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We demonstrate Zn-diffusion InAs-based high-speed photodiodes (PDs) fabricated on semi-insulating (S.I.) GaAs substrates. The Zn-diffusion profile in our PDs is used to minimize the influence of the surface-state on the dark current, which is an issue for small (high-speed) InAs PDs with a large surface-to-volume ratio. Compared to control without Zn-diffusion, our device exhibits a much lower dark current. In addition, as compared to the performance reported for InAs PDs on conductive InAs substrates, our PDs on S.I. substrates demonstrate a lower parasitic capacitance and have a superior capability for being integrated with other microwave components. The measured optical-to-electrical (O-E) bandwidth of our device can be as wide as 20 GHz with a reasonable dark current density ( ~ 50 A/cm2) at room temperature. Based on our modeling results, the measured bandwidths are limited by the internal electron drift/diffusion time due to the intervalley scattering effect under 1.55-μm wavelength excitation.