AbstractWurtzite GaN samples containing one, three and five 4nm thick high temperature (HT) AlN Interlayers (IL) have been grown on (0001) sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). N-polar as well as Ga-polar thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and electrical measurements.All samples under consideration show excellent AFM rms surface roughness below 1nm. Previously, we published a reduction of the threading dislocation (TD) density by a factor of seven due to the introduction of one AlN-IL. When introducing multiple AlN-IL a reduction by a factor of 5.2 is achieved.Hall measurements show a rise in electron mobility due to possible 2DEG formation at the interface between GaN and the AlN-ILs. Significant growth mode differences between Ga-polar and N-polar samples result in drastically higher electron mobility values for N-polar material. For N-polar samples the exceptional mobility increase from 68 (no AlN-IL) to 707 cm2/Vs (one AlN-IL) as well as the extremely low intrinsic carrier density of 1 x 1017 cm-3 prove the applicability of AlN barriers in inverted FET devices.