Lecture 20 OUTLINE The pn junction cont d reverse breakdown Load line analysis method Diode models Diode applications Reference Reading Rabaey et al finish Chapter 3 2 Howe Sodini Chapter 6 4 1 6 4 2 Schwarz and Oldham finish Chapter 13 2 EECS40 Fall 2003 Lecture 20 Slide 1 Prof King pn 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 2x105 V cm the reverse current shows a dramatic increase reverse leakage current ID A forward current breakdown voltage EECS40 Fall 2003 VBD Lecture 20 Slide 2 VD V Prof King 1 Circuit Analysis with a Nonlinear Element Since the pn junction is a nonlinear circuit element its presence complicates circuit analysis Node and loop equations become transcendental RTh I VTh V EECS40 Fall 2003 Lecture 20 Slide 3 Prof King Load Line Analysis Method 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 I RTh I VTh VTh RTh operating point V V VTh The I V characteristic of all of the circuit except the non linear element is called the load line EECS40 Fall 2003 Lecture 20 Slide 4 Prof King 2 Perfect Rectifier Model of pn Diode Circuit symbol ID I V characteristic ID A VD VD forward bias reverse bias Switch model ID VD V A rectifier passes current only in one direction A perfect rectifier has the following properties when ID 0 VD 0 Diode behaves like a switch closed in forward bias mode open in reverse bias mode when VD 0 ID 0 EECS40 Fall 2003 Lecture 20 Slide 5 Prof King Large Signal Diode Model Circuit symbol ID I V characteristic ID A VD Switch model ID forward bias reverse bias VDon VD V VDon VD For a Si pn diode VDon 0 7 V RULE 1 When ID 0 VD VDon RULE 2 When VD VDon ID 0 EECS40 Fall 2003 Lecture 20 Slide 6 Diode behaves like a voltage source in series with a switch closed in forward bias mode open in reverse bias mode Prof King 3 How to Analyze Circuits with Diodes A 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 guess 4 Repeat steps 1 3 until KCL and KVL are obeyed Example vR t vs t EECS40 Fall 2003 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 Lecture 20 Slide 7 Prof King Application Example 1 using the Perfect Rectifier model vs t vs t vR t t vs t rectified version of input waveform t EECS40 Fall 2003 Lecture 20 Slide 8 Prof King 4 Application Example 2 using the Perfect Rectifier model vs t C R vs t vR t t vR t t EECS40 Fall 2003 Lecture 20 Slide 9 Prof King Zener Diode A Zener diode is designed to operate in the breakdown mode reverse leakage current breakdown voltage VBD ID A forward current VD V Example R vs t VBD 15V EECS40 Fall 2003 Lecture 20 Slide 10 t integrated vo t circuit Prof King 5 Light Emitting Diode LED LEDs are made of compound semiconductor materials Carriers diffuse across a forward biased junction and recombine in the quasi neutral regions optical emission EECS40 Fall 2003 Lecture 20 Slide 11 Prof King Diode Logic Diodes can be used to perform logic functions AND gate OR gate output voltage is high only if both A and B are high output voltage is high if either or both A and B are high Vcc A R A B C C R B Inputs A and B vary between 0 Volts low and Vcc high Between what voltage levels does C vary EECS40 Fall 2003 Lecture 20 Slide 12 Prof King 6
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