Semiconductor device with less power supply noise

1. A semiconductor device comprising:

a first power supply line;

a second power supply line;

a first cell arrangement area in which a first cell is arranged; and

a switch area in which a switching transistor and a decoupling capacitance are arranged,

wherein said first cell is provided in a first well of a first conductive type,

said switching transistor is provided in a second well of the first conductive type,

said decoupling capacitance is provided in a separation area of a second conductive type to separate said first well and said second well from each other,

said switching transistor connects said first power supply line and said second power supply line in response to a control signal,

said first cell operates with power supplied from said second power supply line, and

said decoupling capacitance is connected with said first power supply line.

2. The semiconductor device according to claim 1 , wherein said first well is adjacent to said separation area through a PN junction, and

said second well is adjacent to said separation area through the PN junction.

3. The semiconductor device according to claim 2 , further comprising a third power supply line,

wherein said first power supply line supplies a power supply voltage,

said third power supply line supplies a ground voltage,

said second power supply line supplies said power supply voltage supplied through said switching transistor, to said first cell arrangement area, and

said decoupling capacitance is connected with said first power supply line and said third power supply line.

4. The semiconductor device according to claim 3 , further comprising:

a second cell arrangement area arranged between said first power supply line and said third power supply line,

wherein a second cell is provided in said second cell arrangement area to operate without depending on an operation of said switching transistor.

5. The semiconductor device according to claim 4 , wherein said decoupling capacitance comprises a MOS transistor whose source and drain are connected in common.

6. The semiconductor device according to claim 5 , wherein said first well contains an N-type well, and said second well contains an N-type well,

said separation area contains a P-type well, and

said MOS transistor is an NMOS transistor arranged on said P-type well.

7. The semiconductor device according to claim 5 , wherein said first well contains a P well, said second well contains a P well, and said separation area contains an N-type region, and

said MOS transistor whose source and drain are connected is a PMOS transistor arranged on said n-type region.

8. The semiconductor device according to claim 4 , wherein said decoupling capacitance comprises a MOS capacitor.

9. The semiconductor device according to claim 8 , wherein said first well contains an N well, said second well contains an N well, and said separation area contains a P-type region, and

said MOS capacitor is provided on a P-type diffusion region formed on said P-type region.

10. The semiconductor device according to claim 8 , wherein said first well contains an N well, said second well contains an N well, and said separation area contains a P-type region, and

said MOS capacitor is provided on an N-type diffusion region formed on said P-type region.

11. The semiconductor device according to claim 8 , wherein said first well contains a P well, said second well contains a P well, and said separation region contains an N-type region, and

said MOS capacitor is provided on an N-type diffusion region formed in said N-type region.

12. The semiconductor device according to claim 8 , wherein said first well contains a P well, said second well contains a P well, and said separation area contains an N-type region, and

said MOS capacitor is provided on a P-type diffusion region formed in said N-type region.

13. A semiconductor device comprising:

a first well of a first conductive type, wherein said first well has a switch configured to connect a basic power supply line and a disconnection possible power supply line; and

a separation region configured to separate said first well from a second well of the first conductive type in which a standard cell operating with a voltage supplied from said disconnection possible power supply line is arranged,

wherein said separation region is of a second conductive type and has a decoupling capacitance connected between said basic power supply line and a ground line.