Lecture 15 - pn Junction IV characteristics

Lecture 15 - pn Junction IV characteristics - pn Junction...

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1 pn Junction Electrostatics: Summary Band Diagram Electric Field E =1/ q (d E C /d x ) Charge Density ρ = ε (d E /d x ) Potential V =-1/ q ( E C - E REF ) () V V N N K W A bi D A S + = 0 2 max E EE360 – Lecture 15 = 2 bi ln i D A n N N q kT V N N q D A Diagrams from Pierret. pn Junction under Applied Bias • Apply a bias, V A, across the diode . • Assume the voltage drops across the contacts and neutral regions are negligible. We need only adjust V bi in our results to account for the applied bias . For example, From Pierret. EE360 – Lecture 15 V V N N N N q K W A bi D A D A S + = 0 2 V N N N N q K W bi D A D A S + = 0 2 No bias With applied bias
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2 Effects of Applied Bias: Band Diagrams • Of course, the band diagrams also depends on applied bias • Recall • Therefore, a forward bias lowers the energy of the p-type side with respect to the n-type side. • Likewise, a reverse bias raises the energy of the p-type side with EE360 – Lecture 15 respect to the n-type side. • When we bias the diode, we must use quasi-Fermi levels, because we are no longer in equilibrium. From Pierret. Effects of Applied Bias: Depletion Width Depletion width: ( ) V V N N K W D A S + = 0 2 ε V N N K W D A S + = 0 2 N N q A bi D A N N q bi D A No bias With applied bias x n EE360 – Lecture 15 Depletion width increases with reverse bias and decreases with forward bias. x p From Pierret. From Pierret.
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3 Effects of Applied Bias: Electric Field • With applied bias ρ , x n , and x p change, so E changes as well E max increases in reverse bias and decreases in forward bias • If E changes then V must 0 0 max ε S p A S n D K x qN K x qN = = E From Pierret. EE360 – Lecture 15 If changes then must change also From Pierret. Applied Bias Example EE360 – Lecture 15
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4 pn Junction Current-Voltage Characteristics Objective : Understand the physics and models of semiconductor devices including diodes . Questions to be answered : • Why do we observe large diode currents under forward bias and small diode currents under reverse bias? EE360 – Lecture 15 • How do we define an “ideal” diode? • How do arrive at a mathematical expression for the I-V relationship for an ideal diode? pn Junction Qualitative I-V (This is critical!) I-V Relationship ( ) 0 1 A qV kT II e = Diagrams from Pierret. EE360 – Lecture 15 Reverse Bias Equilibrium Forward Bias
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5 pn Junction Qualitative I-V: Equilibrium I-V Relationship From Pierret p. 236 EE360 – Lecture 15 Electrons diffuse from n-side to p-side and drift from p-side to n-side.
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This note was uploaded on 04/20/2008 for the course EE 360 taught by Professor Hastings during the Spring '08 term at Kentucky.

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Lecture 15 - pn Junction IV characteristics - pn Junction...

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