A semiconductor device is created in a doped silicon layer at most one-tenth of a micrometer thick formed on and having an interface with a sapphire substrate. An oppositely doped source region is formed in the silicon layer. A gate electrode is formed above part of the silicon layer. A diffusion layer doped with the same type of impurity as the source region but at a lower concentration is formed in the silicon layer, extending into a first area beneath the gate electrode, functioning as a drain region or as a lightly-doped extension of a more heavily doped drain region. The depth of this diffusion layer is less than the thickness of the silicon layer. This comparatively shallow diffusion depth reduces current leakage by inhibiting the formation of a back channel.