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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 asymmetric response of the junction to an electric field, it will mangle it, re-emitting some of it on multiples of the illumination frequency. The detector has a sensitive receiver tuned to these harmonics, as well as appropriate processing and displays to make their presence known to the user of the device.Because the basis of almost all semiconductor electronics is the p-n junction, an NLJD is correspondingly capable of detecting almost any unshielded electronic device containing semiconductors, whether the electronics are actively powered or not.
In its basic form, an NLJD can also detect things that are not themselves electronic in nature, so the use of the device requires a modicum of skill and experience. For example, a rusty nail inside a wall can give a false positive. For this reason, most modern NLJDs examine the ratio between the second and the third harmonic of the illumination frequency. When a true p-n junction is encountered, the second harmonic will generally be stronger than the third.