The Turn-off Behavior of Superjunction MOSFETs Influenced by the Junction Capacitance in the Freewheel Diodes

We investigated the turn-off waveforms of Superjunction (SJ) silicon MOSFETs with respect to their own charge imbalance by experiments as well as by device simulations.
When SJ-MOSFETs are used for chopper circuits we find a unique hump in the current waveform during the turn-off transient. The magnitude of the hump shape waveform drastically depends on the charge imbalance (CIB) in the SJ structure.

For N-type rich CIB we find larger current hump while the hump is reduced or disappear for P-type rich CIB. Furthermore the junction capacitance in the freewheel diode (FWD) influences on the magnitude of the current hump. Larger junction capacitance enhances the magnitude of the current hump.

These experiments and simulations indicate that the internal physics in the semiconductor directly affects the switching waveforms.

In this article we also propose the novel SJ device structure to stabilize the variation of switching waveforms and turn-off loss (Eoff).