Transistor devices and methods of forming transistor devices

A method of forming a transistor device, the method comprising:

• providing a substrate;
• forming a buffer layer over the substrate;
• forming a source terminal, a drain terminal, and a barrier layer directly on the buffer layer, wherein the barrier layer comprises a recess;
• wherein the barrier layer is disposed laterally between the source terminal and the dram terminal;
• forming a passivation layer over the barrier layer, wherein the passivation layer extends into the recess of the barrier layer overlapping vertically and horizontally with the barrier layer;
• forming a gate terminal over the buffer layer, wherein the gate terminal is arranged laterally between the source terminal and the drain terminal, wherein a part of the gate terminal is arranged over the passivation layer, and wherein the recess of the barrier layer is arranged laterally between the gate terminal and the drain terminal, and
• forming a gate dielectric layer arranged over the passivation layer, the gate dielectric layer overlapping vertically and horizontally with the passivation layer, wherein the gate dielectric layer and the passivation layer are made of different materials.