PIN Diode
Sorry, it may seems to be a stupid question :)
A forward biased pin diode behaves like a variable resistor at microwave frequencies. They are used for limiters, attenuators and switches.
They are both identical in structure. Both have a big "I" (intrinsic) region of lightly doped material sandwiched between to heavily doped regions.
The microwave PIN diode is used with a forward AND reverse DC current applied, to switch between an open and closed "switch" state. When reverse biased, the I region has no charge carriers, and looks like a very small RF Capacitance. The RF voltage is not big enough to cause charge carriers to be formed in the I region. RF PIN diodes are usually silicon.
The optical PIN dioide is always reversed biased. It is packaged so that light can shine onto the I region. When a photon of light impinges on the I region, an electron is generated and is accelerated toward postively biased side. If enough photons hit the I region, then an appreciable current flows across the I region. Optical PIN diodes are made of various compounds, usually containing GaAs. The type of compound has to be matched to the wavelength of light being detected.
Microwave InGaAs PIN Diodes for Millimeter-Wave Applications
http://www.eecs.umich.edu/dp-group/PIN/
Material for I region and parameters of diode are different construction is the same.
http://www.pedasoft.com
Here is a link of how to design a switch using a tool called EM-Supreme:
http://www.pedasoft.com/PedaSoftEMS.pdf