Field effect transistor having nitride semiconductor layer
A field effect transistor includes a nitride semiconductor layer; an In x Al y Ga 1-x-y N layer (wherein 0<x<1, 0<y<1 and 0<x+y<1) formed on the nitride semiconductor layer; and a source electrode and a drain electrode formed on and in contact with the In x Al y Ga 1-x-y N layer. The lower ends of the conduction bands of the nitride semiconductor layer and the In x Al y Ga 1-x-y N layer are substantially continuous on the interface therebetween.
1. A field effect transistor comprising:
a nitride semiconductor layer;
an In x Al y Ga 1-x-y N layer (wherein 0<x<1, 0<y<1 and 0<x+y<1) formed on said nitride semiconductor layer; and
a source electrode and a drain electrode formed on and in contact with said In x Al y Ga 1-x-y N layer,
wherein lower ends of conduction bands of said nitride semiconductor layer and said In x Al y Ga 1-x-y N layer are substantially continuous on an interface therebetween,
said nitride semiconductor layer includes Al, and
Al composition in said nitride semiconductor layer is changed to be larger as a distance from the interface between said nitride semiconductor layer and In x Al y Ga 1-x-y N layer is larger.
2. The field effect transistor of claim 1 ,
wherein polarization of said In x Al y Ga 1-x-y N layer is equal to or larger than polarization of said nitride semiconductor layer.
3. The field effect transistor of claim 1 ,
wherein a lattice constant of said nitride semiconductor layer is substantially equal to a lattice constant of said In x Al y Ga 1-x-y N layer on the interface therebetween.
4. The field effect transistor of claim 1 ,
wherein a band gap of said nitride semiconductor layer is substantially equal to a band gap of said In x Al y Ga 1-x-y N layer on the interface therebetween.
5. The field effect transistor of claim 1 ,
wherein the Al composition in said nitride semiconductor layer is linearly increased as the distance from the interface is larger.
6. The field effect transistor of claim 1 ,
wherein the Al composition in said nitride semiconductor layer is increased in a stepwise manner as the distance from the interface is larger.
7. The field effect transistor of claim 1 ,
wherein the Al composition in said nitride semiconductor layer is increased in a shape of a quadratic curve upward convex as the distance from the interface is larger.
8. The field effect transistor of claim 1 , further comprising, between said nitride semiconductor layer and said In x Al y Ga 1-x-y N layer, a multilayered film including at least one cycle of a GaN layer and an AlGaN layer alternately stacked.
9. The field effect transistor of claim 1 , further comprising, between said nitride semiconductor layer and said In x Al y Ga 1-x-y N layer, a multilayered film including at least one cycle of an InAlGaN layer and an AlGaN layer alternately stacked.
10. The field effect transistor of claim 1 ,
wherein said nitride semiconductor layer includes a portion doped with an impurity for providing a conductivity type of said nitride semiconductor layer in such a manner as to form a pulse-shaped profile along a direction vertical to the interface between said nitride semiconductor layer and said In x Al y Ga 1-x-y N layer.
11. The field effect transistor of claim 10 ,
wherein a peak concentration of said impurity in said portion of said nitride semiconductor layer is 1×10 19 atoms/cm 3 or more.
12. The field effect transistor of claim 1 , further comprising a gate electrode formed in a recess that is provided in a given portion of said In x Al y Ga 1-x-y N layer and reaches said nitride semiconductor layer.