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We have proposed and analyzed MOSFET-mode operation of ultra-thin wafer PTlGBTs in ISPSD02[1]. The present paper, for the first time, presents analytical theory of MOSFET-mode operation, and shows that the SOA is determined by a mechanism similar to the second breakdown of npn bipolar transistors.demonstrates, for the first time, that the MOSFETmode IGBTs are strongly effective for sofl switching application. The developed MOSFET-mode 9OOV 60A thin wafer Trench Gate PTlGBTs have reduced turn-off loss by 55% at 125OC, compared with the conventional (4" generation) sofl switching PTIGBTs. MOSFET-mode OperationMOSFET-mode operation is defined in such a way that the anode efficiency y is less than yuos, which represents pd(pn +h). The anode efficiency y is defined as the ratio of the hole current over the total current at the n-base n-bufferjunction, being identical to the product of pemitter injection efficiency Y~~ and transport factor in the n-buffer ar. (Please note that y in the present paper is different from the conventional injection efficiency of the p-emitter.)The ywosvalue dynamically changes as the electric field changes inside the device because the mobility p is a function of the electric field. Figure I shows yMos as a function of electric field. IGBTs may change its operation mode as the forward voltage increases and the electric field inside the device increases.The present paper also experimentally 0.5 I 1 g0.4 E f o . 4 1 M;FET-Mode., 1 '1E2 1E3 1E4 1E6.Fig.1 Area for MOSFET-mode operation.In the MOSFET-mode operation, several unique characteristic features are distinguished. The features are very similar to the operation of pure MOSFETs. Elwtcic Field (V/cmcm)