A power supply system that coverts a DC voltage having a given voltage value into a DC voltage having a voltage value different from the given voltage value is known as so-called DC/DC converter. A DC/DC converter is used in various electronic devices, one example of which is a computer. In a computer, for example, a supply voltage supplied to a microprocessor (hereinafter “CPU”) is generated by a DC/DC converter.
A power supply system like a DC/DC converter has multiple switch elements that cyclically change the direction of a current flowing from one coil to another coil. Each of the multiple switch elements is composed of a transistor, such as metal-oxide-semiconductor field effect transistor (hereinafter “MOSFET”). Various structures of the MOSFET used in the DC/DC converter have been proposed. Patent Documents 1 and 2 disclose various examples of MOSFET structures. In Patent Document 1, a MOSFET structure is shown in, for example, FIG. 1D in which a first gate electrode 10 and a second gate electrode 12 are stacked vertically. In Patent Document 2, another MOSFET structure is shown in, for example, FIG. 4K in which a gate electrode 26 (hereinafter “first gate electrode”) and a gate electrode 30 (hereinafter “second gate electrode”) are stacked vertically.