In this work, we investigate the temperature dependence of electrical switching properties of back-gated, undoped Si-nanowire field-effect transistors with Ni-silicided source/drain contacts. A simple, phenomenological model illustrates the leading order temperature dependence of the source-drain current, which originates predominantly from charge carrier injection by tunneling through the Schottky junction. Drain current versus gate voltage measurements have been performed for various temperatures and several drain voltages on a single nanowire device. The temperature dependence of the drain-source current for specific gate and drain voltages is analysed within the framework of voltage dependent effective barrier heights. As a result, the temperature dependence of the tunnelling current is not only important for the sub-threshold region, but also plays a significant role in the transistor "on-state". In addition, the effective barrier heights for electrons and holes tend towards the natural Schottky barriers of the NiSi₂-Si interface, if the applied external fields generate the case of flat band condition at the injection Schottky barrier, i.e. in the deep "off-state" region.