Semiconductor device with photonics

A semiconductor structure having a transistor region and an optical device region includes a transistor in a first semiconductor layer of the semiconductor structure, wherein the first semiconductor layer is over a first insulating layer, the first insulating layer is over a second semiconductor layer, and the second semiconductor layer is over a second insulating layer. A gate dielectric of the transistor is in physical contact with a top surface of the first semiconductor layer, and the transistor is formed in the transistor region of the semiconductor structure. A waveguide device in the optical device region and a third semiconductor layer over a portion of the second semiconductor layer.