Power MOSFET having a trench gate electrode and method of making the same

A trench-gated power MOSFET (20) contains a highly doped region (222) in the body region (218) which forms a PN junction diode (D1) with the drain at the center of the MOSFET cell. This diode has an avalanche breakdown voltage which is lower than the breakdown voltage of the drain-body junction (217) near to the wall of the trench (208). Thus the MOSFET breaks down in the center of the cell avoiding the generation of hot carriers that could damage the gate oxide layer. The drain-body junction is located at a level which is above the bottom of the trench, thereby avoiding any deep diffusion that would increase the cell width and reduce the cell packing density. This compact structure is achieved by limiting the thermal budget to which the device is exposed after the body region is implanted. As a result, the body and its highly doped region do not diffuse significantly, and dopant from the highly doped region does not get into the channel region of the device so as to increase its threshold voltage.