A programmable UJT (PUT) is not a unijunction transistor at all but.a four-layer device. Its name is derived from the fact that its characteristic curve and many of its applications are similar to those of a UJT. Figure 18-53 shows the constructionand schematic symbol of a PUT, which resembles an SCR more than a UJT. Like an SCR,…
As shown in Figure 18-46, a unijunction transistor consists of a bar of lightly doped (high-resistivity) N material to which a heavily doped, P-type rod is attached on one side. Ohmic contacts are made at opposing ends of the N-type bar, which are called base / (BI) and “me 2 (B2) of the transistor. The Pv type rod…
An optocoupler combines a light-emitting device with a light-sensitive device in one package. One of the simplest examples is an LED packaged with a phototransistor. The LEO is illuminated by an input circuit and the photolransistor responding to the light drives an output circuit. Thus the input and output circuits are coupled by light energy alone. The principal advantage…
A solar cell is simply a large photodiode operated in its photovoltaic mode. In its most common application, it is used to convert light energy to electrical energy, and it thereby serves as a source of de power. Unlike the photodiode, the solar energy converter is usually constructed in the form of a circular wafer with a very thin…
A photo diode is a PN junction constructed so that it can be exposed to light. When reverse biased, it behaves as a photo conductive device because its resistance changes with light intensity. In this case, the change in resistance manifests itself as a change in reverse leakage current. Recall that reverse leakage current in a conventional diode is…
Optoelectronic devices are those whose characteristics are changed or controlled by light or those that create andlor modify light. In recent years, important technological advances in the design and fabrication of solid-state optoelectronic devices have been responsible for what are now widespread applications in areas such as visual displays, alarm systems, fiber-optic communications, optical readers, lasers, and solar energy converters.…
A silicon controlled switch (SCS) is essentially an SCR equipped with a second gate terminal. Like a conventional switch, and unlike an SCR, an SCS can be switched either on or off through external gate control. Figure lR-16 shows the structure, symbol, and two-transistor equivalent of an SCS. Note that the gate terminal at the base of 02 is…
Four-layer devices, also called thy ristors, comprise a class of semiconductor components whose structure is characterized by alternating layers of P and N material. By altering the terminal configuration anti geometry of the basic structure, it is possible to construct a variety of useful devices- including silicon controlled reotifiers, silicon controlled switches, diacstriacs, and Shockley diodes. These are. widely used…
ZENER DIODES In.earlier chapters we discussed several zener-diode applications, perhaps the most important of which is the zener voltage reference used in regulator circuits. We wish now to examine the characteristics and limitations of these devices in more detail, so we can interpret and apply specifications for some of their important parameters. Figure 18-1 shows a typical 1-…
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