Patent ID: 8853666

Claim:
A multilayered epitaxial film epitaxial grown on a substrate, which is usable in fabrication of a field-effect transistor, characterized in that the multilayered epitaxial film is formed out of compound semiconductors which exhibit spontaneous polarization and piezo polarization effects, or semiconductor alloys thereof, and comprises layered structure in which a heterojunction that is made up of an electron supply layer and a channel layer is formed on a buffer layer; the multilayered epitaxial film optionally comprises a barrier layer between the channel layer and the buffer layer; said buffer layer comprises a region made up of a semiconductor material whose composition monotonically changes along a direction from a substrate surface to the channel layer, wherein the composition of the semiconductor material is selected so that energy of a conduction band edge of the semiconductor material in the region reduces monotonically; the composition of the semiconductor material in the region is selected so that the energy of the conduction band edge of the semiconductor material in the region is higher in comparison with the energy of the conduction band edge of the semiconductor material which makes up of the channel layer at an interface between the region and the channel layer or between the barrier layer and the channel layer; and the energy of the conduction band edge in the region exhibits a shape convex toward a side where electronic energy is higher, along a direction from the substrate surface to the channel layer, wherein the substrate has high resistance, the substrate is selected from the group consisting of C plane of 6H—SiC substrate, C plane of 4H—SiC substrate and a (111) plane of 3C—SiC substrate, the compound semiconductors which exhibit spontaneous polarization and piezo polarization effects, or semiconductor alloys thereof are group III nitride-type compound semiconductors or semiconductor alloys thereof that are grown in the C axis on the substrate, the multilayered epitaxial film further comprises an AlN nucleation layer between the buffer layer and the substrate, the AlN nucleation layer is an insulating AlN layer, the AlN nucleation layer is grown in the C axis on the substrate, the channel layer, the buffer layer and the barrier layer are un-doped, and discontinuity ΔE C of conduction band energy at the interface between the channel layer and the buffer layer or between the channel layer and the barrier layer is selected at least ΔE c >2 kT, where k denotes a Boltzmann's constant and T denotes temperature of about 300K.