Semiconductor device and manufacturing method thereof

1. A semiconductor device, comprising:

a substrate;

a gate structure disposed in the substrate;

a channel layer sandwiched between the gate structure and the substrate, wherein a material of the channel layer is selected from the group consisting of silicon-germanium epitaxial material, silicon-carbon epitaxial material, and a combination thereof, and the content of germanium, carbon or the both of the channel layer is abruptly or gradually increased from a side of the channel layer contacting the substrate to another side of the channel layer contacting the gate structure; and

a first active region and a second active region disposed in the substrate and respectively disposed at opposite sides of the gate structure.

2. The semiconductor device of claim 1 , wherein the content of germanium, carbon or the both of the channel layer has a maximum between a side of the channel layer contacting the substrate and another side of the channel layer contacting the gate structure.

3. The semiconductor device of claim 1 , wherein a memory cell comprises the first active region, the gate structure, and the second active region.

4. The semiconductor device of claim 3 , further comprising a plurality of isolation structures disposed in the substrate, and the memory cell disposed between adjacent two of the isolation structures.

5. The semiconductor device of claim 4 , further comprising an epitaxial layer disposed between each of the isolation structures and the substrate.

6. The semiconductor device of claim 5 , wherein a material of the epitaxial layer is consisting essentially of silicon-germanium epitaxial material, silicon-carbon epitaxial material, or silicon-carbon-germanium epitaxial material.

7. The semiconductor device of claim 6 , wherein the materials of the epitaxial layer and the channel layer are individually Si1-xGex, 0<x≦0.8; Si1-yCy, 0<y≦0.3; or Si1-a-bGeaCb, 0<a≦0.8, 0<b≦0.3.

8. The semiconductor device of claim 6 , wherein the content of germanium, carbon or the both of the epitaxial layer is abruptly or gradually increased from a side of the epitaxial layer contacting one of the isolation structures to another side of the epitaxial layer contacting the substrate.

9. The semiconductor device of claim 6 , wherein the content of germanium, carbon or the both of the epitaxial layer has a maximum between a side of the epitaxial layer contacting one of the isolation structures and another side of the epitaxial layer contacting the substrate.

10. A semiconductor device, comprising:

a substrate;

a gate structure disposed in the substrate;

an active region disposed in the substrate and disposed at a side of the gate structure;

an isolation structure disposed in the substrate and disposed between two adjacent of the active regions; and

an epitaxial layer overlying a surface of the isolation structure, wherein a material of the epitaxial layer is selected from the group consisting of silicon-germanium, silicon-carbon, and a combination thereof.

11. The semiconductor device of claim 10 , wherein the materials of the epitaxial layer and the channel layer are individually Si1-xGex, 0<x≦0.8; Si1-yCy, 0<y≦0.3; or Si1-a-bGeaCb, 0<a≦0.8, 0<b≦0.3.

12. The semiconductor device of claim 10 , wherein the content of germanium, carbon or the both of the epitaxial layer is abruptly or gradually increased from a side of the epitaxial layer contacting one of the isolation structures to another side of the epitaxial layer contacting the substrate.

13. The semiconductor device of claim 10 , wherein the content of germanium, carbon or the both of the epitaxial layer has a maximum between a side of the epitaxial layer contacting one of the isolation structures and another side of the epitaxial layer contacting the substrate.