Field effect transistor
A field effect transistor includes a nitride-based semiconductor multi-layer structure, a source electrode ( 108 ), a drain electrode ( 109 ), a protective film ( 110 ), and a gate electrode ( 112 ) that is provided in a recess structure, which is formed by etching, directly or with a gate insulating film interposed therebetween. The nitride-based semiconductor multi-layer structure includes at least a base layer ( 103 ) made of Al X Ga 1-X N (0≦1), a channel layer ( 104 ) made of GaN or InGaN, a first electron supply layer ( 105 ), which is an undoped or n-type Al Y Ga 1-Y N layer, a threshold value control layer ( 106 ), which is an undoped Al Z Ga 1-Z N layer, and a second electron supply layer ( 107 ), which is an undoped or n-type Al W Ga 1-W N layer, epitaxially grown in this order on a substrate ( 101 ) with a buffer layer ( 102 ) interposed therebetween. The Al composition of each layer in the nitride-based semiconductor multi-layer structure satisfies 0<X≦Y≦1 and 0<Z≦Y≦1.
1. A field effect transistor comprising:
a nitride-based semiconductor multi-layer structure;
a source electrode;
a drain electrode;
a protective film; and
a gate electrode,
wherein the nitride-based semiconductor multi-layer structure includes at least a base layer made of Al X Ga 1-X N (0≦x≦1), a channel layer made of GaN or InGaN, a first electron supply layer, which is an undoped or n-type Al y Ga 1-y N layer, a threshold value control layer, which is an undoped Al Z Ga 1-Z N layer, and a second electron supply layer, which is an undoped or n-type Al w Ga 1-w N layer, epitaxially grown in this order over a substrate with a buffer layer interposed therebetween,
the source electrode, the drain electrode, and the protective film are provided over a surface of the nitride-based semiconductor multi-layer structure, the gate electrode is provided in a recess structure, which is formed by etching the entire protective film, the entire second electron supply layer, and a portion of the threshold value control layer in a portion of a space between the source electrode and the drain electrode, directly or with a gate insulating film interposed therebetween, and the Al composition of each layer in the nitride-based semiconductor multi-layer structure satisfies 0≦X≦Y<1 and 0<Z≦Y≦1.
2. The field effect transistor according to claim 1 , wherein the Al composition of each layer in the nitride-based semiconductor multi-layer structure further satisfies Y−X<0.1.
3. The field effect transistor according to claim 1 , wherein the Al composition of each layer in the nitride-based semiconductor multi-layer structure further satisfies Y−Z<0.1.
4. The field effect transistor according to claim 1 , wherein the Al composition of each layer in the nitride-based semiconductor multi-layer structure further satisfies Z−X<0.1.
5. The field effect transistor according to claim 1 , wherein the first electron supply layer has a thickness equal to or more than 2 nm and equal to or less than 15 nm, and the threshold value control layer has a thickness equal to or more than 2 nm and equal to or less than 15 nm.
6. The field effect transistor according to claim 1 , wherein a difference between the Al composition Z of the threshold value control layer and the Al composition X of the base layer satisfies Z−X<0.03, and a difference between the Al composition Y of the first electron supply layer and the Al composition Z of the threshold value control layer satisfies Y−Z<0.03.
7. The field effect transistor according to claim 1 , wherein the Al composition Z of the threshold value control layer is equal to the Al composition X of the base layer.
8. The field effect transistor according to claim 1 , wherein the Al composition Y of the first electron supply layer is equal to the Al composition Z of the threshold value control layer.
9. The field effect transistor according to claim 1 , wherein the Al composition Y of the first electron supply layer and the Al composition W of the second electron supply layer satisfy Y≦W≦1.
10. The field effect transistor according to claim 1 , wherein the nitride-based semiconductor multi-layer structure further includes a cap layer that is made of a nitride-based semiconductor and is provided over the second electron supply layer.