A buffer layer of aluminum nitride (AlN) about 25 nm thick is provided on a
sapphire substrate. An n.sup.+ layer of a high carrier density, which is
about 4.0 .mu.m thick and which is made of GaN doped with silicon (Si), is
formed on the buffer layer. An intermediate layer of non-doped In.sub.x
Ga.sub.1-x N (0
