Answer 6—A PIN diode gets its name from its three-layer structure, in which an undoped, or Intrinsic, layer is inserted between the usual P-type and N-type silicon layers. The thickness of this layer adds directly to the depletion region created when the diode is reverse-biased, giving it an extremely low capacitance value in that mode. This makes it a good choice for switching RF signals, as in a transmit-receive (TR) switch, or steering different antennas to a receiver for automatic direction finding (ADF).

PIN diodes are also used as high-speed photo-detectors because the intrinsic layer reduces both the leakage current and the shunt capacitance.

For additional information, refer to Ken Davidson’s “ConnecTime: Excerpts from the Circuit Cellar BBS” (Circuit Cellar 62).

Contributor: David Tweed