A method of fabricating a semiconductor device comprising: forming a trench on the face of a silicon substrate of a first conductivity type; depositing a conformal silicon layer of a second conductivity type into the trench; etching away the silicon layer of a second conductivity type selectively to leave portions of the silicon layer in the trench; annealing to drive dopant from the portions of the silicon layer through the walls of the trench into adjacent areas of the silicon substrate; and forming a gate structure in the trench, and source and drain diffusion regions in said silicon substrate on opposing sides of said gate structure.