A semiconductor device includes a single crystalline substrate, an electrical element and an optical element. The electrical element is disposed on the single crystalline substrate. The electrical element includes a gate electrode extending in a crystal orientation and source and drain regions adjacent to the gate electrode. The source region and the drain region are arranged in a direction substantially perpendicular to a direction in which the gate electrode extends. The optical element is disposed on the single crystalline substrate. The optical element includes an optical waveguide extending in a crystal orientation .