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Prof. B. JAYANT BALIGA Director, Power Semiconductor Research Center NCSU Centennial Campus, Box 7924 Raleigh, NC 27695. ABSTRACT Progress made in the development of high performance power rectifiers and switches from silicon carbide are reviewed with emphasis on approaching the 100-fold reduction in the specific on-resistance of the drift region when compared with silicon devices with the same breakdown voltage. The highlights are: (a) Recently completed measurements of impact ionization coefficients in SiC indicate an even higher Baliga's figure of merit than projected earlier. (b) The commonly reported negative temperature co-efficient for breakdown voltage in SiC devices has been shown to arise at defects, allaying concerns that this may be intrinsic to the material. (c) Based upon fundamental considerations, it has been found that Schottky rectifiers offer superior on-state voltage drop than P-i-N rectifiers for reverse blocking voltages below 3000 volts. (d) Nearly ideal breakdown voltage has been experimentally obtained for Schottky diodes using an argon implanted edge termination. (e) Planar ion-implanted junctions have been successfully fabricated using oxide as a mask with high breakdown voltage and low leakage currents by using a filed plate edge termination. (f) High inversion layer mobility has been experimentally demonstrated on both 6H and 4H-SiC by using a deposited oxide layer as gate dielectric. (g) A novel, high-voltage, normally-off, accumulation-channel, MOSFET has been proposed and demonstrated with 50x lower specific on-resistance than silicon devices in spite of using logic-level gate drive voltages. These results indicate that SiC based power devices could become commercially viable in the 21st century if cost barriers can be overcome.

INTRODUCTION As stated in a recent review article [1], 'much of the current interest and activity in SiC power devices stems from a 1989 paper by B.J. Baliga [21', which is based upon an original derivation performed in 1978 and published in 1982 [3]. Based upon the application of Poisson's equation to the drift region of vertical discrete devices, it has been shown that the specific onresistance of the drift region is given by:

(1)

4

WD q Vn ND

E

3nE:

where BV is the breakdown voltage, ,, is the semiconductor dielectric constant, P-t, is the mobility, and E. is the critical electric field for breakdown. More recently, a equation for the specific resistance of the drift region of lateral RESURF devices was also derived [4]:

-

BV 2 Es 'n Ec 77

Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society

(2)

In both cases, the specific on-resistance is found to be inversely proportional to the product (s, vt, EC3), which is now commonly referred to as Baliga's Figure of Merit (BFOM) for selecting semiconductor materials applicable to power devices. In order to obtain an accurate assessment of the benefits of replacing silicon with silicon carbide, it is essential to have reliable data on the impact ionization coefficients measured in defec