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 call 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 DeltaE(g) between interfacial material and silicon as zero. Comparison between experimental and theoretical results was made and they are accordant. Through equivalent model DeltaE(g) of a RCA transistor can be estimated as 101 meV. (C) 2003 Elsevier Ltd. All rights reserved.