Semiconductor switches are everywhere. Any product that uses an electrically controlled system uses one or more switches as part of the system design. As the markets for these types of products increase, so does the demand for more switches.
RonCoff is the figure of merit that is used to rate the performance of an RF switch.
A mechanical switch's "on/off" state is determined by physically changing the switch's position. RF Switches use an electrical input at the gate of the RF transistor to turn "on" or "off". A positive voltage turns the switch on and a negative voltage turns it off. In design schematics, the "on" state is represented as a resistor and the "off" state as a capacitor.
A low resistance allows more of the signal to travel from one port to another when the switch is on. This metric is called Ron. Ron is inversely proportional to the width of the transistor. The wider the transistor the lower the resistance, which allows more of the signal to get through.
A low capacitance will stop more of the signal from traveling from one port to another when the switch is off. This metric is called Coff. Coff is directly proportional to the width of the transistor. The wider the transistor the higher the capacitance, and the more the signal leaks through the switch when it's off.
RonCoff is the product of the Ron and Coff values of the transistor and is used to measure overall switch performance. It is important that both metrics are as low as possible, as both will affect the performance of the switch.