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Unformatted text preview: Chapter 3 SolidState Diodes and Diode Circuits Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Chap 3 1 Chapter Goals • Understand diode structure and basic layout • Develop electrostatics of the pn junction • Explore various diode models including the mathematical model, the ideal diode model, and the constant voltage drop model • Understand the SPICE representation and model parameters for the diode • Define regions of operation of the diode (forward bias, reverse bias, and reverse breakdown) • Apply the various types of models in circuit analysis • Explore different types of diodes • Discuss the dynamic switching behavior of the pn junction diode • Explore diode applications • Practice simulating diode circuits using SPICE Chap 3 2 Diode Introduction • A diode is formed by joining an ntype semiconductor with a ptype semiconductor. • A pn junction is the interface between n and p regions. Diode symbol Chap 3 3 pn Junction Electrostatics Donor and acceptor concentration on either side of the junction. Concentration gradients give rise to diffusion currents. Chap 3 4 Drift Currents • Diffusion currents lead to localized charge density variations near the pn junction. • Gauss’ law predicts an electric field due to the charge distribution: • Assuming constant permittivity, • Resulting electric field gives rise to a drift current. With no external circuit connections, drift and diffusion currents cancel. There is no actual current, since this would imply power dissipation, rather the electric field cancels the diffusion current ‘tendency.’ ∇⋅ E = ρ c ε s E ( x ) = 1 ε s ρ ( x ) dx ∫ Chap 3 5 SpaceCharge Region Formation at the pn Junction Chap 3 6 Potential Across the Junction Charge Density Electric Field Potential φ j =  E ( x ) dx ∫ = V T ln N A N D n i 2 , V T = kT q Chap 3 7 Width of Depletion Region w d = ( x n + x p ) = 2 ε s q 1 N A + 1 N D φ j Combining the previous expressions, we can form an expression for the width of the spacecharge region, or depletion region. It is called the depletion region since the excess holes and electrons are depleted from the dopant atoms on either side of the junction. Chap 3 8 Width of Depletion Region (Example) Problem: Find builtin potential and depletionregion width for given diode Given data :On ptype side: N A = 10 17 /cm 3 on ntype side: N D = 10 20 /cm 3 Assumptions : Roomtemperature operation with V T = 0.025 V Analysis: m 113 . 1 1 2 μ φ ε = + = j D N A N q s d w ( 29 ( 29 ( 29 V 979 . /cm 10 /cm 10 /cm 10 V)ln (0.025 ln 6 20 3 20 3 17 2 = = = i D A T j n N N V φ Chap 3 9 Diode Electric Field (Example) • Problem: Find the electric field and size of the individual depletion layers on either side of a pn junction for a given diode • Given data : On the ptype side:...
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This note was uploaded on 03/29/2009 for the course ECE 51 taught by Professor Martinbrooke during the Fall '08 term at Duke.
 Fall '08
 MartinBrooke

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