For the self-aligned AlGaAs/GaAs HBTs, the surface states at the interface between the extrinsic base surface and the passivation nitride is a major cause of degradation of dc characteristics. In this paper the degradation mechanisms of self-aligned AlGaAs/GaAs HBT were analyzed, and GaAs HBTs, which employed an InGaP ledge emitter structure formed by the nonself-aligned process to cover the surface of the extrinsic base and reduce the surface states, produced high reliability. Accoridng to the acceleration lifetime test, the nonself-aligned InGaP/GaAs HBTs produced very reliable dc characteristics comparing with the self-aligned AlGaAs/GaAs HBTs. The activation energy was 1.97eV and MTTF $4.8{ imes}10^{8}$ hrs at $140^{circ}C$ which satisfied the MIL standard.