The present disclosure provides a semiconductor device. The semiconductor device includes a substrate having isolation regions, a gate region, source and drain regions separated by the gate region, a first fin structure in a gate region. The first fin structure includes a first semiconductor material layer as a lower portion of the first fin structure, a semiconductor oxide layer as an outer portion of a middle portion of the first fin structure, the first semiconductor material layer as a center portion of the middle portion of the first fin structure and a second semiconductor material layer as an upper portion of the first fin structure. The semiconductor device also includes a source/drain feature over the substrate in the source/drain region between two adjacent isolation regions and a high-k (HK)/metal gate (MG) stack in the gate region, wrapping over a portion of the first fin structure.