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Unformatted text preview: Topic 15 : Transient and AC Analysis of PN 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 pn junctions. How a pn junction responds to a current step turnoff transient and a reverse recovery transient. The sources of capacitance in a pn junction and what dominates in forward and reverse bias. How to calculate the capacitance of a pn junction under different biasing conditions. Charge Control Approach for PN Junction DC Analysis An alternative approach to determining the current flow in a PN 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, τ n τ p Charge Control Approach to PN Junction Analysis (cont’d) Using this approach, the total current flowing into the nregion of a PN 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 = ∞ ∫ δ ( ) τ p Charge Control Approach to PN Junction Analysis (cont’d) Since we can express the hole current as: Now since we can express the hole current as: δ p x p e n n x L n p ( ) = ∆ I Q qAL p p p p p p n = = τ τ ∆ ∆ 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 PN Junction Analysis (cont’d) Using the same approach for the electron current into the pregion gives: Thus, the total current through the pn 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 PN Junction Two cases considered so far: Thermal Equilibrium Steady State (constant applied bias) Now we consider: Transient (timevarying applied bias)...
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 Spring '07
 singh
 Trigraph, Pn junction, Qp, Deeplevel transient spectroscopy, pn junction

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