Transistors at High Frequencies

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The gallium arsenide field-effect transistor (GaAsFET) and the bipolar-junction transistor (BJT) are the two most commonly used devices in the design of amplifiers, oscillators and mixers at high frequencies. BJTs used in UHF and microwaves are usually of planar npn silicon type. The advantages of silicon planar BJTs over other types of transistors at high frequencies are that they represent mature technology both in the understanding of the physics and the device design, low cost and proven reliability. Compared with its microwave BJT counterpart, the GaAsFET has high gain and lower noise figure and can operate at a higher maximum frequency. The difference in frequency-handling capacity between BJTs and GaAsFET is due to the slower minority carriers in the base region of the BJT whereas conduction in a GaAsFET depends mainly on majority carriers. GaAsFETs can be used from below 1 GHz to beyond 18 GHz whereas the BJTs can operate up to about 10 GHz. Research on high electron mobility transistors (HEMT) and GaAs heterojunction bipolar transistors (HBT) in recent years has promised the potential application of ‘transistors’ at millimetre-wave frequencies.