Non-linear junction detectors
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 rectify 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.