The current transport in AlGaAs GaAs heterojunction bipolar transistors (HBTs) operating in the temperature range of 300–500 K is investigated based on an analytical model including the lattice heating effect. It is found that an uneven increase in the base and collector currents gives rise to the current gain degradation at high temperatures. In addition, the study suggests that the effect of lattice heating becomes more important if the HBT area is reduced and/or the ambient temperature is increased. Experimental data and results simulated from a device simulator are included in support of our finding.