A power transistor structure includes a planar cell forming a horizontal MOS channel 15 and a plurality of trench recesses 11, which are arranged orthogonally to the longitudinal direction of the planar cells. Planar horizontal MOS channels and vertical trench MOS channels in a single MOS cell structure, or planar MOS channels alone, while utilising the trenches to improve the current spreading of the planar MOS channels. The extent of the body region 8 below the source region 7 controls the extent of vertical channel. Floating P-doped regions 18 at the bottom of the trench recesses protect the device against high peak electric fields. The orthogonal trench recesses are discontinued in their longitudinal direction to allow the planar channels to form. The device structure can be applied to both IGBTs and MOSFETs based on silicon or wide bandgap materials such as silicon carbide SiC, zinc oxide (ZnO), gallium oxide (Ga2O3), gallium nitride (GaN), diamond.