Ga0.51In0.49P/GaAs tunneling emitter bipolar transistors (TEBTs) grown by gas source molecular beam epitaxy with different barrier thicknesses (50 and 100 Å) have been fabricated and measured at different temperatures (77 and 300 K) for the first time. A current gain of 141 and an offset voltage of 50 mV at room temperature were achieved in the thick barrier (100 Å) device with heavily doped p+ (1×1019 cm-3) base. The thinner device had a lower gain (∼53) than the thick barrier device at room temperature. The functional dependence of gain on thickness was attributed to carrier tunneling through the barrier inserted between the base and the emitter. The low-temperature (77 K) results showed that the gain of the thick barrier device was almost the same as that at 300 K while that of the thin barrier device increased, which was explained by the nonequilibrium electron transport in the base.