Design issues for a high-performance bipolar technology with Si or SiGe epitaxial base are discussed. Narrow and shallow polysilicon emitter formation and extrinsic base design for low base resistance are investigated using selective epitaxy emitter window (SEEW) transistors. In spite of a close proximity of the extrinsic base diffusion to the intrinsic device a cut-off frequency (fτ) up to 50 GHz has been achieved for an emitter width of 0.35 μm. The depth of the extrinsic base diffusion did affect the collector-base capacitance and indicated that the extrinsic base should be designed in a manner consistent with the circuit operating current density. Further, we discuss in detail the emitter window tolerance for SEEW formation.