1Slide 1EE40 Fall 2006Prof. Chang-HasnainEE40Lecture 25Prof. Chang-Hasnain10/31/07Reading: Chap. 10 andSupplementary ReaderSlide 2EE40 Fall 2006Prof. Chang-HasnainChapter 10• OUTLINE– Diode Current and Equation– Some Interesting Circuit Applications– Load Line Analysis– Solar Cells, Detectors, Zener Diodes– Circuit Analysis with Diodes– Half-wave Rectifier– Clamps and Voltage Doublers using Capacitors• Reading– Hambley 10.1-10.8– Supplementary Notes Chapter 2Slide 3EE40 Fall 2006Prof. Chang-HasnainI-V CharacteristicsIn forward bias (+ on p-side) wehave almost unlimited flow(very low resistance).Qualitatively, the I-Vcharacteristics must look like:VFIcurrent increasesrapidly with VVFIThe current is closeto zero for anynegative biasIn reverse bias (+ on n-side)almost no current can flow.Qualitatively, the I-Vcharacteristics must look like:Slide 4EE40 Fall 2006Prof. Chang-HasnainDiode Physical Behavior and EquationNtypePtypeSchematic Device+−VIISymbol+−VQualitative I-V characteristics:IVV positive,easyconductionV negative,noconductionQuantitative I-V characteristics:)1e(IIkTqV0!=In which kT/q is 0.026V and IO is aconstant depending on diode area.Typical values: 10-12 to 10-16 A.Interestingly, the graph of thisequation looks just like the figure tothe left.A non-ideality factor n times kT/q is often included.2Slide 5EE40 Fall 2006Prof. Chang-HasnainThe pn Junction I vs. V EquationIn EECS 105, 130, and other courses you will learn why the I vs. Vrelationship for PN junctions is of the form)1e(IIkTqV0!=where I0 is a constant proportional to junction area and dependingon doping in P and N regions,k is Boltzman constant, and T is absolute temperature. a typical value for I0 is,106.1hargec electronicq19!"==,Kat300 0.026VqKT °=A10101512 !!!We note that in forward bias, I increases exponentially and is inthe µA-mA range for voltages typically in the range of 0.6-0.8V.In reverse bias, the current is essentially zero.I-V characteristic of PN junctionsSlide 6EE40 Fall 2006Prof. Chang-HasnainDiode Ideal (Perfect Rectifier) ModelThe equationis graphed below for1)kTqVexp(II0!=A10I150!=The characteristic is described asa “rectifier” – that is, a device thatpermits current to pass in only onedirection. (The hydraulic analog isa “check value”.) Hence thesymbol:+! VISimple “Perfect Rectifier” ModelIf we can ignore the small forward-bias voltage drop of a diode, asimple effective model is the“perfect rectifier,” whose I-Vcharacteristic is given below:VIReverse bias0Vany ,0I <!Forward bias0Iany ,0V >! A perfect rectifier0246810-5 0 5 10Current in mAForward Voltage in VSlide 7EE40 Fall 2006Prof. Chang-HasnainDiode Large-Signal Model (0.7 V Drop)Improved “Large-Signal Diode” Model:If we choose not to ignore the smallforward-bias voltage drop of adiode, it is a very goodapproximation to regard the voltagedrop in forward bias as a constant,about 0.7V. the “Large signalmodel” results.Reverse bias0Vany ,0I <!Forward bias0Iany ,0.7V>≅0100200300400-5 -3 -1 1forward bias (V)Current (microamp)+−VIThe Large-SignalDiode Model- 0.7+VI0.7Slide 8EE40 Fall 2006Prof. Chang-HasnainRectifier CircuitVS(t)t+−VR(t)+−VS(t)VR(t)“rectified” version ofinput waveformtAssume the ideal(perfect rectifier)model.3Slide 9EE40 Fall 2006Prof. Chang-HasnainPeak Detector Circuit+−VC(t)Vi(t)+−CVi(t)VitVC(t)VC+−Assume the ideal (perfect rectifier) model.Key Point:The capacitor chargesdue to one way currentbehavior of the diode.Slide 10EE40 Fall 2006Prof. Chang-Hasnainpn-Junction Reverse Breakdown• As the reverse bias voltage increases, the peak electricfield in the depletion region increases. When the electricfield exceeds a critical value (Ecrit ≅ 2x105 V/cm), thereverse current shows a dramatic increase:ID (A)VD (V)reverse (leakage) currentforward currentbreakdown voltageVBDSlide 11EE40 Fall 2006Prof. Chang-HasnainintegratedcircuitA Zener diode is designed to operate in the breakdown mode.Zener DiodeID (A)VD (V)reverse (leakage) currentforward currentbreakdown voltageRVBD = 15VVBD+vs(t)–t+vo(t)–Example:Slide 12EE40 Fall 2006Prof. Chang-HasnainLoad Line Analysis Method1. Graph the I-V relationships for the non-linearelement and for the rest of the circuit2. The operating point of the circuit is found fromthe intersection of these two curves.VTh+−+V–RThIIVThe I-V characteristic of all of the circuit exceptthe non-linear element is called the load lineVThVTh/RThoperating point4Slide 13EE40 Fall 2006Prof. Chang-HasnainSolar cell: Example of simple PN junction• What is a solar cell?– Device that convertssunlight into electricity• How does it work?– In simple configuration, it is adiode made of PN junction– Incident light is absorbed bymaterial– Creates electron-hole pairs thattransport through the materialthrough• Diffusion (concentration gradient)• Drift (due to electric field)PN Junction DiodeSlide 14EE40 Fall 2006Prof. Chang-HasnainopticalkTVqSDIeII !!= )1(DPhotovoltaic (Solar) CellID (A)VD (V)with incident lightin the darkOperating pointThe load line a simple resistor.Slide 15EE40 Fall 2006Prof. Chang-HasnainI-V characteristics of the device• I-V characteristics of a PNjunction is given bywhere Is is the saturation intensitydepending on band gap and dopingof the material and IL is thephotocurrent generated due to light• Efficiency is defined asVocIsc(Vm, Im)LSIkTeVII !"#$%&'!= 1)exp(IntensityLightIVFFIntensityLightVIscocmm**.==!Voc - Open circuit voltageIsc - Short circuit currentImp , Vmp- Current and voltageat maximum powerFF is the Fill FactorSlide 16EE40 Fall 2006Prof. Chang-HasnainExample 2: Photodiode• An intrinsic region is placedbetween the p-type and n-typeregions♣ Wj ≅ Wi-region, so that most of theelectron-hole pairs are generatedin the depletion region◊ faster response time (~10 GHz operation)ID (A)VD (V)with incident lightin the darkoperating point5Slide 17EE40 Fall 2006Prof. Chang-HasnainPhotodetector Circuit Using Load LineIVload lineVThVTh/RThoperatingpoints underdifferent lightconditions.As light shines on the photodiode, carriersare generated by absorption. These excesscarriers are swept by the electric field at thejunction creating drift current, which is samedirection as the reverse bias current andhence negative current. The current isproportional to light intensity and hence canprovide a direct measurement of lightintensity ◊ photodetector.- What happens when Rth is too large?- Why
View Full Document
Unlocking...