A PIN Diode has favorable characteristics, in that it has a quite linear resistor at high frequencies when forward biased. The resistor can be controlled over a wide range by varying IF. When the current is constantly varied, it can be used as a current controlled resistance or amplitude modulator. On the other hand, when the current through the diode has been switched on and off, it can be used as a Switch, a pulse modulator or as a phase-shifter.