In this paper, we discuss the device performance of atomic monolayer semiconductor FETs composed of Si, Ge and C elements. First, we present the performance potentials of silicene nanoribbon (SiNR), germanene nanoribbon (GeNR) and graphene nanoribbon (GNR), which all have a sufficient band gap to switch off, as a field-effect transistor (FET) channel material. We demonstrate that comparing at the same band gap of ~ 0.5 eV, GNR FET maintains the advantage over SiNR or GeNR FETs under an ideal transport situation, but SiNR and GeNR are attractive channel materials for high performance FETs as well. Next, we compute the electronic band structure and the electron mobility of germanane, which is a hydrogen-terminated Ge monolayer. We demonstrate that germanane has a band gap larger than 1 eV without a nanoribbon structure, and an effective mass smaller than bulk Ge. Therefore, germanane is also a promising two-dimensional material as a FET channel.