Chapter 10 OUTLINE EE40 Lecture 25 Prof Chang Hasnain 10 31 07 Reading Chap 10 and Supplementary Reader EE40 Fall 2006 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 2 Slide 1 Prof Chang Hasnain EE40 Fall 2006 I V Characteristics Prof Chang Hasnain Diode Physical Behavior and Equation I In forward bias on p side we have almost unlimited flow very low resistance Qualitatively the I V characteristics must look like Slide 2 Schematic Device current increases rapidly with V VF Symbol I Qualitative I V characteristics I V positive easy conduction VF I V V I In reverse bias on n side almost no current can flow Qualitatively the I V characteristics must look like The current is close to zero for any negative bias N P type type Quantitative I V characteristics I I 0 eqV kT 1 In which kT q is 0 026V and IO is a constant depending on diode area Typical values 10 12 to 10 16 A Interestingly the graph of this equation looks just like the figure to the left V V negative no conduction A non ideality factor n times kT q is often included EE40 Fall 2006 Slide 3 Prof Chang Hasnain EE40 Fall 2006 Slide 4 Prof Chang Hasnain 1 Diode Ideal Perfect Rectifier Model The pn Junction I vs V Equation The equation I I 0 exp qV kT 1 I V characteristic of PN junctions In EECS 105 130 and other courses you will learn why the I vs V relationship for PN junctions is of the form If we can ignore the small forwardbias voltage drop of a diode a simple effective model is the perfect rectifier whose I V characteristic is given below 10 Current in mA 8 I I 0 eqV kT 1 6 4 where I0 is a constant proportional to junction area and depending on doping in P and N regions q electronic charge 1 6 10 19 k is Boltzman constant and T is absolute temperature 12 10 15 A KT q 0 026V at300 K a typical value for I0 is10 We note that in forward bias I increases exponentially and is in the A mA range for voltages typically in the range of 0 6 0 8V In reverse bias the current is essentially zero EE40 Fall 2006 Slide 5 Prof Chang Hasnain 2 Forward Voltage in V 0 5 0 5 0 7 Current microamp 400 I 300 200 V 100 0 3 1 forward bias V 1 The Large Signal Diode Model I Improved Large Signal Diode Model If we choose not to ignore the small Forward bias Reverse bias forward bias voltage drop of a V 0 7 any I 0 I 0 any V 0 diode it is a very good V approximation to regard the voltage 0 7 drop in forward bias as a constant about 0 7V the Large signal model results I 10 The characteristic is described as a rectifier that is a device that permits current to pass in only one direction The hydraulic analog is a check value Hence the symbol I EE40 Fall 2006 V Diode Large Signal Model 0 7 V Drop 5 Simple Perfect Rectifier Model is graphed below for I 0 10 15 A Reverse bias I 0 any V 0 Forward bias V 0 any I 0 V A perfect rectifier Slide 6 Prof Chang Hasnain Rectifier Circuit Assume the ideal perfect rectifier model VS t VS t VR t t VR t rectified version of input waveform t EE40 Fall 2006 Slide 7 Prof Chang Hasnain EE40 Fall 2006 Slide 8 Prof Chang Hasnain 2 pn Junction Reverse Breakdown Peak Detector Circuit As the reverse bias voltage increases the peak electric field in the depletion region increases When the electric field exceeds a critical value Ecrit 2x105 V cm the reverse current shows a dramatic increase Assume the ideal perfect rectifier model Vi t Vi t C Vi VC t reverse leakage current forward current t VC t Key Point The capacitor charges due to one way current behavior of the diode EE40 Fall 2006 ID A breakdown voltage VBD VD V VC Slide 9 Prof Chang Hasnain EE40 Fall 2006 Slide 10 Prof Chang Hasnain Zener Diode Load Line Analysis Method A Zener diode is designed to operate in the breakdown mode 1 Graph the I V relationships for the non linear element and for the rest of the circuit 2 The operating point of the circuit is found from the intersection of these two curves reverse leakage current breakdown voltage VBD ID A forward current VD V I RTh Example R vs t VBD 15V EE40 Fall 2006 I Slide 11 t VTh operating point V integrated vo t circuit Prof Chang Hasnain VTh RTh V VTh The I V characteristic of all of the circuit except the non linear element is called the load line EE40 Fall 2006 Slide 12 Prof Chang Hasnain 3 Solar cell Example of simple PN junction Photovoltaic Solar Cell What is a solar cell I D I S e qVD Device that converts sunlight into electricity How does it work 1 I optical ID A In simple configuration it is a diode made of PN junction Incident light is absorbed by material Creates electron hole pairs that transport through the material through Diffusion concentration gradient Drift due to electric field EE40 Fall 2006 kT Slide 13 in the dark VD V with incident light Operating point The load line a simple resistor PN Junction Diode Prof Chang Hasnain EE40 Fall 2006 I V characteristics of the device Slide 14 Prof Chang Hasnain Example 2 Photodiode An intrinsic region is placed between the p type and n type regions I V characteristics of a PN junction is given by eV I I S exp 1 I L kT Wj Wi region so that most of the electron hole pairs are generated in the depletion region Voc where Is is the saturation intensity depending on band gap and doping of the material and IL is the photocurrent generated due to light Isc Vm I m faster response time 10 GHz operation ID A Efficiency is defined as I m Vm FF Voc I sc Light Intensity Light Intensity Isc Short circuit current Imp Vmp Current and voltage FF is the Fill Factor EE40 Fall 2006 in the dark Voc Open circuit voltage VD V operating point at maximum power with incident light Slide 15 Prof Chang Hasnain EE40 Fall 2006 Slide 16 Prof Chang Hasnain 4 Photodetector Circuit Using Load Line I RTh I operating points under different light conditions VTh V As light shines on the photodiode carriers are generated by absorption These excess V Th carriers are swept by the electric field at the junction creating drift current which is same direction as the reverse bias current and hence negative current The current is proportional to light intensity and hence can provide a direct measurement …
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