The ability of diodes to conduct with the forward bias, and to insulate with the reverse bias, makes them very useful for switching in some of the electronic applications. Diodes can switch at very high rates, much faster than any mechanical device.
One type of diode, which is made for use as an RF switch, has a special type of semiconductor layer sandwiched in between P-type and N-type material. This layer, is called as intrinsic semiconductor, reduces capacitance of diode, such that it can work at the higher frequencies than an ordinary diode. The intrinsic material is called as I type. A diode having I-type semiconductor is called as PIN diode.
Figure- The PIN diode has a layer of intrinsic (I- type) semiconductor at P-N junction.
Direct current bias, which is applied to one or more PIN diodes, permits RF currents to be effectively channeled without using complicated relays and cables. A PIN diode makes a good RF detector, especially at the frequencies over 30 MHz.