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A new ultrafast photodiode, uni-traveling-carrier photodiode (UTC-PD) is proposed, and its photoresponse characterization on fabricated devices is presented. The prime feature of this PD is much higher output saturation current than that in a conventional pin-PD. This is achieved by reducing the space charge effect by utilizing electron velocity overshoot in the carrier collecting layer. A 20 μm 2-area UTC-PD fabricated with MOVPE-grown InP/InGaAs heterostructure generated an output voltage as high as 2 V for a 25 Ω load while maintaining an f 3dB of 80 GHz. Proper device operations at high photocurrent densities up to 400 kA/cm2 were observed.