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Operation frequency is discussed for near-infrared photodiodes (PDs) using Ge layers on Si, which are indispensable for the photonic-electronic convergence on an Si chip. Based on the formula derived from the continuity equation, Ge pin PDs on Si are found to operate with the 3-dB cut-off frequency as high as 80 GHz, which is limited by the slow diffusion current from the n and p layers. In order to increase the frequency, a new structure is examined, which is composed of a p-Ge/i-Si/n-Si heterojunction. In this structure, electrons generated in the p layer of Ge are collected by the i layer of wider gap Si, being similar to uni-traveling-carrier PDs of InGaAs/InP in III-V systems. Reflecting the larger saturation velocity of carriers for i-Si in comparison with i-Ge, higher operation frequencies as large as 100 GHz are expected by optimizing the layer thicknesses.