ee201-semiconductor devices CHAPTER 2 DIODES asalamsaad,pkt,2011ee201-semiconductor devicesA 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 kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist.Figure 2.1: Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.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, thediodeissaidtobereverse-biased.Adiode maybe thoughtofaslikeaswitch:closedwhen forward-biased and open when reverse-biased.Figure 2.2: Diode representations: PN-junction model, schematic symbol, physical part.The schematic symbol of the diode is shown in Figure 2.2 (b) such that the anode (pointing end) corresponds to the P-type semiconductor at (a). The cathode bar, non-pointing end, at (b) corresponds to the N-type material at (a). Also note that the cathode stripe on the physical part (c) corresponds to the cathode on the symbol.asalamsaad,pkt,2011ee201-semiconductor devicesFigure 2.3: Increasing forward bias from (a) to (b) decreases depletion region thickness.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 forwardvoltages. Forwardvoltagedropremainsapproximatelyconstant forawiderangeofdiode currents, meaning that diode voltage drop is not like that of a resistor or even a normal (closed) switch. For most simplifiedcircuit analysis,the voltage drop across a conducting diode may be considered constant at the nominal figure and not related to the amount of current.Figure 2.4: Diode curve: showing knee at 0.7 V forward bias for Si, and reverse breakdown.Now we come to the most popular application of the diode: rectification. Simply defined, rectification is the conversion of alternating current (AC) to direct current (DC). This involves a device that only allows one-way flow of electrons. As we have seen, this is exactly what a semiconductor diode does.asalamsaad,pkt,2011ee201-semiconductor devicesHalf-Wave RectifierThe simplest way to produce a D.C. voltage from an A.C. voltage is the half-wave rectifier. Figure 2.5 A Half-wave rectifierFigure 2.6(a) Sinusoidal input voltage, (b) Half-wave rectified output voltageVin is a sinusoidal A.C. Voltage. The diode (ideally) passes current when the voltage across it is positive, and stops current when VD is negative, producing a chopped-off sine wave across R. Heavy low-pass filtering is needed to produce a usable D.C. Voltage, usually with a slight ripple (Figure 2.7)Figure 2.7 Filtering smoothed the output voltageThe size of this D.C. output voltage is simply the average value of the rectified voltage:So the D.C. voltage is only 31% of Vm!asalamsaad,pkt,2011( )4V 2 s i n21 4Vd 2 c o s 121 2Vd s i n 2Vd 0 d s i n V 21d t VT1V2P02P02P022P2 02 2pT02 2r m s]]]

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+ ee201-semiconductor devicesNow the maximum available power from the source is the power that would be drawn without any rectifier in the middle just the A.C. source and the load. Its value isSo the power delivered to the load through the half-wave rectifier is onlySo the half-wave rectifier can pass only 20% of the sources available powerto the load not very effective!Full-Wave Rectifierasalamsaad,pkt,2011ee201-semiconductor devicesBridge RectifierThe full wave rectifier produces a positive output voltage for both positive andnegative input voltages.asalamsaad,pkt,2011[ ]V 2 c o sVd s i n V1d t VT1VP0P0pT0D C ( )2V s i n 2Vd 2 c o s 121Vd s i n V1d t VT1VP0PP02pT02r m s220222221]]]

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.| ee201-semiconductor devicesHeres how it works:asalamsaad,pkt,2011ee201-semiconductor devicesThis is 64% of Vm -- twice the average output voltage for the half-wave case.The power drawn isasalamsaad,pkt,2011Vp0- VP 2 Vi( t )tVp- VD0 2 Vo( t )tee201-semiconductor devicesDiode Limiters Figure 2-13(a) shows a diode limiter (also called clipper) that limits or clips the positive part of the input voltage.Astheinput voltagegoespositive, thediodebecomesforward-biasedandconducts current.Becausethecathode isat ground potential (0 V), the anode cannot exceed 0.7V (assuming silicon). Sopoint Aislimitedto+0.7V when the input voltage exceeds this value. When the input voltage goes back below 0.7 V, the diode is reverse- biased and appears as an open. The output voltage lookslikethenegativepart oftheinput voltage, but withamagnitudedeterminedbythevoltage divider formed by R1 and the load resistor, RL as follows: If R1 is small compared to RLthen Vout= Vin. If the diode is turned around, as in Figure 2- 13(b), the negative part of the input voltage is clipped off. When the diode is forward-biased during the negative part of the input voltage, point A is held at -0.7 V by the diode drop. When the input voltage goes above -0.7 V, the diode is no longer forward-biased; and a voltage appears across RL proportional to the input voltage. 2-13(a) Limiting of the positive alternation. The diode is forward-biased during the positive alternation (above 0.7 V) and reverse-biased during the negative alternationasalamsaad,pkt,2011ee201-semiconductor devices2.13(b) Limiting of the negative alternation. The diode is forward-biased during the negative alternation (below -0.7 V) and reverse-biased during the positive alternation. There are two types of clipper circuits, the series and parallel diode clipping circuits.Series Diode Clipping CircuitIn the first type, the voltage source of Em ( positive or negative) is connected through output terminals as in Fig. 2.14. Depending on the diode connection (normal or reverse), the values smaller (Fig.2.14.a) or greater (Fig.2.14.b) than Em is clipped and assigned as Em.Figure 2.14.In the second type of thresholded series clipping, the voltage source is applied between the input and asalamsaad,pkt,2011ee201-semiconductor devicesoutput terminals, series with the diode. This time, the clipped values are assigned to zero and the net output voltage equals to the difference between the input and threshold values.