RF Transistors

RF transistors are designed to handle high-power radio frequency (RF) signals in devices such as stereo amplifiers, radio transmitters, and television monitors. Like other semiconductor devices, they are made of materials such as silicon (Si) or germanium (Ge) and doped with impurities to induce changes in electrical properties. There are several basic types of RF transistors. Bipolar RF transistors consist of an N-type or P-type layer sandwiched between two layers of the opposite type. Both NPN and PNP configurations are available. MOSFET RF transistors are metal-oxide field effect transistors (MOSFETs) with a channel made of either an N-type or P-type material. Heterojunction field effect transistors (HFETs) require a negative power supply and are used mainly for driver or power amplification applications. Pseudomorphic high electron mobility transistors (PHEMTs) are used mainly in wireless devices and satellite communication systems. 

Selecting RF transistors requires an analysis of performance specifications. 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). Other performance specifications for RF transistors include output power, operating frequency, and operating temperature. Some RF transistors support a temperature range and feature mechanical and electrical specifications that are suitable for commercial or industrial applications. Other devices meet screening levels for military specifications (MIL-SPEC).

Basic integrated circuit (IC) package types for RF transistors are transistor outline (TO), small outline (SO), and small outline transistor (SOT). For each package type, many variants are available. Transistor outline packages include TO-92, a single in-line package often used for low power devices; TO-220, which is suitable for high power, medium current, and fast-switching power devices; and TO-263, the surface-mount version of the TO-220 package. Small outline transistor packages include SOT23, which is often used in home appliances, office and industrial equipment, personal computers, printers, and communication equipment; SOT89, a plastic, surface mounted package with three leads and a collector pad for good heat transfer; and SOT223, an encapsulated package that provides excellent performance in environments with high temperatures and humidity levels. IC package types that use flat packaging (FPAK) are also available.  

Packing methods for RF transistors consist of tape reel, rail, bulk pack, and tube technologies. The tape reel method packs components in a tape system by reeling specified lengths or quantities for shipping, handling, and configuration in industry-standard automated board-assembly equipment. Rail, another standard packing method, is typically used only in production environments. Bulk pack devices are distributed as individual parts, while tray components are shipped in trays. Typically, the tube or stick magazine method is used to feed bipolar RF transistors into automatic placement machines for through-hole or surface mounting.

• AN721: Impedance Matching Networks Applied to RF Power Transistors, Courtesy of Motorola
  Some graphic and numerical methods of impedance matching will be reviewed here. The examples given will refer to high frequency power amplifiers. Although matching networks normally take the form of...
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• AN215A: RF Small Signal Design Using Two Port Controllers, Courtesy of Motorla
  Design of the solid-state, small-signal RF amplifier using two-port parameters is a systematic, mathematical procedure, with an exact solution (free from approximation) available for the complete...
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• Silicon RF Power MOSFETS
  Silicon RF Power MOSFETS Providing general guidelines for understanding the design and operation of the transistors, this book describes the physics, design considerations and RF performance of...
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Part Numbers for RF Transistors