Abstract: A vertical semiconductor device, for example a trench-gate MOSFET power transistor (1), has a drift region (12) of one conductivity type containing spaced vertical columns (30) of the opposite conductivity type for charge compensation increase of the device breakdown voltage. Insulating material (31) is provided on the sidewalls only of trenches (20) in the drift region (12) and the opposite conductivity type material is epitaxially grown from the bottom of the trenches (20). The presence of the sidewall insulating material (31) reduces the possibility of defects during the epitaxial growth and hence excessive leakage currents in the device (1). The insulating material (31) also prevents epitaxial growth on the trench sidewalls and hence substantially prevents forming voids in the trenches which would lessen the accuracy of charge compensation. The epitaxial growth by this method can be well controlled and may be stopped at an upper level (21) below the top major surface (10a).