In other words, two types of charge carriers - electrons and holes - comprise this main current through the transistor.Īs you can see, the controlling current and the controlled current always mesh together through the emitter wire, and their electrons always flow against the direction of the transistor's arrow. Small electron base current controls large collector electron current flowing against emitter arrow.īipolar transistors are called bipolar because the main flow of electrons through them takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or vice versa). According to the standards of semiconductor symbology, the arrow always points against the direction of electron flow. The small current that controls the main current goes from base to emitter, or from emitter to base, once again depending on the kind of transistor it is (PNP or NPN, respectively). The main current that is controlled goes from collector to emitter, or from emitter to collector, depending on the type of transistor it is (PNP or NPN, respectively). In other words, transistors restrict the amount of current passed according to a smaller, controlling current. For any given state of operation, the current directions and voltage polarities for each kind of transistor are exactly opposite each other.īipolar transistors work as current-controlled current regulators. The functional difference between a PNP transistor and an NPN transistor is the proper biasing (polarity) of the junctions when operating. The schematic symbols are shown in Figure below(a) and (d).īJT transistor: (a) PNP schematic symbol, (b) physical layout (c) NPN symbol, (d) layout. Each layer forming the transistor has a specific name, and each layer is provided with a wire contact for connection to a circuit. In taking this approach, however, I assume that the reader possesses a certain minimum knowledge of semiconductors: the difference between “P” and “N” doped semiconductors, the functional characteristics of a PN (diode) junction, and the meanings of the terms “reverse biased” and “forward biased.” If these concepts are unclear to you, it is best to refer to earlier chapters in this book before proceeding with this one.Ī bipolar transistor consists of a three-layer “sandwich” of doped (extrinsic) semiconductor materials, either P-N-P in Figure below(b) or N-P-N at (d). I don't mean to downplay the importance of understanding semiconductor physics, but sometimes an intense focus on solid-state physics detracts from understanding these devices' functions on a component level. Here I want to explore how to use these components, not analyze their intimate internal details. Discussions of holes and electrons are better left to another chapter in my opinion. My intent here is to focus as exclusively as possible on the practical function and application of bipolar transistors, rather than to explore the quantum world of semiconductor theory. Understanding how transistors function is of paramount importance to anyone interested in understanding modern electronics. This revolution made possible the design and manufacture of lightweight, inexpensive electronic devices that we now take for granted. Technical feats previously requiring relatively large, mechanically fragile, power-hungry vacuum tubes were suddenly achievable with tiny, mechanically rugged, power-thrifty specks of crystalline silicon. The invention of the bipolar transistor in 1948 ushered in a revolution in electronics. Thermal mismatch (problem with paralleling transistors).Lessons In Electric Circuits - Volume III Chapter 4 BIPOLAR JUNCTION TRANSISTORS Lessons In Electric Circuits - Volume III (Semiconductors) - Chapter 4
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