MOSFET RF transistors are metal-oxide field effect transistors (MOSFETs) that are designed to handle high-power radio frequency (RF) signals from devices such as stereo amplifiers, radio transmitters, and television monitors. They are turned on and off by input voltages and function as miniature electronic switches. Like other semiconductor devices, MOSFET RF transistors are made of materials such as silicon (Si) or germanium (Ge) and doped with impurities to induce changes in electrical properties. Voltage is applied between the gate and source terminals, modulating the current between the source and drain. Typically, a thin layer of oxide insulation is used to prevent current from flowing between the gate and a conductive channel in the semiconductor substrate. There are two basic types of MOSFET RF transistors: N-channel and P-channel. N-channel devices conduct through electrons. P-channel devices conduct through “holes”. With both types of devices, the polarity of the electric field that controls the current in the channel is determined by the majority of carriers in the channel. 

Selecting RF MOSFET Transistors

Selecting MOSFET RF transistors requires an analysis of performance specifications. Drain-source breakdown voltage is the maximum drain-to-source voltage before breakdown with the gate grounded. Power gain, a measure of power amplification, is the ratio of output power to input power. Noise figure, a measure of the amount of noise added during normal operation, is the ratio of the signal-to-noise ratio at the input and the signal-to-noise ratio at the output. Both power gain and noise figure are expressed in decibels (dB). Power dissipation, a measure of total power consumption, is expressed in watts (W) or milliwatts (mW). Other performance specifications for MOSFET RF transistors include maximum drain saturation, common-source forward transconductance, operating frequency, and output power. Some bipolar MOSFET RF transistors are suitable for automotive, commercial, or general industrial applications. Others meet U.S. military specifications (MIL-SPEC).

MOSFET RF transistors vary in terms of operating mode, packaging, and packing methods. Devices that operate in depletion mode can increase or decrease their channels by an appropriate gate voltage. By contrast, devices that operate in enhancement mode can only increase their channels by an appropriate gate voltage. In terms of packaging, MOSFET RF transistors are available in small outline (SO), transistor outline (TO), small outline transistor (SOT), and flat packaging (FPAK). Devices use either surface mount technology (SMT) or through hole technology (SMT) and vary in terms of the number of leads. Packing methods for MOSFET RF transistors include tape reels, rails, bulk packs, tubes, and trays.