Semiconductor device and method of manufacturing the same

Abstract

A semiconductor device includes a first semiconductor layer, a second semiconductor layer formed over the first semiconductor layer, a third semiconductor layer formed over the second semiconductor layer, a gate electrode formed over the third semiconductor layer, and a gate insulating film formed between the third semiconductor layer and the gate electrode. The second semiconductor layer includes an Al y α 1-y N layer (α includes Ga or In, and 0≦y<1), and the third semiconductor layer includes an Al z α 1-z N layer (0≦z<1). y of the Al y α 1-y N layer forming the second semiconductor layer increases from the third semiconductor layer to the first semiconductor layer at least in a region under the gate electrode. There is a relationship “z>y” at an interface between the second nitride semiconductor layer and the third nitride semiconductor layer.

What is claimed is: 
     
         1 . A semiconductor device, comprising:
 a first semiconductor layer;   a second semiconductor layer formed over the first semiconductor layer;   a third semiconductor layer formed over the second semiconductor layer;   a gate electrode formed over the third semiconductor layer; and   a gate insulating film formed between the third semiconductor layer and the gate electrode,   wherein the second semiconductor layer comprises an Al y α 1-y N layer (a comprises Ga or In, and 0≦y<1), and the third semiconductor layer comprises an Al z α 1-z N layer (0≦z<1),   wherein y of the Al y α 1-y N layer forming the second semiconductor layer increases from the third semiconductor layer to the first semiconductor layer at least in a region under the gate electrode, and   wherein there is a relationship “z>y” at an interface between the second nitride semiconductor layer and the third nitride semiconductor layer.   
     
     
         2 . The semiconductor device according to  claim 1 , wherein the first semiconductor layer comprises an Al x α 1-x N layer (0<x<1). 
     
     
         3 . The semiconductor device according to  claim 1 , further comprising:
 a source electrode and a drain electrode formed on the third semiconductor layer.   
     
     
         4 . The semiconductor device according to  claim 3 , wherein a first distance from the drain electrode to the gate electrode is longer than a second distance from the source electrode to the gate electrode. 
     
     
         5 . The semiconductor device according to  claim 1 , wherein the second semiconductor layer is formed by a first sub-layer and a second sub-layer formed on the first sub-layer. 
     
     
         6 . The semiconductor device according to  claim 5 , wherein the first sub-layer comprises an Al y1 Ga 1-y1 N layer, and the second sub-layer comprises an Al y2 Ga 1-y2 N layer, and
 wherein there is a relationship “y 1 >y 2 ”.   
     
     
         7 . The semiconductor device according to  claim 1 , wherein an Al composition ratio of the second semiconductor layer changes stepwise.