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We present numerical simulations of the ultrafast transport dynamics in an ultrahigh-speed double-heterostructure p-i-n photodetector. Nonlinear saturation behaviors under high field and high power illumination are investigated with the external circuit response considered. Damping constants and diffusion constants are both treated as electric-field- and carrier-concentration dependent in our model in order to take into account the effect of carrier scattering. We have also considered the carrier trapping at the heterostructure interfaces for the first time. Besides the drift-induced space charge screening effect, we find that saturation of external circuit and carrier-trapping-induced screening effect are also the dominant mechanisms contributed to the nonlinear bandwidth reduction under high power illumination. On the other hand, previously reported plasma oscillations are found to be greatly suppressed by including strong carrier diffusion effect in the model.