Optimum doping profile of power MOSFET epitaxial layer

The epitaxial layer resistance of a MOSFET can be slightly reduced by using an optimum doping profile, which exhibits a minimum in the upper half of the layer when the layer thickness is large compared to the cell-to-ceU spacing. A gradual transition from the n epitaxial layer to the n+ substrate is desirable. When current spreading is significant, the resistance may rise as V
B
²
rather than V
B
^2.5
.