High electron mobility transistors

A transistor, comprising:

• a channel layer of GaN disposed over a substrate;
• a donor bi-layer of AlGaN contacting the channel layer of GaN, and comprising:
• a mobility-enhancing layer of AlzGa(1-z)N disposed over the channel layer of GaN, wherein a third molar fraction z is less than approximately 0.4 and greater than approximately 0.25;
• and
• a resistance-reducing layer of AlxGa(1-x)N disposed over the mobility-enhancing layer, wherein a first molar fraction x is less than approximately 0.15 and greater than approximately 0.1;
• a source ohmic contact, which contacts the resistance-reducing layer;
• a drain ohmic contact, which contacts the resistance-reducing layer;
• a gate structure, which contacts the resistance-reducing layer over a channel region of the donor bi-layer;
• and
• wherein the donor bi-layer has a conduction band energy that monotonically decreases from the source and drain ohmic contacts to the channel layer of GaN.

A method to form a high electron mobility transistor (HEMT), comprising:

• forming a III-V compound channel layer over a substrate;
• forming a donor bi-layer contacting the III-V compound channel layer, wherein forming the donor bi-layer comprises:
• forming a mobility-enhancing layer of AlzGa(1-z)N on and abutting the III-V compound channel layer, wherein a third molar fraction z is less than approximately 0.4 and greater than approximately 0.25;
• forming a resistance-reducing layer of AlxGa(1-x)N on and abutting the mobility-enhancing layer, wherein a first molar fraction x is less than approximately 0.15 and greater than approximately 0.1;
• forming a source ohmic metal contact, which contacts the resistance-reducing layer;
• forming a drain ohmic metal contact, which contacts the resistance-reducing layer;
• forming a gate material, which contacts the resistance-reducing layer over a channel region of the HEMT;
• and
• wherein the donor bi-layer has a conduction band enemy that monotonically decreases from the source and drain ohmic contacts to the III-V compound channel layer.