The RF MOSFETs are a metal-oxide-semiconductor field effect transistor that is designed to operate at high frequencies, typically between 100MHz and well into the GHz range.  Often, to perform at these high frequencies, the transistor must remain in the linear region meaning that it is still operating similar to a resistor between drain and source contacts.  If the device reaches saturation, the operational speed is reduced.

Most usages for RF MOSFETs are in the power amplification mode.  In linear mode operation, the input signal does not typically have to be modified to a large extent.  This means that the input signal to be amplified adjusts the output current carrying capacity of the drain through the source of the MOSFETs proportionally to itself. 

If the RF MOSFETs is used as a switching device, then pulse-width modulation techniques must be used to translate the signal through the MOSFETs into the output.  Since an on-off switching mode of a transistor cannot directly transport an AC or sinusoidal signal, the data in the input signal must be translated into pulse width data, sent through the MOSFETs amplifier, and then translated back into a stronger AC signal.