A Si/Si p–n junction with very low doping level was made via a standard device fabrication process by implanting As ions at 25 keV into a p-type Si substrate with a boron concentration of 1015 cm−3, followed by heat annealing at 1035 °C for 33 s. To characterize this junction, a pair of 45° wedge-shape cross sections was prepared simultaneously by focused-ion-beam milling and examined using off-axis electron holography. The reconstructed phase images clearly show the phase shift induced by the electrostatic potential drop across the p–n junction, indicating that the junction has been mapped successfully. Quantitative measurements from the phase images give the potential values of 12.21±0.40 and 11.50±0.27 V, respectively, for the n- and p-type sides of the junction, 0.71±0.05 V for the potential drop across the junction and 50.10±3.88 nm for the total electric dead layer thickness. This work demonstrates that electron holography is a powerful technique for characterizing low dopant level p–n junctions in practical devices. © 2002 American Institute of Physics.