Bipolar RF transistors consist of an N-type or P-type layer sandwiched between two layers of the opposite type. They 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, bipolar RF transistors are made of materials such as silicon (Si) or germanium (Ge) and doped with impurities to induce changes in electrical properties. The junctions between the semiconductor sections cause a weak input to be amplified. Varying the current between the base and the emitter varies the current flow between the emitter and the collector. In normal operation, the emitter-base junction is forward-biased and the base-collector junction is reverse-biased. In the linear region, the collector-emitter current is approximately proportional to the base current, but many times larger. In the cut-off region, the base-emitter voltage is too small for any significant amount of current to flow.
Selecting bipolar RF transistors requires an analysis of performance specifications. Collector-to-emitter breakdown voltage is the maximum voltage than can be applied continuously in the reverse direction of the collector junction when the emitter is open. Similarly, collector-to-base breakdown voltage is the maximum voltage that can be applied continuously in the reverse direction of the collector junction when the base is open. 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 or milliwatts. Other performance specifications include maximum collector current, current gain bandwidth, operating frequency, and output power. Some bipolar 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).