ISDRS 2003

Xiaohu Zhang, N.Goldsman, J.B.Bernstein,

J.M.McGarrity and S. Powell

Dept. of Electrical and Computer EngineeringUniversity of Maryland, College Park, MD 20742 Xiaohu@glue.umd.edu neil@eng.umd.edu

Numerical and Experimental Characterization

of 4H-SiC Schottky Diodes

Introduction

4H-SiC appears to be the structure that seems especially promising for the development of Schottky Diodes:

Wide bandgap (3.26eV)High critical electric field (2.2×106V)Thermal conductivity (3.2-3.8W/cm·K) Saturated electron drift velocity (2.0×107cm/sec).

Introduction

A methodology was developed to extract key physical parameters for 4H-SiC Schottky diode operation including:

•Temperature dependent mobility•Mobility versus position and doping•Schottky barrier height versus temperature•Device performance dependence on geometry and doping

Methodology

Combined simulation and experimental methods to extract the key parameters.

•Simulations were achieved by developing a drift-diffusion based CAD tool tailored for SiC Schottky diode analysis•Experiments involved measuring current-voltage characteristics under different controlled external temperatures.•Coordinated use of simulation and experiment facilitated the parameter extractions.

Device Structure

Metal Schottky contact

Epitaxial n Drift Layer

Epitaxial n+ Layer

The schematic cross section of the Simulated SiC Schottky diode

Simulation and Physical Models

A. Drift-diffusion model  

B. Intrinsic Carriers and Band Gap Narrowing

C. Mobility Modeling

0 Cpnq

graddiv

)( nnn GRqt

nqJdiv

)( nnp GRqt

pqJdiv

gradnDgradnqJ nnn gradpDgradpqJ ppp

KT

TEmm

KTTn

gvci 2

)(exp

22)( 4/3

2/3

2 )300()( TdT

dEETE

ggog

pn

pn

refpn

pnpn

pnLFpn

C

C

T

CT,

,

)(1

)300

(),(

,

min,,

min,,

pnpn

satpn

LFpn

LFpnHF

pnE

CT

,, /1

,

,

,,

))(1(

),(

Results

The experiment (dot curve) and simulation results (solid curve) of the forward current-voltage (IV) characteristics of the Ti/4H-SiC Schottky diode under four different temperatures show a very good agreement.

Results

The simulation result shows a much more accurate result than the analytical analysis

Mobility

A T-2.4 variation of 4H-SiC mobility was obtained.

Mobility variation from n+ epilayer to n-drift epilayer under different temperature

Average mobility for different doping density under different temperature

Barrier Height

Simulation results indicate that the barrier height displays negative temperature dependence.

Temperature(K)

Barrier Height(eV)

Temperature(K)

Barrier Height(eV)

298.15 1.14 373.15 0.97

323.15 1.06 423.15 0.91

Device improvement

It shows clearly that the current can be increased more than five times by changing the length of the n-drift epilayer from 4µm to 1µm

Better IV characteristic of 4H-SiC Schottky diodes can be shown in this simulation program by changing the length and doping density in two epilayers.

Device improvement

The doping density of n-epilayer changing from 1×1015 cm-3 to 1×1018 cm-3

The doping density of n+ epilayer Changing from 1×1015 cm-3to 1×1018 cm-3

Summary and Conclusion

• High temperature 4H-SiC Schottky Barrier Diode measurements were performed

•A 4H-SiC Schottky diode device simulator was developed

•Simulations and experiments were performed in conjunction to extract key diode parameters.

•Results show mobility values ranging from 1000 to 200cm2/Vsec temperatures ranging from 273 to 573K for doping of 1015/cm3

Summary and Conclusion

•Results show mobility values ranging from 250 to 50cm2/Vsec temperatures ranging from 273 to 573K for doping of 1018/cm3

•Schottky barrier height ranged from 1.14 to 0.91eV for temperatures ranging from 298 to 423K

•Shorter drift regions give rise to larger diode forward currents.