1Lecture 20, Slide 1EECS40, Fall 2003 Prof. KingLecture #20OUTLINE• The pn junction (cont’d)– reverse breakdown• Load line analysis method• Diode models• Diode applicationsReference Reading• Rabaey et al.(finish Chapter 3.2)• Howe & Sodini– Chapter 6.4.1 - 6.4.2• Schwarz and Oldham(finish Chapter 13.2)Lecture 20, Slide 2EECS40, Fall 2003 Prof. Kingpn-Junction Reverse Breakdown• As the reverse bias voltage increases, the peak electric field in the depletion region increases. When the electric field exceeds a critical value (Ecrit≅ 2x105V/cm), the reverse current shows a dramatic increase:ID(A)VD(V)reverse (leakage) currentforward currentbreakdown voltageVBD2Lecture 20, Slide 3EECS40, Fall 2003 Prof. KingCircuit Analysis with a Nonlinear ElementSince the pn junction is a nonlinear circuit element, its presence complicates circuit analysis.(Node and loop equations become transcendental.)VTh+−+V–RThILecture 20, Slide 4EECS40, Fall 2003 Prof. KingLoad Line Analysis Method1. Graph the I-V relationships for the non-linear element and for the rest of the circuit2. The operating point of the circuit is found from the intersection of these two curves.VTh+−+V–RThIIVThe I-V characteristic of all of the circuit except the non-linear element is called the load lineVThVTh/RThoperating point3Lecture 20, Slide 5EECS40, Fall 2003 Prof. Kingreverse biasforward bias•A rectifier passes current only in one direction. •A perfect rectifier has the following properties:• when ID> 0, VD= 0• when VD< 0, ID= 0Perfect Rectifier Model of pn DiodeID(A)VD(V)ID+VD–+VD–IDCircuit symbol I-V characteristicDiode behaves like a switch: • closed in forward bias mode • open in reverse bias modeSwitch modelLecture 20, Slide 6EECS40, Fall 2003 Prof. KingLarge-Signal Diode Modelreverse biasforward biasID(A)VD(V)ID+VD–+VD–IDCircuit symbol I-V characteristicSwitch modelVDon+−VDonRULE 1: When ID> 0, VD= VDonRULE 2: When VD< VDon, ID= 0Diode behaves like a voltage source in series with a switch: • closed in forward bias mode • open in reverse bias mode For a Si pn diode, VDon≅ 0.7 V4Lecture 20, Slide 7EECS40, Fall 2003 Prof. KingA diode has only two states:• forward biased: ID> 0, VD= 0 V (or 0.7 V)• reverse biased: ID=0, VD< 0 V (or 0.7 V)Procedure:1. Guess the state(s) of the diode(s)2. Check to see if KCL and KVL are obeyed.3. If KCL and KVL are not obeyed, refine your guess4. Repeat steps 1-3 until KCL and KVL are obeyed.Example: vs(t)If vs(t) > 0 V, diode is forward biased (else KVL is disobeyed – try it)If vs(t) < 0 V, diode is reverse biased (else KVL is disobeyed – try it)How to Analyze Circuits with Diodes+−+vR(t)–Lecture 20, Slide 8EECS40, Fall 2003 Prof. King“rectified” version of input waveform:Application Example #1(using the Perfect Rectifier model)vs(t)+−+vR(t)–vs(t)vs(t)tt5Lecture 20, Slide 9EECS40, Fall 2003 Prof. KingApplication Example #2(using the Perfect Rectifier model)vs(t)+−+vR(t)–vs(t)tRCvR(t)tLecture 20, Slide 10EECS40, Fall 2003 Prof. KingintegratedcircuitA 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:6Lecture 20, Slide 11EECS40, Fall 2003 Prof. KingLight Emitting Diode (LED)•LEDs are made of compound semiconductor materials– Carriers diffuse across a forward-biased junctionand recombine in the quasi-neutral regions Æ optical emissionLecture 20, Slide 12EECS40, Fall 2003 Prof. KingDiode Logic• Diodes can be used to perform logic functions:AND gateoutput voltage is high only if both A and B are highABRVccCOR gateoutput voltage is high if either (or both) A and B are highABRCInputs A and B vary between 0 Volts (“low”) and Vcc(“high”)Between what voltage levels does C
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