We investigated the temperature dependent characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) at temperatures down to 20 K. With decreasing temperature, the threshold voltage increased, the field effect mobility and the on-current decreased. The measured on-currents versus inverse temperature above 80 K are represented as the sum of two exponentially varied currents. It is concluded that on-current is nearest-neighbour hopping between 120 K and 80 K. Below this temperature, the temperature dependence of on-current is explained by variable range hopping and below about 30 K on-current becomes nearly independent of temperature. At very low temperature hopping probability may be governed not by temperature but by temperature independent tunneling, depending on the overlap of the wave function. The explanation of threshold voltage increase at low temperature is given.