Self-aligned AlGaAs/GaAs Single HBTs were fabricated using different epilayers with identical layer structure and processing technology. These HBTs manifested different long-term reliability characteristics despite their identical device design and similar DC characteristics. The low-frequency noise characterization of these devices revealed generation-recombination centers with activation energies from 120 meV to 200 meV. The base-emitter region 1/f noise of these devices was found to be in correlation with the long-term reliability.