ch15 - Topic 15 : Transient and AC Analysis of P-N...

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Unformatted text preview: Topic 15 : Transient and AC Analysis of P-N Junctions Fall 2007 ECE 3500 - Semiconductor Materials and Devices Learning Objectives In this topic you will learn: How to use the charge control approach to analyze p-n junctions. How a p-n junction responds to a current step turn-off transient and a reverse recovery transient. The sources of capacitance in a p-n junction and what dominates in forward and reverse bias. How to calculate the capacitance of a p-n junction under different biasing conditions. Charge Control Approach for P-N Junction DC Analysis An alternative approach to determining the current flow in a P-N junction is the charge control approach . In this method, the current flowing into the p- or n regions is taken to be the total charge stored in the excess carrier distribution, n p Charge Control Approach to P-N Junction Analysis (contd) Using this approach, the total current flowing into the n-region of a P-N junction = Q p / , where Q p represents the total charge stored in the excess hole profile. This is given by: Q qA p x dx p n n = ( ) p Charge Control Approach to P-N Junction Analysis (contd) Since we can express the hole current as: Now since we can express the hole current as: p x p e n n x L n p ( ) =- I Q qAL p p p p p p n = = p p e n n qV kT = - ( ) 1 I x qA D p L e p n p n p qV kT ( ) ( )( ) = = - 0 1 Charge Control Approach to P-N Junction Analysis (contd) Using the same approach for the electron current into the p-region gives: Thus, the total current through the p-n junction is given by: This is the same diode equation as found earlier. I x qA D n L e n p n p n qV kT ( ) ( )( ) = = - - 0 1 I qA D L p D L n e p p n n n p qV kT = + - ( )( ) 1 Transient Analysis of P-N Junction Two cases considered so far: Thermal Equilibrium Steady State (constant applied bias) Now we consider: Transient (time-varying applied bias)...
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ch15 - Topic 15 : Transient and AC Analysis of P-N...

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