An MOS-type semiconductor device comprises two semiconductors separated by an insulator. The two semiconductors comprise monocrystal semiconductors, each having a crystallographic orientation with respect to the insulator (or other crystallographic/semiconductor property) different to the crystallographic orientation (or other respective property) of the other semiconductor. This arrangement of crystallographic orientations (and other crystallographic/semiconductor properties) can yield reduced unintended electron tunneling or current leakage through the insulator vis a vis a semiconductor device in which such an arrangement is not used. Methods for forming the MOS-type semiconductor devices of the invention are also provided.