Transistor with oxidized cap layer

1. A semiconductor device, comprising:

a substrate;

a channel layer disposed on or above the substrate, wherein a two dimensional electron gas channel exists in the channel layer;

a spacer layer disposed on the channel layer and in contact with the channel layer;

a barrier layer disposed on the spacer layer and in contact with the spacer layer;

an oxidized cap layer disposed on the barrier layer and in contact with the barrier layer, wherein the oxidized cap layer is made of aluminum oxynitride;

a source and a drain disposed on or above the barrier layer;

a gate disposed at least on or above the oxidized cap layer and disposed between the source and the drain, wherein the spacer layer has an oxidation segment, the oxidized cap layer has a first recess, and the barrier layer has a second recess, the first recess and the second recess together expose at least a portion of the oxidation segment, and at least a portion of the gate is disposed in the first recess and the second recess; and

a passivation layer disposed on the oxidized cap layer, wherein at least a portion of the passivation layer is disposed between the oxidized cap layer and the gate, and at least a portion of the passivation layer is in contact with the barrier layer.

2. The semiconductor device of claim 1 , wherein a thickness of the spacer layer is less than 5 nm.

3. The semiconductor device of claim 1 , wherein the spacer layer is made of aluminum nitride.

4. The semiconductor device of claim 1 , wherein a thickness of the oxidized cap layer is less than 5 nm.

5. The semiconductor device of claim 1 , wherein the barrier layer is made of aluminum gallium nitride (Al x Ga (1-x) N), and 0.1≦x≦0.4.

6. The semiconductor device of claim 1 , wherein the passivation layer is conformally disposed in the first recess and the second recess, and at least a portion of the passivation layer is disposed between the gate electrode and the oxidation segment of the spacer layer.