Ultra thin (100-200 A) AlGaAs emitter layers have been incorporated in AlGaAs/GaAs Heterojunction Bipolar Transistors (HBTs). Fabricated using a self-aligned process technology, this novel structure has yielded transistors with submicron emitter widths. The AlGaAs emitter layer serves to passivate the base surface resulting in constant current gain values of 25 for all geometries independent of emitter area. The maximum cutoff frequency obtained was 35 GHz with a corresponding 38 GHz for the maximum frequency of oscillation for a 1.3?m x 9 ?m emitter area device.