A semiconductor device, a method for fabricating the same, and a transformer circuit using the same are disclosed. The semiconductor device includes a trench metal oxide semiconductor (MOS) transistor for switching a load of current supplied from a power source, and a boost controller for controlling driving of the trench MOS transistor, the boost controller being formed with the trench MOS transistor on a single semiconductor device to form an integrated structure. In this structure, the physical space of the semiconductor device is reduced, thereby reducing the size of a DC-DC transformer circuit using the semiconductor device. It is possible to obtain finely-adjusted output values by controlling values of the ripple current and ripple voltage. A desired operational stability according to a variation in temperature can also be secured.