Static and Dynamic Effects of the Incomplete Ionization in Superjunction Devices

The incomplete ionization of impurity atoms affects the free carrier concentration of several wide-bandgap (WBG) semiconductor materials even at room temperature, thus modifying the electrical properties of power devices. In this paper, the influence of the partial ionization of the dopants on the static and dynamic behaviors of WBG semiconductor-based superjunction (SJ) devices has been numerically investigated through extensive 2-D finite-element simulations. While this physical effect has only a minor impact on the static device characteristics, if a reverse bias pulse with a rise time comparable or smaller than the ionization time constant is applied to the structure, a “dynamic ionization” phenomenon can take place. The onset of this time-dependent ionization is the cause of charge unbalance effects in the device structure, due to the temporal dependence of the activated number of dopants. Electrothermal simulations, which have been carried out for a 4H-silicon carbide SJ diode, show a nonuniform temperature distribution during the transients which leads, in turn, to the self-heating of the device. Through an accurate redesign of the cross-sectional view of the device, the drawbacks of the incomplete ionization have been mitigated and the device performance enhanced.