Semiconductor device and manufacturing method thereof

A semiconductor device, comprising:

• a substrate having a first portion and a second portion thinner than the first portion;
• a plurality of first horizontal nanowires over the first portion of the substrate;
• and
• a plurality of second horizontal nanowires over the second portion of the substrate, wherein the second horizontal nanowires comprises a material different from the first horizontal nanowires, and one of the first horizontal nanowires is substantially level with one of the second horizontal nanowires.

A method, comprising:

• forming a recess in a substrate;
• forming an epitaxy layer into the recess in the substrate, wherein the epitaxy layer comprises a semiconductor material different from the substrate;
• and
• forming a stacked structure of a plurality of first semiconductor layers and a plurality of second semiconductor layers alternately stacked over the substrate and the epitaxy layer.