NpnAlGaAs/GaAs heterojunction bipolar transistors (HBTs) are fabricated to examine the critical passivation ledge thickness needed to prevent current gain degradation due to extrinsic base surface recombination current. The current gains of these molecular‐beam epitaxially grown HBTs with Be‐doped base layers are measured as the passivation ledge is gradually etched away. Experimental results indicate that partial passivation is achieved with a ledge thickness of 300 Å and a maximum current gain is measured with a ledge thickness of 900 Å. The ideality factor values of the base currents at various etch depths are also examined and compared with published results.