Electron holography has been used to image electrostatic potential variations across an AlGaN/ InGaN/AlGaN heterojunction diode. Features in the energy profile have been interpreted in terms of the expected built-in voltage across the p-n junction of the diode as well as contributions from the spontaneous and piezoelectric polarization fields within the material. Additional profile features indicate the presence of two-dimensional electron gas at the upper interface of the InGaN well, and evidence for hole accumulation close to the lower interface. © 2000 American Institute of Physics.