The base current of AlGaAs/GaAs heterojunction bipolar transistor subjected to a long burn-in test often exhibits an abnormal characteristic with an ideality factor of about 3, rather than a normal ideality factor between 1 and 2, in the midvoltage range, We develope an analytical model to investigate the physical mechanisms underlying such a characteristic. Consistent with the finding of an experimental work reported recently, our model calculations show that the recombination current in the base has an ideality factor of about 3 in the midvoltage range and that such a current is responsible for the observed abnormal base current in heterojunction bipolar transistor after a long burn-in test. Post-burn-in data measured from two different heterojunction bipolar transistors are also included in support of the model