The PIN photodiode was invented by Jun-ichi Nishizawa and his colleagues in 1950. It is a semiconductor device that operates as a variable resistor at RF and microwave frequencies and consists of an intrinsic (lightly doped) region that is sandwiched between a p-type and an n-type layer. PIN diode finds its application in RF switches, attenuators, photodetectors, and phase shifters. The RC time constant determines the frequency response of the PIN device. PIN photodiodes may have a broad spectral range from the near infrared to ultraviolet and even to high-energy regions. They are also known for their high-speed response, high sensitivity, and low noise.