The present disclosure provides a semiconductor structure, including a substrate having a front surface, a first semiconductor layer proximal to the front surface, a second semiconductor layer over the first semiconductor layer, a gate having a portion between the first semiconductor layer and the second semiconductor layer, a spacer between the first semiconductor layer and the second semiconductor layer, contacting the gate, and a source/drain (S/D) region, wherein the S/D region is in direct contact with a bottom surface of the second semiconductor layer, and the spacer has an upper surface interfacing with the second semiconductor layer, the upper surface including a first section proximal to the S/D region, a second section proximal to the gate, and a third section between the first section and the second section.