The electronic band structure of a modulation-doped p-type AlGaN/AlN/GaN heterostructure, where the AlGaN layer is compositionally graded, has been studied by electron holography. The electrostatic potential energy profile and the two-dimensional hole gas (2DHG) distribution have been measured with high spatial resolution across the heterostructure. A positive curvature in the potential profile has been observed, and it is considered as evidence for the accumulation of holes in a 2DHG at the AlN/GaN interface. It is also observed that the potential barrier for the 2DHG is greatly affected by acceptor ionization in the p-AlGaN layer. This knowledge of nature of the energy barriers for hole transfer between adjacent channels is important in the optimization of vertical conductivity of p-type AlGaN/GaN heterostructures, which can be used as current spreading layers in GaN-based light emitting diodes or laser diodes. © 2009 The Japan Society of Applied Physics.