An field effect transistor includes a first semiconductor region, a gate electrode insulatively disposed over the first semiconductor region, source and drain electrodes between which the first semiconductor region is sandwiched, and second semiconductor regions each formed between the first semiconductor region and one of the source and drain electrodes, and having impurity concentration higher than that of the first semiconductor region, the source electrode being offset to the gate electrode in a direction in which the source electrode and the drain electrode are separated from each other with respect to a channel direction, and one of the second semiconductor regions having a thickness not more than a thickness with which the one of second semiconductor regions is completely depleted in the channel direction being in thermal equilibrium with the source electrode therewith.