Drain Extended MOS Body Region Engineering for Switching Reliability Under Unclamped Inductive Load Conditions

Abstract

This work presents physical insights on drain extended NMOS (DeNMOS) transient switching reliability under unclamped inductive load (UIL) conditions. The dependence of UIL switching dynamics on the rate of carrier injection after parasitic bipolar junction transistor (P-BJT) triggering and subsequent space charge modulation (SCM) is demonstrated using a 2D TCAD simulation study. In DeNMOS, reduced body resistance suppresses the P-BJT while increasing the back gate bias delays the SCM. We show that the increased body contact length of the proposed DeNMOS offers the dual benefit of P-BJT mitigation and delayed SCM without degrading the DC electrical performance. Furthermore, it is shown that mitigation of P-BJT and delayed SCM enhance the turn-off reliability against thermal runaway when DeNMOS is subjected to transient switching conditions in the presence of UIL. Finally, we show that when external body resistance and bias are individually applied, they result in limited improvement in switching reliability, whereas body layout engineering could achieve optimum switching reliability. Subsequently, a comparative investigation of the body, drain and drift layout optimization techniques shows the highest enhancement in avalanche ruggedness and robustness against thermal runaway for body contact optimized DeNMOS.