Abstract : We present an impulse response of traveling-wave photodetector considering the photo-excited carrier transit time as well as the velocity mismatch between lightwave and microwave. The transit time effect on the bandwidth is found to be more significant than the velocity mismatch effect, if the thickness of the intrinsic absorption region is over 0.2 μm. The effect of the intrinsic absorption region thickness is also examined. The optimal thickness of the intrinsic absorption region is determined considering the transit time, the optical confinement factor, and the velocity mismatch.