The thin film transistor comprises source and drain regions, a channel forming region formed between the source and drain regions, a first (main) gate for turning on or off the transistor, and in particular at least one second (sub-) gate for reducing turn-off leakage current. When the n-channel transistor is turned off, for example, a negative voltage is applied to the main gate to form a p-channel layer in the channel forming region under the main gate and a positive voltage is applied to the subgate to form an n-channel layer in the channel forming region under the subgate, for instance, so that a pn junction can be formed between under the main gate and the subgate to reduce the turn-off leakage current. The above-mentioned disclosure can be clearly applied to p-channel transistors. Further, the above four-terminal transistor can be simply modified to a three-terminal transistor by connecting the main gate to the subgate via a diode or a capacitor or by directly connecting the drain region to the subgate. Further, the above three-terminal transistor can be manufactured in accordance with only the ordinary device manufacturing process.