Semiconductor device and method of manufacturing the same

1. A semiconductor device in which current flows in a vertical direction, comprising:

a semiconductor layer including a lower semiconductor layer and an upper semiconductor layer formed on the lower semiconductor layer and having a width different from a width of the lower semiconductor layer;

a gate layer formed in a step portion between the lower semiconductor layer and the upper semiconductor layer;

a source layer formed on the upper semiconductor layer and contacted to the gate layer through an insulating film, and

a resistance decreasing structure decreasing resistance between a source electrode and a drain electrode along with a current path at a position different from a position having highest electric field intensity between the source electrode and the drain electrode.

2. The semiconductor device according to claim 1 ,

wherein the resistance decreasing structure is formed in a side surface of the lower semiconductor layer.

3. The semiconductor device according to claim 2 , further comprising:

a drain electrode provided in an upper layer of the semiconductor layer; and

a drain contact electrode layer connected to the highly-concentrated impurity layer and extended from the highly-concentrated impurity layer to the drain electrode.

4. A semiconductor device, comprising:

a semiconductor substrate;

a semiconductor layer including a first conductive type first highly-concentrated impurity layer formed on the semiconductor substrate, a first conductive type lower semiconductor layer formed on the first highly-concentrated impurity layer and having a width narrower than a width of the first highly-concentrated impurity layer, and a first conductive type upper semiconductor layer formed on the lower semiconductor layer and having a width narrower than a width of the lower semiconductor layer;

a gate layer formed in a step portion between the lower semiconductor layer and the upper semiconductor layer;

an insulating film formed between the lower semiconductor layer and the gate layer;

a first conductive type source layer formed in the upper semiconductor layer and contacted to the gate layer through the insulating film;

a source contact layer contacted to the source layer;

a second conductive type base layer formed in the upper semiconductor layer, contacted to the source contact layer, and opposed to the gate layer through the insulating film; and

a first conductive type second highly-concentrated impurity layer formed in a side surface of the lower semiconductor layer.

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

a drain electrode provided in an upper layer of the semiconductor layer; and

a drain contact layer connected to the first highly-concentrated impurity layer and extended from the lower semiconductor layer to the drain electrode.

6. The semiconductor device according to claim 4 , wherein the base layer includes a second conductive type highly-concentrated base layer in a part contacted to the source contact layer.

7. A semiconductor device according to claim 2 , wherein the resistance decreasing structure is a highly-concentrated impurity layer.