Transistors of a semiconductor device are fabricated by forming a plurality of gate electrodes on a semiconductor substrate. The gate electrodes are used as an ion implantation mask. A first impurity is ion implanted below the exposed surface of the semiconductor substrate to form first impurity regions. A second impurity is ion implanted in two directions by tilting the implantation to a predetermined angle to thereby form second impurity regions separated from the first impurity regions. The second impurity regions are formed below the channel region under the gate electrodes. The second impurity regions may overlap to provide a higher impurity concentration below a portion of the channel.