What characteristic of a PIN diode makes it useful as an RF switch?

A PIN diode is constructed with a wide, undoped 'intrinsic' semiconductor layer sandwiched between P-type and N-type regions. This wide intrinsic layer significantly increases the physical distance between the P and N regions, which directly results in a very low junction capacitance. In RF engineering, a solid-state switch must be able to completely block a signal when turned off. Because capacitors naturally pass high-frequency alternating currents, a standard diode with high junction capacitance would leak RF energy even when reverse-biased (switched off). The remarkably low junction capacitance of a PIN diode prevents this leakage. It allows the diode to effectively block high-frequency signals when reverse-biased, making it an excellent and highly reliable RF switch.