For HFET application a series of samples with 30 nm AlxGa1-xN (x = 0.02-0.4) layers deposited at 1040 degreesC onto optimised 2 mum thick undoped GaN buffers were fabricated. The AlxGa1-xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the AlxGa1-xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz-10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A nondestructive, characterisation technique for verification of device heterostucture quality from the measured C - V and G - V versus frequency characteristics of the heterostructure is proposed.