Semiconductor device

A semiconductor device, comprising:

• a substrate;
• a source and drain material layer formed on the substrate, wherein the source and drain material layer contains a first trench extending there-through;
• a channel material layer, including a U-shape portion conformally formed on a bottom and sidewalls of the first trench and two L-shape portions formed on a top surface of the source and drain material layer, wherein the two L-shape portions connects with two ends of the U-shape portion, respectively, and the two L-shape portions form a second trench, and each of the two L-shape portions of the channel material layer includes:
• a first planar portion levelled on the top surface of the source and drain material layer, and a second planar portion directly on the top surface of the source and drain material layer, perpendicular to the first planar portion, and extending in a direction perpendicular to the top surface of the source and drain material layer;
• a source region and a drain region formed in the source and drain material layer respectively on sides of the channel material layer, wherein the second planar portion has a planar surface coplanar with a side surface of the source region and the drain region, and an undoped portion of the source and drain material layer is located between the source region and the channel material layer and another undoped portion of the source and drain material layer is located between the drain region and the channel material layer;
• and a gate structure conformally formed on the channel material layer, on the bottom and the sidewalls of each of the first trench and the second trench, wherein the gate structure has a recess and the recess has a symmetrical step structure.