MIT6_012S09_lec16 - Lecture 16 The pn Junction Diode(III...

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Lecture 16 The pn Junction Diode (III) Outline I-V Characteristics (Review) Small-signal equivalent circuit model • Carrier charge storage –Diffusion capacitance Reading Assignment: Howe and Sodini; Chapter 6, Sections 6.4 - 6.5 1 6.012 Spring 2009 Lecture 16
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1. I-V Characteristics (Review) Diode Current Equation: V D ( ) I = I e V th 1 D o 2 6.012 Spring 2009 Lecture 16
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Physics of forward bias: Diode Current equation: I D = I o exp qV D 1 kT Junction potential φ J (potential drop across SCR) reduced by |V D | minority carrier injection into QNRs Minority carrier diffusion through QNRs Minority carrier recombination at contacts to the QNRs (and surfaces) Large supply of carriers injected into the QNRs I D exp qV D kT 3 6.012 Spring 2009 Lecture 16
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Physics of reverse bias: I D = I o exp qV D 1 kT Junction potential φ J (potential drop across SCR) increased by |V D | 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 D I ≈ − I D o 4 6.012 Spring 2009 Lecture 16
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2. Small-signal equivalent circuit model If v d small enough, linearize exponential characteristics: ( ) d o D d v kT I I q i + = Examine effect of small signal overlapping
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