The electrostatic potential profiles and charge distributions in modulation-doped n-type and p-type AlGaN/AlN/GaN hetero-structures have been measured by electron holography with high spatial resolution. For n-type two-dimensional electron gas structure a negative curvature and for p-type two-dimensional hole gas structure a positive curvature in the potential profile at the AlN/GaN interface were observed, which demonstrated the accumulation of two-dimensional carriers. The measured electrostatic potential profiles were also compared with the calculated band diagram in the heterostructures. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.