Selecting a diode for a particular RF/microwave application is more a matter of understanding which type of diode performs what type of function. For higher power amplification, GaN is usually the semiconductor material of choice at present for microwave frequencies, typically in the form of a flange-packaged power GaN HEMT device (Fig. Such transistors are characterized for use in their linear, small-signal regions, with the tradeoff being that output power is limited to generally less than 1 W. Thus, GaAs FETs might be the best option, since they are available with impressively low (under 1 dB) noise figure at microwave frequencies. For example, in receiver applications, signal sensitivity is critical and the noise figure of a receiver front-end transistor as used in a low-noise amplifier must be as low as possible. A transistor with two positive layers surrounding a negative layer is known as a pnp transistor, and a device with two negative layers around a positive layer is an npn transistor.Ĭhoosing a transistor for a high-frequency application generally hinges on frequency range as well as performance. Current flows laterally in an FET and vertically in a bipolar transistor.Ī bipolar transistor essentially consists of two junction diodes on semiconductor material having positive (p) and negative (n) polarities. ![]() In either type of transistor, charge is transferred in a controlled manner between two terminals: between the source and the drain in an FET and between the emitter and the collector in a bipolar transistor. The three FET terminals are called the gate, drain, and source, while the three bipolar transistor terminals are referred to as the emitter, collector, and base. The two terminals of a diode are known as the anode and the cathode. High-power microwave HEMTs are being made affordable and easier to handle in surface-mount housings. ![]() Transistor types essentially include field-effect transistors (FETs), such as metal-oxide-semiconductor FETs (MOSFETs) and metal-epitaxial-semiconductor FETs (MESFETs), and bipolar junction transistors (BJTs), such as heterojunction bipolar transistors (HBTs).ġ. Diode types include Schottky-barrier, positive-intrinsic-negative (PIN), Gunn, Impatt, and varactor-tuning diodes. Depending on the application, a transistor may be operated a low power levels in its small-signal or linear region, or at higher power levels in its large-signal or nonlinear region.ĭiodes and transistors have been developed and fabricated in many different forms. With the three terminals, a voltage or current applied between one pair of terminals can affect the voltage or current between another pair of terminals to produce gain as needed for an amplifier or an oscillator. They can be used as amplifiers or switches, with frequency dependent on device structure, dimensions, and material properties. Transistors are versatile semiconductor devices fabricated from many different chemical elements and compounds, such as silicon (Si), gallium arsenide (GaAs), and gallium nitride (GaN). Often, diodes also function as rectifiers to convert alternating current (ac) to direct current (dc) in a circuit. The device works well as a switch, and is useful for limiting signal levels, frequency multiplication, tuning, and protecting circuitry from the flow of current in a given direction. A diode can allow the flow of current in one direction while blocking the flow of current in the other direction. ![]() Put simply, a diode is a two-terminal semiconductor device and a transistor is a three-terminal semiconductor device. Knowing how the different semiconductors behave can simplify the task of specifying disparate diodes and transistors for a wide range of RF/microwave applications. In analog, digital, and mixed-signal circuits, diodes and transistors both provide different types of switching functions, while transistors also supply signal amplification when needed.ĭiodes and transistors of various types are used at RF and microwave frequencies, depending on required function and frequency range. ![]() Many different active circuits are based on two essential semiconductor function blocks-diodes and transistors. This file type includes high-resolution graphics and schematics when applicable.
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