The PIN(Positive-Intrinsic-Negative)Diode: Unlike normal
diodes(PN-junction diode), the PN-junction is separated by
a resistive(intrinsic) layer(this is why it is called a PIN diode).
The PIN diode has a low resistance in the forward direction
and a low capasitance in the backward direction. It has
a relatively slow switching. This means that the diode
won't cause distortion to high frequency signals.
The resistance of the diode changes dependant on the current
that flows through it. This means that by varying the current
the diode can be used as a variable attenuator.