CORRELATION BETWEEN FORWARD VOLTAGE DROP AND LOCAL CARRIER LIFETIME FOR A LARGE-AREA SEGMENTED THYRISTOR

The forward voltage drop for individual segments of a large area thyri stor has been correlated to the local, bulk carrier lifetime by lifeti me mapping of the wafer after final device processing, The Lifetime ma pping was performed under high injection conditions using an all-optic al technique where carriers were generated by a short YAG laser pulse and the subsequent carrier decay was monitored by an IR laser beam usi ng free carrier absorption, The lateral resolution was similar to 100 mu m. The lifetime map revealed heavily contaminated areas where the l ifetime was reduced by more than an order of magnitude, The forward vo ltage drop for corresponding thyristor segments was high and, for some areas, no stable turn-on could be achieved, Deep Level Transient Spec troscopy characterization of contaminated areas confirmed the lifetime measurement results and suggest that the contamination is most likely due to metal impurities introduced in the first extended-time/high-te mperature drive-in of the p-base, Device simulations showed qualitativ e agreement between the bulk carrier lifetime and the corresponding vo ltage drop.