Compound semiconductor device and method of manufacturing the same
A compound semiconductor device is provided with a substrate, an AlN layer formed over the substrate, an AlGaN layer formed over the AlN layer and larger in electron affinity than the AlN layer, another AlGaN layer formed over the AlGaN layer and smaller in electron affinity than the AlGaN layer. Furthermore, there are provided an i-GaN layer formed over the latter AlGaN layer, and an i-AlGaN layer and an n-AlGaN layer formed over the i-GaN layer.
1. A method of manufacturing a compound semiconductor device, comprising:
forming a first buffer layer of a first nitride semiconductor over a substrate;
forming over the first buffer layer a second buffer layer of a second nitride semiconductor larger in electron affinity than the first buffer layer;
forming over the second buffer layer a third buffer layer of a third nitride semiconductor smaller in electron affinity than the second buffer layer;
forming a channel layer over the third buffer layer; and
forming an electron supply layer over the channel layer.
2. The method of manufacturing the compound semiconductor device according to claim 1 , further comprising, between the forming the third buffer layer and the forming the channel layer, forming a fourth buffer layer larger in electron affinity than the third buffer layer.
3. The method of manufacturing the compound semiconductor device according to claim 1 , wherein, as the first buffer layer, the second buffer layer, and the third buffer layer, layers containing at least aluminum are formed.
4. The method of manufacturing the compound semiconductor device according to claim 1 , wherein
as the second buffer layer, a layer having a composition represented by Al x1 Ga 1-x1 N (0.1≦x 1 ≦10.5) is formed;
as the third buffer layer, a layer having a composition represented by Al x2 Ga 1-x2 N (0.3≦x 2 ≦1) is formed; and
a value of x 1 is smaller than a value of x 2 .
5. The method of manufacturing the compound semiconductor device according to claim 1 , wherein
as the second buffer layer, a layer having a composition represented by In y1 Al 1-y1 N (0.1≦y 1 ≦0.25) is formed;
as the third buffer layer, a layer having a composition represented by In y2 Al 1-y2 N (0.1≦y 2 ≦0.25) is formed; and
a value of y 1 is smaller than a value of y 2 .
6. The method of manufacturing the compound semiconductor device according to claim 5 , wherein
as the second buffer layer, a layer having a lattice constant larger than a lattice constant of gallium nitride is formed; and
as the third buffer layer, a layer having a lattice constant smaller than the lattice constant of gallium nitride is formed.
7. A method of manufacturing a compound semiconductor device, comprising:
forming a first buffer layer of a first nitride semiconductor containing aluminum over a substrate;
forming over the first buffer layer a second buffer layer of a second nitride semiconductor containing a lower concentration of aluminum than that in the first buffer layer;
forming over the second buffer layer a third buffer layer of a third nitride semiconductor containing a higher concentration of aluminum than that in the second buffer layer;
forming a channel layer over the third buffer layer; and
forming an electron supply layer over the channel layer.