Measurement of impact ionization coefficients in silicon carbide

Silicon carbide has been recognized as a potential semiconductor for high temperature, high frequency and high power applications. If SiC is to be widely used as the semiconductor for fabricating power devices, it is essential that the parameters governing its electrical performance be determined accurately. One of the most important parameters of a SiC power device is its breakdown voltage. In order to obtain a clear understanding of this breakdown characteristics, it is important to have an exact knowledge of the impact ionization coefficients for SiC.; In this work, hole impact ionization coefficients have been accurately measured as a function of temperature in both 4H and 6H-SiC using the pulsed electron beam induced current (PEBIC) technique. Using Chynoweth's equation ({dollar}alpha{dollar} = a e{dollar}sp{lcub}rm {lcub}-{rcub}b/E{rcub}),{dollar} our measurements gave an a{dollar}sb{lcub}rm p{rcub}{dollar} value of {dollar}(2.5pm 0.2)times 10sp6{dollar}/ cm and a b{dollar}sb{lcub}rm p{rcub}{dollar} value of {dollar}(1.5pm 0.2)times 10sp7{dollar} V/cm for 6H-SiC at room temperature while the values of a{dollar}sb{lcub}rm p{rcub}{dollar} and b{dollar}sb{lcub}rm p{rcub}{dollar} for 4H-SiC were found to be {dollar}(3.5pm 0.4)times 10sp6{dollar}/ cm and {dollar}(1.8pm 0.4)times 10sp7{dollar} V/cm, respectively, at room temperature. Simulations performed using the new measured data indicated that the measured data was more accurate than the data available in the literature. An analytical solution to the variation of the ionization coefficients as a function of the electric field of the form {dollar}alpha{dollar} = mE{dollar}sp{lcub}rm n{rcub}{dollar} was derived in 6H and 4H-SiC, which will allow us to obtain close-form solutions for avalanche breakdown voltage for abrupt junctions with the same values as the obtained using the Chynoweth's equation. The coefficient a{dollar}sb{lcub}rm p{rcub}{dollar} was found to decrease with increasing temperature for both polytypes while the coefficient b{dollar}sb{lcub}rm p{rcub}{dollar} remained constant. Based upon this data, the breakdown voltage of the 4H and 6H-SiC devices is predicted to increase with temperature which is an important desirable characteristic for power devices.; Electron Beam Induced Current (EBIC) techniques were employed in order to understand the role of defects on the breakdown characteristics of SiC. EBIC images revealed that ceratin defects caused enhanced multiplication leading to the catastrophic failures in SiC diodes. The impact ionization coefficients for holes measured at the defective site {dollar}(alphasb{lcub}rm p,eff{rcub}){dollar} were found to be higher than those measured at a non-defective site. Also, {dollar}alphasb{lcub}p,eff{rcub}{dollar} measured at the defective site was found to increase with increasing temperature in contrast with a defect free diode where {dollar}alphasb{lcub}rm P{rcub}{dollar} decreases with increasing temperature, clearly indicating that the defects produce the observed negative temperature coefficient of breakdown voltage in SiC. This work provides the first conclusive evidence that defects are responsible for the observed decrease in breakdown voltage in SiC diodes. From our results, it can be concluded that SiC devices, with the desired positive temperature coefficient for breakdown essential for stable operation, can be fabricated if the defects can be eliminated.