Semiconductor device

1. A semiconductor device comprising:

a first well provided in a semiconductor substrate;

a second well provided in the semiconductor substrate, so as to be isolated from the first well;

a Schottky barrier diode formed in the first well; and

a first PN junction diode formed in the second well, with an impurity concentration of the PN junction thereof set higher than an impurity concentration of the Schottky junction of the Schottky barrier diode, acid being connected antiparallel with the Schottky barrier diode.

2. The semiconductor device of claim 1 , further comprising:

a first signal line connected to one terminal of an antiparallel set of the Schottky barrier diode and the first PN junction diode, and through which a source voltage or a reference voltage is applied; and

a second signal line connected to the other terminal, of the antiparallel set, and through which a voltage different from the source voltage and from the reference voltage is applied.

3. The semiconductor device of claim 2 , further comprising:

a third well provided in the semiconductor substrate, so as to be isolated from the first, well and the second well; and

a first transistor formed in the third well,

the second signal line being connected to the third well.

4. The semiconductor device of claim 3 ,

wherein the semiconductor substrate has, in the surficial portion thereof, an epitaxially-grown semiconductor layer, and

the third well is located deeper than the epitaxially-grown semiconductor layer.

5. The semiconductor device of claim 3 , further comprising:

a fourth well provided in the semiconductor substrate, so as to be isolated from the first well and the second well;

a second transistor provided in the fourth well, and having a gate insulating film, thicker than that of the first transistor.

6. The semiconductor device of claim 5 ,

wherein the semiconductor, substrate has, in the surficial portion thereof, an epitaxially-grown semiconductor layer, and

the fourth well extends from surface of the epitaxially-grown semiconductor layer.

7. The semiconductor device of claim 1 ,

wherein the semiconductor substrate has, in the surficial portion thereof, an epitaxially-grown semiconductor layer,

the first well extends from the surface of the epitaxially-grown semiconductor layer, and

the second well is located deeper than the epitaxially-grown semiconductor layer.

8. The semiconductor device of claim 1 ,

wherein the Schottky diode is formed by a Schottky junction between the first well, and an electrode formed over the first well.

9. The semiconductor device of claim 1 ,

wherein the first PN junction diode is formed by a PN junction between the second well, and an imparity layer formed in the surficial portion of the second well and having a conductivity type reverse to chat of the second well.

10. The semiconductor device of claim 1 , further comprising:

a fifth well provided in the semiconductor substrate, so as to be isolated from the first well and the second well; and

a second PN junction diode formed in the fifth well, with an impurity concentration of the PN junction thereof set higher than an impurity concentration of the Schottky junction of the Schottky barrier diode, and being connected antiparallel with the Schottky barrier diode,

a series-connected body of the first PN junction diode and the second PN junction diode, and the Schottky barrier diode being connected antiparallel.

11. The semiconductor device of claim 1 , further comprising:

a sixth well provided in the semiconductor substrate, so as to be isolated from the first well and the second well, and

a third PN junction diode formed in the sixth well, with an impurity concentration of the PN junction thereof set higher than an impurity concentration of the Schottky junction of the Schottky barrier diode, and being connected antiparallel with, the Schottky barrier diode.