Abstract: An over-etched defect in a semiconductor wafer is detected by applying an electrical field to the contacts in a first area and comparing the intensity measured with the intensity from a reference area. In one embodiment, one of the contacts in each of the first and reference areas is a gate contact in an MOS device and a second contact is either a source or drain contact. The selected charging field forward biases the pn junctions between the source and drain regions and the well in which they are formed. As a result, defects caused by gate contacts shorted to one of the source and drain contacts are visible using voltage contrast imaging techniques.