Diode with shortened reverse recovery time without reducing impurity concentration

Abstract

In a pn junction diode having a conductivity modulating element provided on a first principal surface of a semiconductor substrate, when an impurity concentration of a p type impurity region is lowered to shorten a reverse recovery time, hole injection is suppressed, thereby causing a problem that a forward voltage value is increased at a certain current point. Moreover, introduction of a life time killer to shorten the reverse recovery time leads to a problem of increased leak current. On an n− type semiconductor layer that is a single crystal silicon layer, a p type polycrystalline silicon layer (p type polysilicon layer) is provided. Since the polysilicon layer has more grain boundaries than the single crystal silicon layer, an amount of holes injected into the n− type semiconductor layer from the p type polysilicon layer in forward voltage application can be suppressed. Moreover, a natural oxide film formed between the n− type semiconductor layer and the p type polysilicon layer in formation of the p type polysilicon layer can also reduce the amount of holes injected into the n− type semiconductor layer. Thus, a time to extract the holes in reverse voltage application, that is, a reverse recovery time can be shortened without using a life time killer.