Formation of ohmic contacts to ultra-thin channel AlN/GaN HEMTs

p s s current topics in solid state physics c status solidi 1 Introduction AlGaN/GaN high electron mobility transistor (HEMT) technology is rapidly advancing into the 100-200 GHz operation regime with the scaling of gate length, and the reduction of parasitic elements. Scaling of vertical het-erostructure dimensions is needed to support further improvements in HEMT performance. Therefore, ultra-thin barrier layers under the gate have been of increasing interest , formed either by gate recess or by direct epitaxy [1, 2]. The large bandgap of AlN (6.2 eV) allows efficient carrier confinement and lowers gate leakage current, and the absence of alloy disorder (compared to AlGaN barriers) results in improvement of both low-and high-field carrier transport. A handful of variations on AlN/GaN HEMTs have been reported with notable success [2-5]. However, in the early years, AlN was exploited as an insulating layer for a doped GaN channel, i.e. metal-insulator-semiconductor field effect transistors. Therefore, the epitaxial quality of the AlN layer deposited was largely unknown; neither was reported the existence and transport properties of the two-dimensional electron gas (2DEG), which can be produced at the AlN/GaN heterojunction due to the large polarization effect similar to that in the AlGaN/GaN heterostruc-tures. Recently Higashiwaki et al. [2] reported a f t of 107