(If Em is negative, then E0 = E Em = E + |Em|)Parallel Diode Clipping CircuitInthistypeofclippers, thediodeisconnected between output terminals. Theon/off state of diode directly affects the output voltage. These type of clippers may also have a non-zero threshold voltage by addition of a voltage series with diode. Following figures illustrate the clipping process.asalamsaad,pkt,2011ee201-semiconductor devicesClamper CircuitsClamper Circuits, or briefly clampers are used to change the D.C. level of asignal to a desired value.asalamsaad,pkt,2011ee201-semiconductor devicesBeing different from clippers, clamping circuits uses a capacitor and a diode connection. When diode is in its on state, the output voltage equals to diode drop voltage (ideally zero) plus the voltage source, if any.The circuit for a positive clamper is shown in the figure. During the negative half cycle of the input signal, the diode conducts and acts like a short circuit. The output voltage Vo=0 volts. The capacitor is charged to the peak value of input voltage Vm. and it behaves like a battery. During the positive half of theinput signal, thediodedoesnot conductandactsasanopencircuit. Hencetheoutput voltage Vo=Vm+Vm. This gives a positively clamped voltage. There exist again modified versions of this circuit in which a threshold value isinserted for clamping. Following figures illustrate these modifications and resulting outputs.asalamsaad,pkt,2011ee201-semiconductor devicesZener diodeA zener diode is a silicon p-n junction device that is designed for operation in the reverse-breakdown region. If azener diodeisforward-biased, it operatesthesameapower diode. Zener diodesare designedtooperateinreversebreakdown. Twotypesofreversebreak-downinazener diodeare avalancheandzener.Theavalanchebreakdown, occursinbothpowerdiodeandzenerdiodesat a asalamsaad,pkt,2011ee201-semiconductor devicessufficiently high reverse voltage. Zener breakdown occurs in a zener diode at low reverse voltages. Figure 2.15(a)Figure 2.15(b)Figure2.15(b)showsthereverseportionofazenerdiode'scharacteristiccurve, Noticethat asthe reverse voltage (V R ) is increased, the reverse current (IR) remains extremely small up to the "knee" of the curve. The reverse current is also called the zener current, lz. At this point, the breakdown effect begins: the internal zener resistance, also called zener impedance (Zz), begins to decrease as the reverse current increases rapidly. From the bottom of the knee. the zener breakdown voltage (V z ) remains essentially constant although it increases slightly as the zener current, lz, increased.Zener Regulation The ability to keep the reverse voltage across its terminals essentially constant is the keyfeatureof thezener diode.Azener diodeoperatinginbreakdownactsasavoltageregulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse-current values. A minimum value of reverse current, IZK must he maintained in order to keep the diode in breakdown forvoltageregulation.Youcanseeonthe curve in Figure 2.15(b)that when thereverse currentis reduced below the knee of the curve, the voltage decreases drastically and regulation is lost. Also, there is a maximum current, IZM above which the diode may be damaged due to excessive power dissipation. So, basically. thezenerdiodemaintains anearly constant voltage across its terminals forvaluesof reverse current ranging from IZK to IZM. asalamsaad,pkt,2011ee201-semiconductor devicesOther type of diodesThe light-Emitting Diode (LED) Whenthedeviceisforward-biased, electronscrossthep-njunctionfromthen-typematerial and recombine with holes in the p-type material. these free electrons are in the conduction band and at a higher energy than the holes in the valence band. When recombination takes place, the recombining electrons release energy in the form of heat and light. A large exposed surface area on one layer of the semi conductivematerial permits thephotons tobeemittedas visiblelight. This process, called electroluminescence, Various impurities are added during the doping process to establish the wavelength of the emitted light. The wavelength determines the color of the light and if it is visible or infrared (IR). PhotodiodeFigure 2.16 asalamsaad,pkt,2011ee201-semiconductor devicesThe photo diode is a device that operates in reverse bias. as shown in Figure 3-33(a), where I is the reversecurrent.Thephotodiodehasasmalltransparentwindowthat allowslight tostrikethepn junction. A photo diode differs from a rectifier diode in that when its p-n junction is exposed to light, thereversecurrent increaseswiththelight intensity. Whenthereisnoincident light. thereverse current, Iisalmost negligibleandiscalledthedarkcurrent.Anincreaseintheamount of light intensity, expressed as irradiance (mW/cm 2 ), produces an increase in the reverse current, Laser diodeFigure 2.17 The termlaser stands for light amplificationbystimulatedemissionof radiation. Laser light is monochromatic. which means that it consists of a single color and not a mixture ofcolors. Laser light is also called coherent light, a single wavelength, as compared to incoherent light, which consists of a widebandofwavelengths.Thelaserdiodenormallyemitscoherent light, whereastheLEDemits incoherent light. ThesymbolsarethesameasshowninFigure2.17(a). Thebasicoperationisas follows. The laser diode is forward-biased by an external voltage source. As electrons move through asalamsaad,pkt,2011ee201-semiconductor devicesthe junction. recombinationoccurs just as inan ordinary diode. As electrons fall intoholes to recombine, photons are released. asalamsaad,pkt,2011