Semiconductor device and method for fabricating the same

Abstract

A semiconductor device has a first nitride semiconductor layer, a second nitride semiconductor layer formed on the first nitride semiconductor layer and having such a composition as to generate a 2-dimensional electron gas layer in the upper portion of the first nitride semiconductor layer, and an electrode having an ohmic property and formed selectively on the second nitride semiconductor layer. The second nitride semiconductor layer includes a contact area having at least one inclined portion with a bottom or wall surface thereof being inclined toward the upper surface of the first nitride semiconductor layer and defining a depressed cross-sectional configuration. The electrode is formed on the contact area.

1 - 6 . (canceled)  
   
   
       7 . A method for fabricating a semiconductor device, the method comprising the steps of: 
 forming, on a first nitride semiconductor layer, a second nitride semiconductor layer having such a composition as to form a 2-dimensional electron gas layer in an upper portion of the first nitride semiconductor layer;    selectively forming, in an upper portion of the second nitride semiconductor layer, a contact area having at least one inclined portion whose bottom or wall surface is inclined toward an upper surface of the first nitride semiconductor layer and defining a depressed cross-sectional configuration; and    forming an electrode having an ohmic property on the contact area.    
   
   
       8 . The method of  claim 7 , wherein the step of forming the contact area includes the steps of: 
 forming a resist film on the second nitride semiconductor layer and exposing a region of the resist film for forming the contact area at a dosage to a degree that the resist film is not exposed completely by using an interference exposure process to form a periodic striped pattern which is projecting and depressed in cross section in the contact area forming region of the resist film; and    performing etching with respect to the second nitride semiconductor layer by using the resist film formed with the periodic pattern as a mask to transfer the periodic pattern onto the contact area of the second nitride semiconductor layer.    
   
   
       9 . The method of  claim 7 , wherein the step of forming the contact area includes the steps of: 
 forming a resist film on the second nitride semiconductor layer and exposing the resist film to an electron beam to cause a proximity effect on a region of the resist film for forming the contact area by using an electron beam exposure process to form a periodic striped pattern which is projecting and depressed in cross section in the contact area forming region of the resist film; and    performing etching with respect to the second nitride semiconductor layer by using the resist film formed with the periodic pattern by a mask to transfer the periodic pattern onto the contact area of the second nitride semiconductor layer.    
   
   
       10 . The method of  claim 7 , wherein the step of forming the contact area includes the step of irradiating a region of the second nitride semiconductor layer to be formed into the contact area with an ion beam to form a periodic striped pattern which is projecting and depressed in cross section in the contact area of the second nitride semiconductor layer.  
   
   
       11 . A semiconductor device comprising: 
 a first nitride semiconductor layer;    a second nitride semiconductor layer formed on the first nitride semiconductor layer, including a contact area exposing a part of the first nitride semiconductor layer, and having such a composition as to generate a 2-dimensional electron gas layer in an upper portion of the first nitride semiconductor layer; and    an electrode having an ohmic property and formed on the contact area, wherein    the contact area is a stepped portion composed of an exposed portion formed by partly lowering an upper surface of the first nitride semiconductor layer to a level reaching the 2-dimensional electron gas layer and an end portion facing the exposed portion of the second nitride semiconductor layer and    the electrode is formed to cover the stepped portion.    
   
   
       12 . The semiconductor device of  claim 11 , wherein an impurity having a conductivity has been introduced into a region of the stepped portion located in the vicinity of an interface between the first and second nitride semiconductor layers.  
   
   
       13 . The semiconductor device of  claim 12 , wherein the impurity is silicon having an n-type conductivity.  
   
   
       14 . The semiconductor device of  claim 12 , wherein the impurity is magnesium having a p-type conductivity.  
   
   
       15 . The semiconductor device of  claim 11 , wherein the electrode is composed of one layer made of a metal selected from the group consisting of titanium, strontium, aluminum, niobium, vanadium, zirconium, hafnium, chromium, tungsten, molybdenum, rhodium, rhenium, cobalt, and lanthanum, of at least two layers each made of a metal selected from the group, of an alloy layer containing at least two metals selected from the group, or of a conductive compound containing at least one metal selected from the group and oxygen, nitrogen, or boron.  
   
   
       16 . The semiconductor device of  claim 11 , wherein 
 a general formula of a material composing the first nitride semiconductor layer is Al x In y Ga 1-x-y N (where x and y satisfy 0≦x≦1, 0≦y≦1, and 0≦x+y≦1),    a general formula of a material composing the second nitride semiconductor layer is Al u In y Ga 1-u-v N (where u and v satisfy 0≦u≦1, 0≦v≦1, and 0≦u+v≦1), and    a composition of the second nitride semiconductor layer and a composition of the first nitride semiconductor layer are such that u representing an Al ratio is larger than x representing an Al ratio and v representing an In ratio is smaller than y representing an In ratio.    
   
