Q. I was reading a magazine article the other day and it mentioned PIN diodes. What are PIN Diodes?
A. PIN diodes have a P-type semiconductor and an N-type semiconductor, as do ordinary diodes. But between the P and N material, there is a very thin layer of undoped, or intrinsic semiconductor. This is what the letter I stands for in "PIN". Undoped semiconductor is a good insulator. The I layer is put between the P and N layers to reduce the capacitance of the diode. The reduced capacitance makes the diode start to conduct and stop conducting faster than regular diodes.
PIN diodes are used in circuits that handle radio-frequency and high speed digital signals.