Lessons In Electric Circuits Volume III Semiconductors Chapter 3 1 of 31 http www faqs org docs electric Semi SEMI 3 html Lessons In Electric Circuits Volume III Chapter 3 DIODES AND RECTIFIERS INCOMPLETE Introduction A 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 as such 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 the diode is said to be reverse biased A diode may be thought of as a kind of switch closed when forward biased and open when reverse biased 5 19 2006 6 01 PM Lessons In Electric Circuits Volume III Semiconductors Chapter 3 2 of 31 http www faqs org docs electric Semi SEMI 3 html Oddly 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 valve allows 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 check valve will close and hold the gate so that no flow occurs Like check valves diodes are essentially pressure operated voltage operated devices The essential difference 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 5 19 2006 6 01 PM Lessons In Electric Circuits Volume III Semiconductors Chapter 3 3 of 31 http www faqs org docs electric Semi SEMI 3 html When 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 and the diode becomes reverse biased it drops all of the battery s voltage and leaves none for the lamp If we consider 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 diode drops 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 a semiconductor 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 acts as an insulator If a reverse biasing voltage is applied across the P N junction this depletion region expands further resisting any current through it 5 19 2006 6 01 PM Lessons In Electric Circuits Volume III Semiconductors Chapter 3 4 of 31 http www faqs org docs electric Semi SEMI 3 html Conversely 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 a sustained 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 forward voltage is only 0 3 volts The chemical constituency of the P N junction comprising the diode accounts for 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 most purposes 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 through 5 19 2006 6 01 PM Lessons In Electric Circuits Volume III Semiconductors Chapter 3 5 of 31 http www faqs org docs electric Semi SEMI 3 html a 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 26 millivolts Knowing this and assuming a nonideality coefficient of 1 we may simplify the diode equation and re write it as such You need not be familiar with the diode equation in order to analyze simple diode circuits Just understand 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 why many 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 circuits intentionally make use of the P N junction s inherent exponential current voltage relationship and thus can only be understood in the context of
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