Steady-state and transient forward current-voltage characteristics of 4H-silicon carbide 5.5 kV diodes at high and superhigh current densities

Abstract

Steady-state and transient forward current-voltage I-V characteristics have been measured in 5.5 kV p/sup +/-n-n/sup +/ 4H-SiC rectifier diodes up to a current density j/spl ap/5.5/spl times/10/sup 4/ A/cm/sup 2/. The steady-state data are compared with calculations in the framework of a model, in which the emitter injection coefficient decreases with increasing current density. To compare correctly the experimental and theoretical results, the lifetime of minority carriers for high injection level, /spl tau//sub ph/, has been estimated from transient characteristics. At low injection level, the hole diffusion length L/sub pl/ has been measured by photoresponse technique. For a low-doped n-base, the hole diffusion lengths are L/sub pl//spl ap/2 /spl mu/m and L/sub ph//spl ap/6-10 /spl mu/m at low and high injection levels respectively. Hole lifetimes for low and high injection levels are /spl tau//sub pl//spl ap/15 ns and /spl tau//sub ph//spl ap/140-400 ns. The calculated and experimental results agree well within the wide range of current densities 10 A/cm/sup 2/<j< 4/spl times/10/sup 3/ A/cm/sup 2/. At j>5 kA/cm/sup 2/, the experimental values of residual voltage drop V is lower than the calculated ones. In the range of current densities 5/spl times/10/sup 3/ A/cm/sup 2/<j<2/spl times/10/sup 4/ A/cm/sup 2/, the minimal value of differential resistance R/sub d/=dV/dj is 1.5/spl times/10/sup -4/ /spl Omega/ cm/sup 2/. At j>25 kA/cm/sup 2/, R/sub d/ increases with increasing current density manifesting the contribution of other nonlinear mechanisms to the formation steady-state current-voltage characteristic. The possible role of Auger recombination is also discussed.