Two different $Ga_{0.51}In_{0.49}P/GaAs$ Heterojunction Phototransistors (HPT) are grown by MOCVD. Both HPTs exhibit optical gain of 6 with input optical power of 500 $\mu$W. The dc gain of the HPT is an increasing function of incident optical power at low incident optical power levels. This result indicates that surface recombination effect is dominant at low incident power levels. The gain-bandwidth product of the sample STR-B is measured to be 3.8 GHz with input optical power of 340 $\mu$W, and that of the sample STR-C is 1.7 GHz with input optical power of 500 $\mu$W. This difference in the gain-bandwidth products is due to the different junction capacitances of the STR-B and STR-C. From the C-V curves, the emitter and collector charging time $\tau_e$ is obtained. Calculated cut-off frequencies are compared with the experimental data.