   
       17 . A semiconductor device comprising: 
 a first nitride semiconductor layer;    a second nitride semiconductor layer formed on the first nitride semiconductor layer, having a contact area forming region defining a depressed cross-sectional configuration, and having such a composition as to generate a 2-dimensional electron gas layer in an upper portion of the first nitride semiconductor layer; and    an electrode having an ohmic property and formed selectively on the second nitride semiconductor layer, wherein    the second nitride semiconductor layer includes a contact area composed of a third nitride semiconductor layer containing a first impurity having a conductivity and buried in the contact area forming region and    the electrode is formed on the contact area.    
   
   
       18 . The semiconductor device of  claim 17 , wherein a second impurity having the same conductivity as the first impurity has been introduced into a bottom portion of the contact area forming region of the second nitride semiconductor layer.  
   
   
       19 . The semiconductor device of  claim 18 , wherein each of the first and second impurities is silicon.  
   
   
       20 . The semiconductor device of  claim 18 , wherein each of the first and second impurities is magnesium.  
   
   
       21 . The semiconductor device of  claim 17 , wherein the electrode is composed of one layer made of a metal selected from the group consisting of titanium, strontium, aluminum, niobium, vanadium, zirconium, hafnium, chromium, tungsten, molybdenum, rhodium, rhenium, cobalt, and lanthanum, of at least two layers each made of a metal selected from the group, of an alloy layer containing at least two metals selected from the group, or of a conductive compound containing at least one metal selected from the group and oxygen, nitrogen, or boron.  
   
   
       22 . The semiconductor device of  claim 17 , wherein 
 a general formula of a material composing the first nitride semiconductor layer is Al x In y Ga 1-x-y N (where x and y satisfy 0≦x≦1, 0≦y≦1, and 0≦x+y≦1),    a general formula of a material composing the second nitride semiconductor layer is Al u In v Ga 1-u-v N (where u and v satisfy 0≦u≦1, 0≦v≦1, and 0≦u+v≦1), and    a composition of the second nitride-semiconductor layer and a composition of the first nitride semiconductor layer are such that u representing an Al ratio is larger than x representing an Al ratio and v representing an In ratio is smaller than y representing an In ratio.    
   
   
       23 . A method for fabricating a semiconductor device, the method comprising the steps of: 
 forming, on a first nitride semiconductor layer, a second nitride semiconductor layer having such a composition as to form a 2-dimensional electron gas layer in an upper portion of the first nitride semiconductor layer;    selectively forming a contact area forming region defining a depressed cross-sectional configuration in an upper portion of the second nitride semiconductor layer;    forming and burying a third nitride semiconductor layer in the contact area forming region of the second nitride semiconductor layer, while introducing a first impurity having a conductivity therein, to form a contact area composed of the third nitride semiconductor layer in the contact area forming region; and    forming an electrode having an ohmic property on the contact area.    
   
   
       24 . The method of  claim 23 , further comprising, prior to forming the contact area, the step of: 
 introducing a second impurity having the same conductivity as the first impurity into a bottom portion of the contact area forming region of the second nitride semiconductor layer.    
   
   
       25 - 27 . (canceled)  
   
   
       28 . A method for fabricating a semiconductor device, the method comprising the steps of: 
 forming, on a first nitride semiconductor layer, a second nitride semiconductor layer having such a composition as to form a 2-dimensional electron gas layer in an upper portion of the first nitride semiconductor layer;    selectively forming a contact area defining a depressed cross-sectional configuration in an upper portion of the second nitride semiconductor layer;    forming an insulating film on the second nitride semiconductor layer to cover an entire surface thereof including the contact area;    performing an etch-back process with respect to the formed insulating film to form a sidewall composed of the insulating film on an inner wall surface of the contact area;    introducing an impurity having a conductivity into a bottom portion of the contact area of the second nitride semiconductor layer; and    forming an electrode having an ohmic property such that an inner side of the contact area including the sidewall is covered therewith.    
   
   
       29 - 32 . (canceled)  
   
   
       33 . A semiconductor device comprising: 
 a semiconductor layer having a carrier concentration that varies along a direction of depth; and    an electrode having an ohmic property and formed selectively on the semiconductor layer, wherein    the semiconductor layer has a contact area formed by partly removing an upper portion thereof and the electrode is formed in the contact area.    
   
   
       34 . The semiconductor device of  claim 33 , wherein the contact area has a lowermost portion thereof positioned at a distance of not less than 1 nm and not more than 50 nm from a peak position of the carrier concentration in the semiconductor layer located therebelow.  
   
   
       35 . The semiconductor device of  claim 33 , wherein the contact area has a lowermost portion thereof disposed at a position coincident with a peak position of the carrier concentration in the semiconductor layer.  
   
   
       36 . The semiconductor device of  claim 35 , wherein the electrode contains platinum or palladium.  
   
   
       37 . The semiconductor device of  claim 33 , wherein the semiconductor layer is made of a group III-V nitride semiconductor represented by a general formula Al x In y Ga 1-x-y N (where x and y satisfy 0≦x≦1, 0≦y≦1, and 0≦x+y≦1).  
   
   
       38 . The semiconductor device of  claim 37 , wherein the electrode contains platinum or palladium.