ECE162-Lecture-12-PN-Junction-Diodes

ECE162-Lecture-12-PN-Junction-Diodes - ECE 162 FUNDAMENTALS...

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ECE 162 FUNDAMENTALS OF MICROELECTRONIC DEVICES Lecture 12 PN-JUNCTION DIODES Professor Hisham Z. Massoud Department of Electrical and Computer Engineering Fitzpatrick Building, Room 3521 Duke University, Durham, NC 27708–0291 [email protected] https://courses.duke.edu/webapps/portal/frameset.jsp ECE 162 Chapter 08 – PN-Junction Diodes Lecture 12.1
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LECTURE 12 - TOPICS 8. Small-Signal Capacitance-Voltage Characteristics 9. PN-Junction Diode Dynamic Charge-Control Model 10. Temperature Effects in PN-Junction Diodes 11. PN-Junction Diode S PICE Model ECE 162 Chapter 08 – PN-Junction Diodes Lecture 12.2
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8. SMALL-SIGNAL C pn ( V P N ) CHARACTERISTICS 8.6. Capacitance of an Abrupt One-Sided PN + -Junction Diode Consider a reverse-biased abrupt one-sided PN + -junction diode ( N + d N - a ). The diode capacitance per unit area under reverse-bias conditions consists mainly of the depletion capacitance and can be approximated for N + d N - a and V PN < 0 as C pn ( V PN ) C pn · dep ( V PN ) , s q Si 2 · N - a N + d ( N - a + N + d ) · 1 ( V bi - V PN ) , s q Si N - a 2 ( V bi - V PN ) . This relationship can be rearranged to read h C pn ( V PN ) i 2 q Si N - a 2 ( V bi - V PN ) . ECE 162 Chapter 08 – PN-Junction Diodes Lecture 12.3
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8. SMALL-SIGNAL C pn ( V P N ) CHARACTERISTICS 8.6. Capacitance of an Abrupt One-Sided PN + -Junction Diode Inverting this relationship yields 1 C pn ( V PN ) 2 2 V bi q Si N - a - 2 q Si N - a V PN . Plotting 1 /C pn ( V PN ) 2 vs V PN yields a straight line whose slope is Slope = - 2 q Si N - a , and intercept with the bias-voltage axis is at Intercept = V bi . ECE 162 Chapter 08 – PN-Junction Diodes Lecture 12.4
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8. SMALL-SIGNAL C pn ( V P N ) CHARACTERISTICS 8.6. Capacitance of an Abrupt One-Sided PN + -Junction Diode A d C pn ( V PN ) 2 Intercept = V bi Slope = - 2 q Si N - a - 4 - 3 - 2 - 1 0 1 2 PN-Junction Bias Voltage, V PN (V) ECE 162 Chapter 08 – PN-Junction Diodes Lecture 12.5
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8. SMALL-SIGNAL C pn ( V P N ) CHARACTERISTICS 8.7. Transit Time and Capacitance of a Short-Base Diode The concentration distribution of minority holes in the n -side quasi-neutral region of a forward-biased, one-sided, abrupt P + N-junction diode with a short-base n -type region is considered. It is of interest to derive an expression for the transit time of a hole in a short-base n -type quasi-neutral region. We assume that the hole velocity is v p · x,N ( x ) at any position x ranging from W d,N to W N . The hole transit time τ tr · p,N in the short-base n -type quasi-neutral region is given by τ tr · p,N = W N Z W d,N dx v p · x,N ( x, V PN ) . ECE 162 Chapter 08 – PN-Junction Diodes Lecture 12.6
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8. SMALL-SIGNAL C pn ( V P N ) CHARACTERISTICS 8.7. Transit Time and Capacitance of a Short-Base Diode The hole velocity is related to the hole current density through the hole flux F p · x,N ( x, V PN ) which is given by F p · x,N ( x, V PN ) = v p,N ( x, V PN ) p N ( x, V PN ) . This flux, multiplied by q , yields J p · x · diff,N ( x, V PN ), the hole diffusion current density. The hole velocity in the short-base n -type quasi-neutral region is then given by v p · x,N ( x, V PN ) = F p · x,N ( x, V PN ) p N ( x, V PN ) = J p · x · diff,N ( x, V PN ) q p N ( x, V PN ) .
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