MIT6_012S09_lec15

# MIT6_012S09_lec15 - Lecture 15 The pn Junction Diode(II...

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Lecture 15 The pn Junction Diode (II) Outline I-V characteristics Forward Bias Reverse Bias Reading Assignment: Howe and Sodini; Chapter 6, Sections 6.4 - 6.5 6.012 Spring 2009 Lecture 15 1

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F n p n F p 1. I-V Characteristics (contd.) Diode Current equation: I = I o e V V th 1 Physics of forward bias: Potential drop across SCR reduced by V minority carrier injection in QNRs Minority carrier diffusion through QNRs Minority carrier recombination at contacts to the QNRs (and surfaces) Large supply of carriers injected into the QNRs V I e [ V th ] 6.012 Spring 2009 Lecture 15 2
F n p n F p Physics of reverse bias: Potential drop across SCR increased by V minority carrier extraction from QNRs Minority carrier diffusion through QNRs Minority carrier generation at surfaces & contacts of QNRs Very small supply of carriers available for extraction I saturates to small value 6.012 Spring 2009 Lecture 15 3

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Development of analytical current model: 1. Calculate the concentration of minority carriers at edges of SCR; 2. Find the spatial distribution of the minority carrier concentrations in each QNR; 3. Calculate minority carrier diffusion current at SCR edge. 4. Sum minority carrier electron and hole diffusion currents at SCR edge. x -x p x n W n - W p contact to contact to p region n p ( x ) p n ( x ) (p-type) (n-type) p n x n ( ) p no e V D V th / = n p x p ( ) n po e V D V / = p n W n ( ) p = << p no << n po n region n p (-W p )=n po Reverse Bias 6.012 Spring 2009 Lecture 15 4
Total and Excess Concentrations Forward Bias (Step 1) x -x p x n W n - W p p n x n ( ) n p x p ( ) n p (x) p n

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MIT6_012S09_lec15 - Lecture 15 The pn Junction Diode(II...

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