LAYER STRUCTURE FOR A GROUP-Ill-NITRIDE NORMALLY-OFF TRANSISTOR

A group-III-nitride layer structure for a normally-off transistor, the layer structure comprising:

• an electron-supply layer made of at least one first group-III-nitride material having a first band-gap energy;
• a back-barrier layer made of at least one second group-III-nitride material having a second band-gap energy;
• and
• a channel layer arranged between the electron-supply layer and the back barrier layer and being made of a third group-III-nitride material having a third band-gap energy that is lower first and second band-gap energies,
• wherein the second group-III-nitride material of the back-barrier layer is of a p-type conductivity, while the first group-III-nitride material of the electron-supply layer and the third group-III-material of the channel layer are not of the p-type conductivity,
• wherein the first band-gap energy of the first group-III-nitride material of the electron-supply layer is smaller than the second band-gap energy of the second group-III-nitride material of the back-barrier layer,
• wherein, in absence of an external voltage applied to the layer structure, a lower conduction band-edge of the third group-III-nitride material in the channel layer is higher in energy than a Fermi level of the third group-III-nitride material in the channel layer,
• wherein the group-III-nitride layer structure further comprises a spacer layer between the channel layer and the electron-supply layer, the spacer layer having a thickness of not more than 3 nanometers and being made of AlN or AlGaN having a higher Al mole fraction than the electron-supply layer, and
• wherein the group-III-nitride layer structure further comprising exactly two recesses reaching vertically down to a channel region of the channel layer, each recess accommodating either the source contact or the drain contact.