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Berkeley ELENG 100 - Lessons In Electric Circuits

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Lessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 3 http://www.faqs.org/docs/electric/Semi/SEMI_3.html1 of 31 5/19/2006 6:01 PM Lessons In Electric Circuits -- Volume IIIChapter 3DIODES AND RECTIFIERS*** INCOMPLETE ***IntroductionA diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common type of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams assuch:When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage:When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is "backward" and the diode blocks current, thediode is said to be reverse-biased. A diode may be thought of as a kind of switch: "closed" whenforward-biased and "open" when reverse-biased.Lessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 3 http://www.faqs.org/docs/electric/Semi/SEMI_3.html2 of 31 5/19/2006 6:01 PMOddly enough, the direction of the diode symbol's "arrowhead" points against the direction of electron flow. This is because the diode symbol was invented by engineers, who predominantly use conventional flow notation in their schematics, showing current as a flow of charge from the positive (+) side of the voltage source to the negative (-). This convention holds true for all semiconductor symbols possessing"arrowheads:" the arrow points in the permitted direction of conventional flow, and against the permitted direction of electron flow.Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valveallows fluid flow through it in one direction only:Check valves are essentially pressure-operated devices: they open and allow flow if the pressure across them is of the correct "polarity" to open the gate (in the analogy shown, greater fluid pressure on the right than on the left). If the pressure is of the opposite "polarity," the pressure difference across the checkvalve will close and hold the gate so that no flow occurs.Like check valves, diodes are essentially "pressure-" operated (voltage-operated) devices. The essentialdifference between forward-bias and reverse-bias is the polarity of the voltage dropped across the diode.Let's take a closer look at the simple battery-diode-lamp circuit shown earlier, this time investigating voltage drops across the various components:Lessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 3 http://www.faqs.org/docs/electric/Semi/SEMI_3.html3 of 31 5/19/2006 6:01 PMWhen the diode is forward-biased and conducting current, there is a small voltage dropped across it, leaving most of the battery voltage dropped across the lamp. When the battery's polarity is reversed andthe diode becomes reverse-biased, it drops all of the battery's voltage and leaves none for the lamp. If weconsider the diode to be a sort of self-actuating switch (closed in the forward-bias mode and open in the reverse-bias mode), this behavior makes sense. The most substantial difference here is that the diodedrops a lot more voltage when conducting than the average mechanical switch (0.7 volts versus tens of millivolts).This forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. When there is no voltage applied across asemiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current through it. The depletion region is for the most part devoid of available charge carriers and so actsas an insulator:If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it:Lessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 3 http://www.faqs.org/docs/electric/Semi/SEMI_3.html4 of 31 5/19/2006 6:01 PMConversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region will collapse and become thinner, so that the diode becomes less resistive to current through it. In order for asustained current to go through the diode, though, the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish, called the forward voltage:For silicon diodes, the typical forward voltage is 0.7 volts, nominal. For germanium diodes, the forwardvoltage is only 0.3 volts. The chemical constituency of the P-N junction comprising the diode accountsfor its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages. Forward voltage drop remains approximately equal for a wide range of diode currents,meaning that diode voltage drop not like that of a resistor or even a normal (closed) switch. For mostpurposes of circuit analysis, it may be assumed that the voltage drop across a conducting diode remains constant at the nominal figure and is not related to the amount of current going through it.In actuality, things are more complex than this. There is an equation describing the exact current throughLessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 3 http://www.faqs.org/docs/electric/Semi/SEMI_3.html5 of 31 5/19/2006 6:01 PMa diode, given the voltage dropped across the junction, the temperature of the junction, and several physical constants. It is commonly known as the diode equation:The equation kT/q describes the voltage produced within the P-N junction due to the action of temperature, and is called the thermal voltage, or Vt of the junction. At room temperature, this is about 26millivolts. Knowing this, and assuming a "nonideality" coefficient of 1, we may simplify the diodeequation and re-write it as such:You need not be familiar with the "diode equation" in order to analyze simple diode circuits. Justunderstand that the voltage dropped across a current-conducting diode does change with the amount of current going through it, but that this change is fairly small over a wide range of currents. This is whymany textbooks simply say the voltage drop across a conducting, semiconductor diode remains constant at 0.7 volts for silicon and 0.3 volts for germanium. However, some


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Berkeley ELENG 100 - Lessons In Electric Circuits

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