barcode in vb.net 2005 Behavior of a P-N junction in Software

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Behavior of a P-N junction
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Simply having a semiconducting material, either P or N type, might be interesting, and a good object of science experiments. But when the two types of material are brought together, the P-N junction develops properties that make the semiconductor materials truly useful as electronic devices. Figure 19-3 shows the schematic symbol for a semiconductor diode, formed by joining a piece of P-type material to a piece of N-type material. The N-type semiconductor is represented by the short, straight line in the symbol, and is called the cathode. The P-type semiconductor is represented by the arrow, and is called the anode. In the diode as shown in the figure, electrons flow in the direction opposite the arrow. (Physicists consider current to flow from positive to negative, and this is in the same direction as the arrow points.) But current cannot, under most conditions, flow the other way. Electrons normally do not flow in the direction that the arrow points. If you connect a battery and a resistor in series with the diode, you ll get a current flow if the negative terminal of the battery is connected to the cathode and the positive
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364 Introduction to semiconductors
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19-2 Pictorial representation of hole flow. Small dots represent electrons, moving one way; open circles represent holes, moving the other way.
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19-3 Schematic symbol for a semiconductor diode.
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terminal is connected to the anode (Fig. 19-4A). No current will flow if the battery is reversed (Fig. 19-4B). The resistor is included in the circuit to prevent destruction of the diode by excessive current. It takes a certain minimum voltage for conduction to occur. This is called the forward breaker voltage of the junction. Depending on the type of material, it varies from about 0.3 V to 1 V. If the voltage across the junction is not at least as great as the forward breaker value, the diode will not conduct. This effect can be of use in amplitude limiters, waveform clippers, and threshold detectors. You ll learn about the various ways diodes are used in the next chapter.
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How the junction works
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When the N-type material is negative with respect to the P-type, as in Fig. 19-4A, electrons flow easily from N to P. The N-type semiconductor, which already has an excess of electrons, gets even more; the P-type semiconductor, with a shortage of
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How the junction works 365
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19-4 Series connection of
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battery B, resistor R, milliammeter mA, and diode D. At A, forward bias results in current flow; at B, reverse bias results in no current.
electrons, is made even more deficient. The N-type material constantly feeds electrons to the P-type in an attempt to create an electron balance, and the battery or power supply keeps robbing electrons from the P-type material. This is shown in Fig. 19-5A and is known as forward bias. When the polarity is switched so the N-type material is positive with respect to the P type, things get interesting. This is called reverse bias. Electrons in the N-type material
19-5 At A, forward bias of a
P-N junction; at B, reverse bias. Electrons are shown as small dots, and holes are shown as open circles.
366 Introduction to semiconductors are pulled towards the positive charge, away from the junction. In the P-type material, holes are pulled toward the negative charge, also away from the junction. The electrons (in the N-type material) and holes (in the P type) are the majority charge carriers. They become depleted in the vicinity of the P-N junction (Fig. 19-5B). A shortage of majority carriers means that the semiconductor material cannot conduct well. Thus, the depletion region acts like an insulator.
Junction capacitance
Some P-N junctions can alternate between conduction (in forward bias) and nonconduction (in reverse bias) millions or billions of times per second. Other junctions are slower. The main limiting factor is the capacitance at the P-N junction during conditions of reverse bias. The amount of capacitance depends on several factors, including the operating voltage, the type of semiconductor material, and the cross-sectional area of the P-N junction. By examining Fig. 19-5B, you should notice that the depletion region, sandwiched between two semiconducting sections, resembles the dielectric of a capacitor. In fact, the similarity is such that a reverse-biased P-N junction really is a capacitor. Some semiconductor components are made with this property specifically in mind. The junction capacitance can be varied by changing the reverse-bias voltage, because this voltage affects the width of the depletion region. The greater the reverse voltage, the wider the depletion region gets, and the smaller the capacitance becomes. In the next chapter, you ll learn how engineers take advantage of this effect.
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