Key electrical characteristics of polysilicon emitter contacts in bipolar transistors, such as contact resistance and recombination velocity, are extremely sensitive to the microstructure of the polysilicon/single-crystal silicon interface. In this study, we correlated the microstructural and electrical characteristics of this interface by performing cross-sectional transmission electron microscopy (XTEM) on actual transistors on the same chip where ringoscillator speeds were measured. The base current and emitter resistance of the fastest devices approached values typical of singlecrystal silicon emitters. Interpretation of these electrical data and of the SIMS impurity profile indicates that significant restructuring of the polysilicon/single-crystal interface had taken place. This conclusion was indeed confirmed by the XTEM results. Although the low-current performance was degraded because of higher junction capacitances, the high-current switching speed was improved because of the minimal emitter contact resistance. Since the current gain was sufficiently high and very uniform, it is concluded from this work that minimization of both junction depth and contact resistance is the most important design consideration for high-performance submicron transistors, rather than maximization of the gain enhancement of the polysilicon/single-crystal interface.^Copyright 1987 by International Business Machines Corporation. Copying in printed form for private use is permitted without payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract, but no other portions, of this paper may be copied or distributed royalty free without further permission by computer-based and other information-service systems. Permission to republish any other portion of this paper must be obtained from the Editor.