AlN/GaN/AlN double heterostructures with thin AlN top barriers

AlN/GaN/AlN double heterostructures grown by PAMBE on sapphire (0001) substrates with different AlN top barrier thicknesses, ranging between 1.5 and 4 nm, were investigated. The two-dimensional electron gas (2DEG) density increased with AlN barrier thickness in agreement with the theoretical values. The highest value of 2.2×1013 cm-2 for the HEMT structure with 4.5 nm AlN barrier thickness and the highest mobility of 900 cm2/Vs for the structure with 3 nm AlN barrier thickness were obtained. HEMT devices, processed with a gate length of 1μm, exhibited a maximum drain current of 1.1 A/mm for 3 and 3.7 nm AlN barrier thickness. The transistor threshold voltage scaled linearly from +0.2V to-2.7V by varying the AlN barrier thickness from 1.5 nm to 4.5 nm respectively showing a normally-off potential use.