The long-term reliability of heterojunction bipolar transistor (HBT) continues to be a subject of great interest due to the increased acceptance of this device in a wide range of applications. The most demanding requirements for long-term reliability include high performance microwave instrumentation, X-band radar, and lightwave communication (OC-192). A significant leap in the long-term reliability performance was observed in HBT as the AlGaAs emitter material was replaced with lattice matched InGaP. A dramatic improvement in the long-term reliability was also observed in AlGaAs emitter HBT's as the turn on voltage (Vbe) was lowered. The typical failure mechanism in HBT devices at high current density and high temperature long-term reliability testing was a dramatic increase in the base current at low current densities. One of the limiting factors in obtaining MTTF in InGaP HBT was the long time required to promote failures in the HBT device. Furthermore, a large sample size is necessary to extract a reliable MTTF. Significant increases in the current density as high as 180 kA/cm2 during reliability testing was used to promote failures in order to obtain an MTTF within a reasonable amount of time. The MTTF at a junction temperature of 334C and at a current density of 180 kA/cm2 was 1159 hours. The extrapolated MTTF at a junction temperature of 150C exceeded 106 hours for all of the tested devices. An attempt to predict the MTTF of AlGaAs and InGaP HBT using a simple model based upon the fitting of the initial Gummel plots of large area devices was made. The model was based upon the estimation of the trap defect density at the base/emitter junction, the hole injection component of the base current, and the turn-on Vbe. Degradation of the HBT was assumed to occur at the base/emitter junction and this corresponded to an increase in the trap density at this heterojunction. A factor of 5 improvement in the MTTF of the reliability of AlGaAs HBT with a lower turn on voltage was estimated based upon the above model, which confirmed the experimental results. These results suggested that the emitter material is primarily responsible in determining the long-term reliability characteristics of HBT. The combination of a high effective hole barrier and a low turn-on Vbe are highly desirable for long-term reliability characteristics.