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Chapter 18: Bipolar and Field-Effect Transistors and Their Circuits

By Ferril A. Losee
From RF Systems, Components, and Circuits Handbook, Second Edition

18.1 Bipolar Junction Transistors

Figure 18.1 shows the geometry, representation, and the symbol for an NPN BJT. Figure 18.1(a) shows the cross-section of a planar BJT. This drawing shows a central volume of n-type doped Si called the emitter, a p-type Si layer around the emitter volume called the base, and an n-type volume of doped Si called the collector. Metallic leads are connected to each of these semiconductor sections to form the transistor.


Figure 18-1: NPN BJT: (a) cross-section of a planar NPN BJT; (b) elements of a NPN BJT; and (c) symbol for the NPN BJT.

Figure 18.1(b) is another representation of an NPN BJT, showing the n-type emitter, the p-type base, and the n-type collector stacked one above the other, with leads exiting from each region. This representation shows the junctions between the elements.

The symbol for the NPN BJT used in schematic diagrams is shown in Figure 18.1(c). The arrow points in the direction of conventional current flow.

The operation of the Si NPN transistor is as follows. This device acts as a current-controlled valve, controlling the flow of electrons between the emitter and the collector. The collector is biased very positive with respect to the base. The combination of base and collector thus acts like a back-biased pn junction diode.

The base may be biased positive or negative with respect to the emitter, depending on whether the device is turned on or off. The combination of base and emitter also acts as a junction diode.

Copyright Artech House, Inc. 2005 under license agreement with Books24x7
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Bipolar RF Transistors
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.
Darlington Transistors
Darlington transistors (Darlington pairs) are semiconductor devices that combine two bipolar transistors in a single device. They provide high current gain (commonly written ß) and require less space than configurations that use two discrete transistors.
Small-Signal Bipolar Transistors (BJT)
Small-signal bipolar transistors (BJT) are semiconductors that amplify small AC or DC signals. They consist of a base n-type or p-type layer sandwiched between emitter and collector layers of the opposite type.
Transistors
Transistors are electronic devices made of semiconductor material that amplify a signal or open or close a circuit.
Power Bipolar Transistors
Power bipolar transistors are semiconductors in which a base n-type or p-type layer is sandwiched between emitter and collector layers of the opposite type. The junctions between the semiconductor sections amplify weak incoming electrical signals.
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