Semiconductor device having two bipolar transistors constituting electrostatic protective element

1. A semiconductor device, comprising:

a pair of transistors formed in a semiconductor substrate of a first conductive type, each of the transistors comprising:

a collector region of a second conductive type, opposite to the first conductive type, formed in the semiconductor substrate;

a base region of the first conductive type formed in the collector region;

an emitter region of the second conductive type formed in the base region, the collector region of one transistor of the pair of transistors being separated from that of the other transistor;

a first region of the first conductive type formed between the collector regions of the pair of transistors; and

a buried layer of the second conductive type formed in the semiconductor substrate under the collector region of one transistor of the pair of transistors, to connect the collector regions of the transistors therethrough.

2. The semiconductor device according to claim 1 ,

wherein the first region is connected to the semiconductor substrate and has a same potential as that of the semiconductor substrate.

3. The semiconductor device according to claim 1 ,

wherein the first region is connected to the collector region and has a same potential as that of the collector region.

4. The semiconductor device according to claim 3 ,

wherein the buried layer covers a bottom face of the first region completely, and

wherein the second region is formed so as to be adjacent to a portion included in a circumference of the first region and not between the collector regions, and so as to surround the first region together with each of the collector regions and formed to reach the buried layer.

5. The semiconductor device according to claim 1 , further comprising:

a third region of the second conducive type formed so as to surround each of the collector regions and be connected to the buried layer, an impurity concentration of the third region being higher than that of each of the collector regions.

6. The semiconductor device according to claim 1 , further comprising:

a signal line connected to the base and emitter regions of one transistor of the pair of transistors, respectively; and

a power line or ground line connected to the base and emitter regions of the other transistor, respectively.

7. A semiconductor device, comprising:

first and second transistors formed in a semiconductor substrate of a first conductive type, each of the first and second transistors comprising:

a collector region of a second conductive type, opposite to the first conductive type, formed in the semiconductor substrate;

a base region of the first conductive type formed in the collector region; and

an emitter region of the second conductive type formed in the base region, the collector region of the first transistor being separated from that of the second transistor;

means for, when a first noise, which has a voltage potential the same or less than a withstand voltage of the second transistor, is applied to an input terminal of the first transistor, evacuating a current generated by a parasitic transistor activated by the first noise to a ground potential; and

means for, when a second noise, which has a voltage potential larger than the withstand voltage of the second transistor, is applied to the input terminal of the first transistor, for responding to the second noise to increase a potential of the collector region of the second transistor so that the second transistor turns ON.