The non-linear junction detector, or an NLJD, is a device that illuminates a small region of space with high-frequency, RF energy. Any "non linear junction" in the vicinity—for example, and particularly, the p-n junction -- will receive this energy, and because of the non-linear nature of the junction, it will rectify it, re-emitting some of it on multiples of the illumination frequency (see harmonic). The detector has a sensitive receiver tuned for these harmonics, as well as appropriate processing and displays to make their presence known to the user of the device.
Since the basis of almost all semiconductor electronics is the p-n junction, an NLJD is correspondingly capable of detecting almost any un-shielded electronics, whether the electronics are on or off. It can also detect things that are not themselves electronic in nature, so the use of the device requires a modicum of skill and experience.