The temperature-dependent characteristics of InGaP/GaAs heterojunction bipolar transistors with different emitter ledge thickness (t) are studied and demonstrated. From experimental results, devices d(3) (t=100 A) and d(4) (t=200 A) show the highest current gains, lowest base current, and base current ideality factors. Devices d(3) and d(4) also exhibit an improved thermal stability on dc current-voltage characteristics. Moreover, device d(4) shows the best reliability performance after a 200 h stress test. Therefore, the data support that the optimum emitter ledge thickness is between 100 and 200 A.