A new theory about minority carrier injection into polysilicon emitter has been proposed in this paper. The minority carrier recombination in grain boundary and its width are considered. The functions of minority carrier mobility and lifetime as polysilicon width are introduced. Not only the conclusion that grain boundary blocks and recombines minority carrier is obtained

but the contribution of different grain boundaries to injecting saturate minority carrier current is distingnished successfully. The injecting saturate minority carrier current may be reduced the most effectively by the first grain boundary. Meanwhile

the relationship between the number of effective grain boundaries

which reduce injecting saturate minority carrier current more than ten percent

and the interfacial state density of grain boundary and grain size has also been presented.