Transistors - Basic Applications, Construction and Types
Transistors are three-terminal semiconductor devices used to regulate current or to amplify an input signal into a greater output signal. Transistors are also used to switch electronic signals. The circulation of electrical current through all types of transistors is adjusted by electron addition. This process creates variations in voltage to cause proportionally larger variations in output current, creating amplification.
Although it varies, most types of transistors are quite small and versatile. Nearly every electronic device contains at least one or more types of transistors. For many, transistors have been deemed one of the key inventions of the modern electrical era because of the transistor’s standard and frequent place among electronic systems and modern circuits.
Most types of transistors are packaged individually but can also be included in an integrated circuit. Within these integrated circuits, the number of transistors can vary greatly depending upon the application.
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A Bipolar Junction Transistor (BJT) is a three-terminal electronic device made of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. A bipolar transistor will have terminals that are labeled: emitter, collector, base. A small current at the base terminal (passing from the base to the emitter) can modify or switch a much larger current between the collector and emitter terminals.
An Insulated Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device typically used as an electronic switch. IGBT's are types of transistors that are capable of switching electric power in many modern appliances such as electric cars, trains, variable speed refrigerators, air-conditioners and even stereo systems with switching amplifiers.
The Darlington Transistor is actually two bipolar transistors, connected in such a way that the current amplified by the first transistor is amplified even further by the second one. This model offers a higher common-emitter current gain than if both types of transistors are separated and can even take up less space because both transistors can share a collector.
A Metal-Oxide-Semiconductor Field-Effect Transistor (MOFET) is used in integrated circuits to control the conductivity of a channel. MOSFETs are highly dependent on negative and positive charges. They have many purposes, including limiting a device's power levels, storing data, and being used as a switch for a wide variety of electronic devices.
Many types of transistors are made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit.
The most basic element of a transistor power module is the silicon chip. Because of the high gain of Darlington configurations, most bipolar types of transistors and transistor modules contain Darlington transistor chips. Some of these chips are planar structures, as illustrated in Figure 1.1. The surface of a planar chip can be easily treated, simplifying mass production. Various manufacturers employ state-of-the-art fine line emitter patterns, resulting in excellent gain and safe operating area performance. High blocking voltages are achieved by using a triple diffusion process and guard rings.
Figure 1.2 illustrates the internal construction of a transistor module. The transistor chip is soldered to a molybdenum base. The molybdenum base alleviates thermal stress on the chip due to the nearly equivalent thermal expansion coefficients of silicon and molybdenum. This assembly is next soldered to a copper collector electrode along with a freewheeling diode chip. The copper electrode is in turn soldered to a ceramic substrate. The ceramic substrate can withstand 2000 to 2500 volts without adding significantly to the device's thermal resistance. The chips are bonded with aluminum wire and then encapsulated with silicone gel to guard the chip surfaces. Finally, the package is back-filled with epoxy resin to increase mechanical and environmental strength.
The proper application of power semiconductors requires an understanding of their maximum ratings and electrical characteristics, information that is presented within the device data sheet. Good design practice employs data sheet limits and not information obtained from small sample lots.
A rating is a maximum or minimum value that sets a limit on device capability. Operation in excess of a rating can result in irreversible degradation or device failure. Maximum ratings represent extreme capabilities of a device. They are not to be used as design conditions.
A characteristic is a measure of device performance under specified operating conditions expressed by minimum, typical, and/or maximum values, or shown graphically.