Semiconductor device with superlattice-structured graded buffer layer and fabrication method thereof

A fabrication method of a semiconductor device comprising the steps of:

• forming a semiconductor compositionally-graded buffer layer on a semiconductor substrate;
• and
• forming an active layer on said compositionally-graded buffer layer,
• said step of forming said compositionally-graded buffer layer comprising the step of:
• alternately stacking first semiconductor sublayers and second semiconductor sublayers on said substrate in a direction perpendicular to said substrate, thereby forming a semiconductor superlattice structure,
• wherein each of said first sublayers is made of a first semiconductor material, and each of said second sublayers is made of a second semiconductor material different in composition from said first semiconductor material,
• and wherein each of said first and second sublayers is formed so that a lattice constant of each of said first and second sublayers varies stepwise from a side nearest said to said active layer.

A semiconductor transistor comprising:

• a semiconductor substrate made of a semiconductor material having a first mole fraction;
• an active channel region made of a semiconductor material having a second mole fraction;
• and
• a graded buffer region between said substrate and said active channel region, said graded buffer region comprising:
• a first plurality of semiconductor layers made of a first semiconductor compound material;
• and
• a second plurality of semiconductor layers made of a second semiconductor compound material, different from said first semiconductor compound material,
• said first plurality of semiconductor layers being interleaved with said second plurality of said semiconductor layers to form an alternating stack between said substrate and said channel region, and
• wherein a material composition of each layer in said stack has a mole fraction graded from that of said first mole fraction in said substrate to that of said second mole fraction in said active channel region.