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Detailed electrical results are presented for self-aligned and non-self-aligned bipolar transistors with polysilicon contacted emitters. The transistors have been fabricated with a thin interfacial oxide layer at the polysilicon-monosilicon interface. It is demonstrated that the enhanced gain obtained from this type of device can be traded for a considerable reduction in sub-emitter sheet resistance, and hence for a potential improvement in switching performance. At very high base doping, an unexpected increase in base current is observed, and this is explained by an increase in the bandgap narrowing in the emitter as a result of partial compensation of the emitter dopant. To fabricate the self-aligned transistors, a new process technology has been employed which utilises selective oxidation of the polysilicon emitter contact. Under certain circumstances it is shown that non-ideal electrical characteristics can be obtained from this type of device, and these are caused by lateral spread of the extrinsic base region beneath the sidewall oxide of the polysilicon. To overcome this problem, a polysilicon base contact has been employed which allows a self-aligned extrinsic base region to be fabricated with negligible lateral movement.