Modulation-doped superlattice AlGaN barrier GaN/AlGaN HFETs

For nitride heterojunction field-effect transistors, the heterojunction forming the electron barrier has to be designed based on trade-offs between lattice mismatch, two-dimensional electron gas concentration, and alloy scattering. In this work, we study the use of a superlattice-based barrier which effectively increases the heterojunction barrier while reducing the lattice mismatch and alloy scattering. The structure studied consists of a five-period superlattice of 2 nm Al_0.2Ga_0.8N + 2 nm Al _0.3Ga_0.7N on GaN grown using metalorganic chemical vapor deposition on a 6 H SiC conducting substrate. The barriers of the second and third period of the superlattice are Si-doped to give the effect of modulation doping. Devices fabricated from the structure exhibit a high g_m of ∼299 mS/mm and an I_{ds max} of 0.85 A/mm. To our knowledge, this is the first investigation of the use of superlattice structure for improvement of the barrier properties in a nitride HFET.