Analysis for rapid tail current decay in IGBTs with low dose p-emitter

A new theory is developed to correctly describe the turn-off mechanism in IGBTs. We found that the p-emitter efficiency (α) in IGBTs dramatically reduces during the fall-time. The hole current component of the tail current in IGBTs with low α instantly decays to zero at the onset of the fall-time, because α becomes negative during the fall-time and the stored holes are removed towards the collector electrodes. An abnormal hole diffusion current flows in the n-drift toward the p-emitter because the decreasing carrier density gradient toward the p-emitter is created. The magnitude of the stored carriers (p_n) at the n-drift near the p-emitter is universally determined by the forward bias (V_Fp) between the p-emitter and the n-drift even in the turn-off transient. It is found that the initial V_Fp is decided only by the p-emitter dose (d_P). Consequently, the turn-off tail current in IGBTs is decided by dP because V_Fp in the fall-time and the stored carriers in the n-drift in the fall-time are decided by d_P. It is difficult to correctly describe the phenomenon with conventional IGBT models because the models assume that the α does not change in the fall-time period.