Compressively Strained InxAl1-xN/Al0.22Ga0.78N/GaN (x = 0.245–0.325) Heterostructure Field Effect Transistors with Regrown AlGaN Contact Layers

Abstract

Compressively strained InxAl1-xN (15 nm)/Al0.22Ga0.78N (3 nm)/GaN heterostructure field effect transistors (FETs) with regrown AlGaN contact layers were fabricated. The increase in compressive strain in InAlN reduced the polarization charge, i.e., the density of two-dimensional electron gas decreased from 6.5 ×1012 to 1.3 ×1012 cm-2 as the In content of InAlN increased from 0.245 to 0.325. With the insertion of the AlGaN layer, electron mobility of as high as 1570 cm2 V-1 s-1 was achieved at the In content of 0.245. Selectively regrown AlGaN contact layers reduced the sheet resistance from 17,000 to 584 Ω/sq. at the access layer for In0.325Al0.675N/Al0.22Ga0.78N/GaN. We fabricated FETs with this structure. The FETs without the regrown AlGaN contact layer did not operate at all owing to the high resistance. In contrast, the devices with the regrown AlGaN did. The maximum transconductance is 60 mS/mm, and the drain current is 0.11 A/mm. The threshold voltage becomes shallower, changing from -3.2 to -0.2 V with the increase in In content from 0.245 to 0.325. These results indicate that it is possible to fabricate enhancement-mode FETs with InAlN/AlGaN/GaN heterostructures.