E E 339 - CN13-pn capacitance and switching

E E 339 - CN13-pn capacitance and switching - P-N...

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1 P-N junctions: Capacitance and Switching P N junctions: 1 Capacitance Depletion capacitance “Diffusion” capacitance Outline Diffusion capacitance Switching “Diffusion charge” and storage delay times Depletion layer charge and switching transients Summary 2
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2 Consider small signal (as opposed to large signal/switching) differential capacitance Q C Capacitance of p-n junctions evaluated about some dc bias ( V dc ). There are two types of capacitance to consider: depletion capacitance ” or (less accurately) “ junction capacitance due to changes in the depletion region size with V changes in voltage, which dominates under reverse bias . diffusion capacitance associated with the excess carrier concentrations δ n ( x ) δ p ( ) in the quasi-neutral regions , which can dominate under forward bias for short diodes . 3 Figure 5—13, modified Effects of bias on a p-n junction: transition region width and peak electric field, Depletion capacitance electrostatic potential, and energy band diagram (a) equilibrium, (b) forward bias, and (c) reverse bias. W ( V ) 4
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3 To obtain the total ( W ) and partial ( x p and n ) depletion widths under bias V we need only take the equilibrium results from before for o , p,o and n,o and replace by –V to obtain the non-equilibrium values , and : V V N N V V 1 1 2 2 and ) ( 2 d o d p N V V W N x d a o d a d a o N N q N N q W ) ( ) ( ) ( d a a d a N N N q N N ) ( ) ( 2 d a d a o d a a n N N N N q V V W N N N x 5 Therefore the stored (negative) charge on the p-side of the de- pletion region (= negative of stored positive charge on n-side) is Therefore ) ( 1 ) ( 2 d a d a o a p depletion N N N N V V q A A N qx Q junction area Therefore, or ) ( ) ( 2 d a d a o depletion depletion N N N N V V q A V Q C A C just as for a parallel plate capacitor of thickness W … because the changes in stored charge occur at the outer edges of the depletion region . W depletion 6
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4 ) ( V Ax qN Q p a ) ( V Ax qN Q n d + h + h + e e ) ( V W V junction )] ( ) ( [ V x V V x A qN Q p p a ) ( V x p ) ( V V x p ) ( V V x n ) ( V x n )] ( ) ( [ V x V V x A qN Q n n d h + h e e “||” only to confirm that illustration is for positive change in applied voltages For future reference, note that the positively charged holes that are supplied to ( extracted from ) the p-side edge of the depletion region with increases (decreases) in the voltage drop from the p- side to n-side are supplied from (extracted to ) the positive p-side contact , and similarly for electrons from the negative n-side contact 7 Note that the differential capacitance is a function of the bias … … which can be used in reverse bias to provide a (dc) voltage variable
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This note was uploaded on 11/07/2011 for the course E E 339 taught by Professor Leonardregister during the Spring '11 term at University of Texas at Austin.

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E E 339 - CN13-pn capacitance and switching - P-N...

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