A transistor includes a substrate and an isolation region disposed in the substrate. The isolation regions defines an active region comprising upper and lower active regions, the upper active region having a first width and the lower active region having a second width greater than the first width. An insulated gate electrode extends through the upper active region and into the lower active region. Source and drain regions are disposed in the active region on respective first and second sides of the insulated gate electrode. The insulated gate electrode may include an upper gate electrode disposed in the upper active region and a lower gate electrode disposed in the lower active region, wherein the lower gate electrode is wider than the upper gate electrode. Related fabrication methods are described.