Interface control in double diffused polysilicon bipolar transistors

The properties of the polysilicon/silicon interface which contribute to variations in DC performance of bipolar transistors are investigated. Measurements of interfacial oxide thickness and epitaxial realignment of the polysilicon are correlated with gain, emitter resistance, and poly sheet resistance from complete devices, two-mask bipolar structures, and unpatterned implanted poly subjected to base-emitter drive cycles. The data obtained suggest that significant benefits may be realized in the fabrication of diffused polysilicon bipolar transistors if rapid thermal processing is used for the emitter drive. Higher gain and potentially lower emitter resistance may be obtained if epitaxial realignment is initiated by minimizing the amount of interfacial oxide introduced prior to deposition