A power supply device includes a control IC fabricated by a bipolar process and a power supply element fabricated by a MOS process, both of them die-bonded on a leadframe, and with a chip edge of one of them kept in intimate contact with a chip edge of the other. Thus, heat conducts via those chip edges with increased efficiency, permitting the heat generated in the power supply element to quickly conduct to the control IC. This prevents heat-induced breakdown to which a MOS semiconductor is susceptible. The power supply element fabricated by a MOS process can be a horizontal structure so that a current flows from one part of the top surface of the chip to another. This makes it easy to reduce power loss. The power supply element and the control IC can be die-bonded with a single type of die-bonding paste.