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.