Semiconductor device that uses a transistor for field shield

1. A semiconductor device comprising:

a semiconductor substrate including first, second and third portions;

the first and second portions being partitioned by a cell gate trench having a bottom surface, a first side surface located on the first portion side, and a second side surface located on the second portion side,

the first and third portions being partitioned by a first field-shield gate trench exposing a third side surface located on the first portion side and a fourth side surface located on the third portion side, and

a distance between the third and fourth surfaces being narrower than a distance between the first and second surfaces,

a first upper diffusion layer that is provided on an upper part of the first portion of the semiconductor substrate;

a second upper diffusion layer that is provided on an upper part of the second portion of the semiconductor substrate;

a third upper diffusion layer that is provided on an upper part of the third portion of the semiconductor substrate;

a lower diffusion layer that is provided in the bottom surface of the cell gate trench;

a first and a second storage element that are electrically connected to the first and second upper diffusion layers, respectively;

a bit line that is electrically connected to the lower diffusion layer;

a first and a second cell gate electrode that cover the first and second side surfaces, respectively, via a gate insulating film; and

a first field-shield gate electrode that is embedded in the first field-shield gate trench via a gate insulating film.

2. The semiconductor device as claimed in claim 1 , wherein

the first cell gate electrode, the first upper diffusion layer and the lower diffusion layer constitute a first cell transistor,

the second cell gate electrode, the second upper diffusion layer and the lower diffusion layer constitute a second cell transistor, and

the first field-shield gate electrode and the first and third upper diffusion layers constitute a first field-shield transistor.

3. The semiconductor device as claimed in claim 2 , wherein the first field-shield gate electrode is supplied with a voltage less than a threshold voltage of the first field-shield transistor.

4. The semiconductor device as claimed in claim 1 , wherein each of the first and second cell gate electrodes comprises a sidewall conducting film formed on corresponding one of the first and second side surfaces.

5. The semiconductor device as claimed in claim 1 , wherein

the first upper diffusion layer is provided so as to be in contact with upper ends of the cell gate trench and the first field-shield gate trench,

the second upper diffusion layer is provided so as to be in contact with an upper end of the cell gate trench, and

the third upper diffusion layer is provided so as to be in contact with an upper end of the first field-shield gate trench.

6. The semiconductor device as claimed in claim 1 , further comprising a bit line contact plug that electrically connects the lower diffusion layer and the bit line, wherein

at least a portion of the bit line contact plug is provided in a region between the first cell gate electrode and the second cell gate electrode in the cell gate trench.

7. The semiconductor device as claimed in claim 1 , wherein

the semiconductor substrate further includes a fourth portion,

the second and fourth portions are partitioned by a second field-shield gate trench having a fifth side surface located on the second portion side and a sixth side surface located on the second portion side, and

a distance between the fifth and sixth surfaces is narrower than the distance between the first and second surfaces,

the semiconductor device further comprising:

a fourth upper diffusion layer that is provided on an upper part of the fourth portion of the semiconductor substrate; and

a second field-shield gate electrode that is embedded in the second field-shield gate trench via a gate insulating film, wherein

the second field-shield gate electrode and the second and fourth upper diffusion layers constitute a second field-shield transistor, and

the second field-shield gate electrode is supplied with a voltage less than a threshold voltage of the second field-shield transistor.

8. A semiconductor device comprising:

a semiconductor substrate having a plurality of cell gate trenches and a plurality of field-shield gate trenches that extend to a first direction in parallel, the cell gate trenches and the field-shield gate trenches are disposed so as to alternately appear in a second direction, each of the cell gate trenches exposing a bottom surface and first and second side surfaces of the semiconductor substrate, and each of the cell gate trenches is wider in the second direction than each of the field-shield gate trenches;

a plurality of upper diffusion layers, each of which is provided on an upper part of the semiconductor substrate provided between an associated one of the cell gate trenches and an associated one of the field-shield gate trenches;

a plurality of lower diffusion layers, each of which is provided in an associated one of the bottom surfaces;

a plurality of storage elements, each of which is electrically connected to an associated one of the upper diffusion layers;

a plurality of bit lines extending to the second direction, each of which is electrically connected to an associated one of the lower diffusion layers;

a plurality of first cell gate electrodes, each of which covers the first side surface exposed on an associated one of the cell gate trenches via a gate insulating film;

a plurality of second cell gate electrodes, each of which covers the second side surface exposed on an associated one of the cell gate trenches via a gate insulating film; and

a plurality of field-shield gate electrodes, each of which is embedded in an associated one of the field-shield gate trenches via a gate insulating film.

9. The semiconductor device as claimed in claim 8 , wherein

each of the first cell gate electrodes and the upper diffusion layer and lower diffusion layer that are each adjacent to the first cell gate electrode constitute a first cell transistor,

each of the second cell gate electrode and the upper diffusion layer and lower diffusion layer that are each adjacent to the second cell gate electrode constitute a second cell transistor, and

each of the field-shield gate electrode and two of the upper diffusion layers that are each adjacent to the field-shield gate electrode constitute a field-shield transistor.

10. The semiconductor device as claimed in claim 9 , wherein each of the field-shield gate electrodes is supplied with a voltage less than a threshold voltage of the corresponding field-shield transistor.

11. The semiconductor device as claimed in claim 8 , wherein each of the first and second cell gate electrodes comprises a sidewall conducting film formed on an associated one of the first and second side surfaces.

12. The semiconductor device as claimed in claim 8 , wherein each of the upper diffusion layers is provided as to be in contact with upper ends of the adjacent cell gate trenches and field-shield gate trenches.

13. The semiconductor device as claimed in claim 8 , further comprising a plurality of bit line contact plugs that are provided for each of the lower diffusion layers to electrically connect the corresponding lower diffusion layers and the corresponding bit lines, wherein

at least a portion of each of the bit line contact plugs is provided in a region between the first cell gate electrode and the second cell gate electrode in the corresponding cell gate trench.