We report the demonstration of a germanium-tin (Ge0.9Sn0.1) multiple-quantum-well p-i-n photodiode on silicon (Si) substrate for 2 μm-wavelength light detection. Characterization of the photodetector in both direct current (DC) and radio frequency (RF) regimes was performed. At the bias voltage of -1 V, a dark current density of 0.031 A/cm2 is realized at room-temperature, which is among the lowest reported values for Ge1-xSnx-on-Si p-i-n photodiodes. In addition, for the first time, a 3 dB bandwidth (f3dB) of around 1.2 GHz is achieved in Ge1-xSnx photodetectors operating at 2 μm. It is anticipated that further device optimization would extend the f3dB to above 10 GHz.