Nitride semiconductor device

1. A nitride semiconductor device, comprising:

a first semiconductor layer made of first nitride semiconductor;

a second semiconductor layer formed on a principal surface of the first semiconductor layer and made of second nitride semiconductor having a bandgap wider than that of the first nitride semiconductor, said second nitride semiconductor includes Al x M 1-x , where M is at least one of the group III elements except Al, and an Al x content of 0<x<0.25;

a control layer selectively formed on an upper portion of the second semiconductor layer and made of third nitride semiconductor having a p-type conductivity;

source and drain electrodes formed on the second semiconductor layer at respective sides of the control layer; and

a fourth semiconductor layer formed on a surface of the first semiconductor layer opposite to the principal surface, having a potential barrier in a valence band with respect to the first nitride semiconductor and made of fourth nitride semiconductor containing aluminum, wherein:

a the channel region is formed in the first semiconductor layer,

the control layer controls electrical conductivity between the source electrode and the drain electrode,

the nitride semiconductor device is of a normally OFF type, and

a thickness of the control layer is greater than a thickness of the second semiconductor layer.

2. The nitride semiconductor device of claim 1 , wherein the first nitride semiconductor contains indium.

3. The nitride semiconductor device of claim 1 , wherein the first semiconductor layer has a thickness larger than 0 nm and equal to or smaller than 30 nm.

4. The nitride semiconductor device of claim 3 , wherein the fourth semiconductor layer is formed so that the holes injected into the channel region can increase in the first semiconductor layer without dissipation.

5. The nitride semiconductor device of claim 1 , wherein the first nitride semiconductor contains indium, and the first semiconductor layer has a thickness larger than 0 nm and equal to or smaller than 30 nm.

6. The nitride semiconductor device of claim 1 , wherein the distance between the control layer and the drain electrode is about 5 μm or more.

7. The nitride semiconductor device of claim 1 , wherein

the control layer has a different Al content from the second semiconductor layer, and

the control layer is made of p-type GaN.

8. The nitride semiconductor device of claim 1 , further comprising a contact layer formed directly on the control layer and made of fifth nitride semiconductor having a p-type impurity concentration higher than that of the control layer; and

a gate electrode formed directly on the contact layer,

wherein the gate electrode is in Ohmic contact with the contact layer.

9. The nitride semiconductor device of claim 1 , wherein the second semiconductor layer includes Al x Ga 1-x .