Semiconductor substrate and device with a surface layer structure

A semiconductor substrate consisting essentially of a III-V compound semiconductor containing Ga and As, and having a surface layer structure provided on a surface of the semiconductor substrate, the semiconductor substrate having a first valence band maximum, and a first conduction band minimum,

• wherein said surface layer structure comprises a strained layer epitaxially grown on the surface of said semiconductor substrate and consisting essentially of at least one first element selected from the group consisting of indium, gallium, aluminum and boron, and at least one second element selected from the group consisting of arsenic and phosphorus, said strained layer having an uppermost surface, and an innermost surface which contacts said surface of the semiconductor substrate, and said strained layer having a composition different from that of said semiconductor substrate;
• said strained layer having a second valence band maximum and a second conduction band minimum, said second valence band maximum being lower in energy level than said first valence band maximum of said semiconductor substrate;
• and
• said strained layer having a thickness small enough to render surface states appearing on said uppermost surface of said strained layer lower in energy level than said first valence band maximum of the surface of said semiconductor substrate when an electric field is applied.

A semiconductor device having a semiconductor substrate and a functional portion provided on the semiconductor substrate and having a p-n junction, the semiconductor substrate consisting essentially of a III-V compound semiconductor containing Ga and As, and having a surface layer structure provided on the semiconductor substrate, the semiconductor substrate having a first valence band maximum, and a first conduction band minimum,

• wherein said surface layer structure comprises a strained layer epitaxially grown on the surface of said semiconductor substrate and consisting essentially of at least one first element selected from the group consisting of indium, gallium, aluminum and boron, and at least one second element selected from the group consisting of arsenic and phosphorus, said strained layer having an uppermost surface, and an innermost surface which contacts said surface of the semiconductor substrate, and said strained layer having a composition different from that of said semiconductor substrate;
• said strained layer having a second valence band maximum and a second conduction band minimum, said second valence band maximum being lower in energy level than that of said first valence band maximum of said semiconductor substrate;
• and
• said strained layer having a thickness small enough to render surface states appearing on said uppermost surface of the strained layer lower in energy level than said first valence band maximum of the surface of the semiconductor substrate when an electric field is applied.