The surface recombination current at the emitter‐base heterojunction of AlGaAs/GaAs heterojunction bipolar transistors prepared either by liquid phase or molecular‐beam epitaxy have been studied. An ideality factor close to 1.5 was observed for those transistors with an undoped AlxGa1−xAs (x=0,0.1,0.2) spacer layer being employed in between the emitter‐base heterojunction. Evidence is provided indicating that the rate‐limiting process for surface current transport is the electron injection through the surface channel near the emitter‐spacer junction surface. It was also found that the emitter edge‐thinning design will be most effective when the emitter‐base junction is graded and the exposed base surface is a wide band‐gap AlGaAs layer. By using compositional grading of the emitter‐base junction and emitter edge‐thinning design, high gain (4200)npnAlGaAs/GaAs heterojunction bipolar transistors prepared by molecular‐beam epitaxy can be obtained.