P-type field effect transistor and method for fabricating the same

1. A method for fabricating a p-type field effect transistor (pFET), comprising:

providing a substrate;

forming a pad layer on the substrate;

performing an ion implantation process to implant germanium (Ge) into the substrate through the pad layer to form a channel region, wherein a concentration of Ge closer to a boundary between the pad layer and the channel region is greater than a concentration of Ge closer to a boundary between the channel region and the substrate;

performing an anneal process while the pad layer is disposed on a top surface of the channel region to separate the channel region into a top portion and a bottom portion, wherein a depth of the top portion is equal to a depth of the bottom portion and the concentration of germanium in the bottom portion is lower than the concentration of germanium in the top portion, and after the anneal process, the top portion and the bottom portion of the channel region is made of SiGe; and

forming a gate structure on the substrate.

2. The method of claim 1 , further comprising:

forming a well in the substrate before performing the ion implantation process.

3. The method of claim 1 , wherein the pad layer comprises silicon oxide.

4. The method of claim 1 , further comprising removing the pad layer after performing the anneal process.

5. The method of claim 4 , further comprising:

forming the gate structure on the substrate after removing the pad layer; and

forming a lightly doped drain adjacent to two sides of the gate structure, wherein the depth of the channel region is less than a depth of the lightly doped drain.

6. The method of claim 1 , further comprising forming a well in the substrate after performing the ion implantation process.