This paper develops a semi-empirical model for predicting the AlGaAs/GaAs heterojunction bipolar transistor (HBT) long-term current instability and mean time to failure (MTTF). The model involves the use of device physics as well as several empirical parameters which need to be determined from the pre- and post-burn-in data measured from a particular stress condition (i.e., stress temperature, current density, and time). Once these parameters are found, the model can then be used to predict the HBT reliability for various stress conditions. Experimental and modeling fitting for different HBTs subjected to thermal stress of 100-300 degrees C and current stress of 10(4) to 5 x 10(4) A/ cm(2) are included in support of the model.