This paper describes the first self-aligned heterojunction bipolar transistor (HBT) process which includes ion implantation to reduce the base resistance. With this substitutional emitter technique, the base implant and the emitter contact patterns are defined with the same mask. Arbitrary contact materials can be used allowing optimization of the contact resistances. Transistors with emitter width down to 1.5 pm have been fabricated. Nonthreshold logic (NTL) ring oscillators made with these transistors had propagation delay times down to 27.2 ps. This is the lowest reported to date for bipolar transistors.