The schematic cross section of the Simulated SiC Schottky diode

 

Substrate Layer (1μm)

Schottky: Results and Discussion I-V and Temperature

I-V Characteristics from experiment and simulation show a very good agreement.

Experimental (dot curve)  and Simulation (solid curve) results of forward IV characteristics of Ti/4H-SiC Schottky diodes under different temperatures.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Forward Voltage (V)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Cu

rre

nt

(A)

298K

453K

323K

373K

• Mobility Temperature dependence

Schottky: Results and Discussion

ncs

Bon

E

VR

3

24

oncs

Bn

RE

V

3

24

)/1( onn R

5.6 5.8 6 6.2 6.4

Tem perature(K)

3.6

4

4.4

4.8

5.2

5 .6

Mo

bili

ty(c

m2 /

v-s

)

3.6

4

4.4

4.8

5.2

5 .6

5 .6 5 .8 6 6.2 6.4

A T-2.38 is obtained from the fitting curve  

1.  

2 . Another relationship derived from Mobility

Fig. 4. Mobility Temperature Dependence

Improvement Due to Computer Modeling

Position and Temperature Dependent Mobility

4H SiC Mobility vs Doping and Temperature

Effect of Doping in Epi Drift Layer

Effect of Doping in Epi Drift Layer

• Barrier Height & Temperature

Results and Discussion

 

A negative temperature dependence of barrier height on n-type Ti/4H-SiC Schottky diode was obtained

Table 3. Barrier Height Temperature Dependence

Temperature (K)Voltage

(V)Barrier Height

(eV)

Voltage(V)

Barrier Height

(eV)298.15 1.6 1.14 1.5 1.14

323.15 1.6 1.04 1.5 1.07

373.15 1.6 0.95 1.5 0.98

423.15 1.6 0.90 1.5 0.92

Schottky Summary and Conclusion

 

•Model agrees with current-voltage experiments vs. temperature.•Mobility and Schottky barrier height models obtained•The on-resistance shows a T2.23 variation with temperature. •The electron mobility temperature dependence T-2.38 was confirmed. •A negative temperature dependence in barrier height. •Better device performance was observed by shortening the epi drift region and increasing the epi doping. 

•Developed and employed full zone Monte Carlo to characterize transport in SiC at high temperatures and at surface.•Developed Drift-Diffusion simulator; 6H-SiC MOSFET simulator

•Combined with experiment to extract interface states•Extracted surface mobility •Explained why current increases with temperature•Extended method to modeling 4H MOSFETs•Compared 4H and 6H performance.

•Developed Method for Predicting Chip Heating•Expanded the Experimental Component with the Merit Program•Extended work in 4H-SiC with Schottky diode

•Developed temperature dependent diode Schottky simulator that agrees with experiment.

•Transferred Software to ARL

Achievement Summary

1) G. Pennington, and N. Goldsman, "Empirical Pseudopotential Band Structure of 3C, 4H, and 6H SiC Using Transferable Semiempirical Si and C Model Potentials,” Phy. Rev. B, vol 64, pp. 45104-1-10, 2001.

2) G. Pennington, N. Goldsman, C. Scozzie, J. McGarrit, F.B. Mclean., “Investigation of Temperature Effects on Electron Transport in SiC using Unique Full Band Monte Carlo Simulation,” International Semiconductor Device Research Symposium Proceedings, pp. 531-534, 2001.

3) S. Powell, N. Goldsman, C. Scozzie, A. Lelis, J. McGarrity, “Self-Consistent Surface Mobility and Interface Charge Modeling in Conjunction with Experiment of 6H-SiC MOSFETs,” International Semiconductor Device Research Symposium Proceedings, pp. 572-574, 2001.

4) S. Powell, N. Goldsman, J. McGarrity, J. Bernstein, C. Scozzie, A. Lelis, “Characterization and Physics-Based Modeling of 6H-SiC MOSFETs”’ Journal of Applied Physics, V.92, N.7, pp 4053-4061, 2002

5) S Powell, N. Goldsman, J. McGarrity, A. Lelis, C. Scozzie, F.B McLean., “Interface Effects on Channel Mobility in SiC MOSFETs,” Semiconductor Interface Specialists Conference, 2002

6) G. Pennington, S. Powell, N. Goldsman, J.McGarrity, A. Lelis, C.Scozzie., “Degradation of Inversion Layer Mobility in 6H-SiC by Interface Charge,” Semiconductor Interface Specialists Conference, 2002.

Very Recent Publications

7) G. Pennington and N. Goldsman, ``Self-Consistent Calculations for n-Type Hexagonal SiC Inversion Layers,” Accepted for publication in Journal of Applied Physics, 2003

8) G. Pennington, N. Goldsman, J. McGarrity, A Lelis and C. Scozzie, ``Comparison of 1120 and 0001 Surface Orientation in 4H SiC Inversion Layers,” Semiconductor Interface Specialists Conference, 2003.

9) S. Potbhare, N. Goldsman, A. Lelis, “Characterization and Simulation of Novel 4H SiC MOSFETs”, UMD Research Review Day Poster, March 2004.

10) G. Pennington, N. Goldsman, J. McGarrity, A. Lelis, C. Scozzie, ``(001) Oriented 4H-SiC Quantized Inversion Layers," International Semiconductor Device Research Symposium, pp. 338-339, 2003.

11) X. Zhang, N. Goldsman, J.B. Bernstein, J.M. McGarrity, S. Powell, ``Numerical and Experimental Characterization of 4H-SiC Schottky Diodes,” International Semiconductor Device Research Symposium, pp. 120-121, 2003.

Very Recent Publications Continued

Future Work• Explain physics of velocity saturation in MOSFET

channel.• Calculate IC Heating• Characterize 4H MOSFET experimentally and

theoretically • Use Monte Carlo to pin-point key physical differences

between 4H and 6H SiC surface mobility.• Extend Monte Carlo to Extract Underlying Mechanisms of Oxide Degradation on Atomic Scale• Develop SiC MOSFET Circuit (SPICE) Model• Extend work to high temperature integrated circuits.• Personnel development (ARL scientists: Advanced

research and advanced degrees.)