Composite device and switching power supply

A composite device in which a first enhancement-mode MOS device and a depletion-mode MOS device are integrated, comprising:

• an epitaxial region of a first doping type, wherein the epitaxial region acts as drains of the first enhancement-mode MOS device and the depletion-mode MOS device;
• a first well region and a second well region formed in parallel on a front side of the epitaxial region, wherein the first well region and the second well region have a second doping type opposite to the first doping type;
• a first doped region of the first doping type formed within the first well region, wherein the first doped region acts as a source of the first enhancement-mode MOS device;
• a gate of the first enhancement-mode MOS device formed on the front side of the epitaxial region, wherein the gate of the first enhancement-mode MOS device covers at least a portion of the first doped region and extends onto the epitaxial region outside the first well region;
• a second doped region of the first doping type formed within the second well region, wherein the second doped region acts as a source of the depletion-mode MOS device;
• a channel region of the first doping type located within the second well region, wherein the channel region extends from a boundary of the second well region to a boundary of the second doped region;
• and
• a gate of the depletion-mode MOS device formed on the front side of the epitaxial region, wherein the gate of the depletion-mode MOS device covers the channel region and extends onto the epitaxial region outside the second well region;
• wherein the epitaxial region is shorted to a first electrode formed on a back side of the epitaxial region, the first well region and the first doped region are shorted via a second electrode, the second well region and the second doped region are shorted via a third electrode, wherein the second electrode and the third electrode are formed on the front side of the epitaxial region.