A high-performance IGBT with new N+ buffer structure

Abstract

We propose a new IGBT structure with a new N+ buffer, and confirm by experiments and numerical simulations that the new IGBT is superior to the conventional one. The following results were obtained. (1) According to our experiments, the new IGBT was able to decrease the total power loss, and the parallel operation became easier, compared with the conventional IGBT. Moreover, the short-circuit ruggedness of the new IGBT was almost the same as that of the conventional one by optimizing the ratio of the N++ buried layer. (2) We clarified why the characteristics of the new IGBT were improved by numerical simulations. (a) When the new IGBT is on, holes injected from the P+ substrate flow through, remaining out of the N++ buried layer. Also, the holes rapidly turn around in the N++ buried layer when passing by, and the hole concentration becomes even. Because the lifetime of the new IGBT is designed to be long, the hole concentrations of the new IGBT increases in the N− layer. Therefore, the saturation voltage of the new IGBT is lower than that of the conventional IGBT. (b) Since the lifetime of the N− layer of the new IGBT can be extended, the temperature dependence of the lifetime becomes small, and IZTC of the new IGBT is improved. (c) In the turn-on state, the holes are injected through the N+ buffer layer with lower concentration from the P+ substrate, thus the turn-on speed of the new IGBT become quicker and the turn-on loss of the IGBT is reduced. (d) In the turn-off state, as the N− layer is depleted completely, the carriers in the N+ buffer layer mainly influence the tail current. There are few carriers in the N++ buried layer of the new IGBT, so the turn-off loss of the new IGBT is reduced. (e) Since the effect to prevent the holes being injected from the P+ substrate affects the N− layer, the number of carriers of the N− layer of the new IGBT is limited in the saturation current region. Therefore, the saturation current is also controlled, and the short-circuit ruggedness of the new IGBT is not diminished.