Groups III-V compound semiconductors and their alloys are the main photodetecting elements for the entire fiber optic telecommunication band. However, the recent successful growth of alloy on Ge virtual substrates on Si platform makes the group IV alloys a potential competitor. alloy shows direct band gap and has an absorption coefficient almost 10 times higher than that of Ge. The photonic devices are complementary metal–oxide–semiconductor compatible. We have considered an heterojunction phototransistor (HPT) and studied the variations of terminal currents by considering the Gummel Poon model of HPT, and values of optical and current gains, photocurrent, and responsivity have been obtained. The performance of the device as a photodetector at fiber optic communication wavelengths seems quite encouraging to justify the use of GeSn-based HPTs as a replacement of III-IV semiconductor-based photodetectors.