A new method was presented in this article to explain high current gain of polysilicon emitter transistor. By regarding interfacial oxide between polysilicon emitter and silicon emitter as a wide band gap material which can limit minority transport from silicon to polysilicon, thus improve emitter injection efficiency and increase current gain, the characteristics of RCA transistor (a ultra-thin interfacial oxide layer exists between polysilicon and silicon emitter) can be explained expediently. The theory also applied to HF transistor by treating difference band gap ∆ Eg between interfacial material and silicon as zero. Comparison between experimental and theoretical results was made and they are accordant. Through equivalent model ∆ Eg of a RCA transistor can be estimated as 101 meV.