Alloys of Ge and Sn grown on Si platform show about tenfold increase in the absorption over Ge at C and L-bands due to the direct nature of the band gap in GeSn. This led us to propose a Ge/GeSn/GeSn heterophototransistor (HPT) structure on a Si-platform and to calculate the terminal currents, current gain, optical gain and responsivity of the device at (Formula presented.). The model developed by Frimel and Roenker for HPTs with InGaAs base is used. In the model, the carrier density in the base is obtained by matching the diffusion-limited current with the current due to thermionic emission and tunneling across the emitter-base heterobarrier, and also considering the carrier density at the collector-end limited by the saturation drift velocity. The contribution of holes to the current is also included. The resulting carrier distribution is used to express the terminal currents and calculate the values of gain. The band gap for GeSn and the band discontinuity in the heterobarrier are calculated by using model solid theory. The calculated values for the GeSn HPT are found to be comparable with and even better than the corresponding values for the InP/InGaAs/InGaAs Npn device for lower values of the base thickness and the base doping. © 2014, Springer Science+Business Media New York.