Lecture 20 OUTLINE pn Junctions contd smallsignal model

Lecture #20 OUTLINE • pn Junctions (cont’d) – small-signal model – transient response • turn-off Reading: Chapters 7, 8 Spring 2007 EE 130 Lecture 20, Slide 1 1

Small-Signal Model of the Diode Small signal equivalent circuit: i + va R=1/G C Small-signal conductance : Spring 2007 EE 130 Lecture 20, Slide 2 2

Review: Charge Storage in pn-Diode Spring 2007 EE 130 Lecture 20, Slide 3 3

2 types of capacitance associated with a pn junction: 1. CJ depletion capacitance 2. CD diffusion capacitance (due to variation of depletion charge) (due to variation of stored minority charge in the quasineutral regions) For a one-sided p+n junction, QP >> QN so Q = QP + QN QP: Spring 2007 EE 130 Lecture 20, Slide 4 4

Depletion Capacitance p n W conductor “insulator” conductor What are three ways to reduce Cdep? Spring 2007 EE 130 Lecture 20, Slide 5 5

Total pn-Junction Capacitance i + va R=1/G • CD dominates at moderate to high forward biases • Cdep dominates at low forward biases, reverse biases Spring 2007 EE 130 Lecture 20, Slide 6 6

CJ-vs. -VA (Reverse Bias) Spring 2007 EE 130 Lecture 20, Slide 7 7

Example If the slope of the (1/Cdep)2 vs. VA characteristic is -2 x 1023 F-2 V-1, the intercept is 0. 84 V, and A is 1 mm 2, find the lighter and heavier doping concentrations Nl and Nh. Solution: Spring 2007 EE 130 Lecture 20, Slide 8 8

Summary: Small Signal Model Depletion capacitance Conductance Diffusion capacitance Spring 2007 EE 130 Lecture 20, Slide 9 9

Transient Response of pn Diode • Suppose a pn-diode is forward biased, then suddenly turned off at time t = 0. Because of CD, the voltage across the pn junction depletion region cannot be changed instantaneously. The delay in switching between the ON and OFF states is due to the time required to change the amount of excess minority carriers stored in the quasi-neutral regions. Spring 2007 EE 130 Lecture 20, Slide 10 10

Turn-Off Transient • In order to turn the diode off, the excess minority carriers must be removed by net carrier flow out of the quasi-neutral regions and/or recombination – Carrier flow is limited by the switching circuitry Spring 2007 EE 130 Lecture 20, Slide 11 11

Decay of Stored Charge Consider a p+n diode (Qp >> Qn): Dpn(x) i(t) ts t v. A(t) For t > 0: Spring 2007 t ts EE 130 Lecture 20, Slide 12 12

Examples (qualitative) Increase IF i(t) ts Spring 2007 Decrease tp Increase IR t i(t) ts EE 130 Lecture 20, Slide 13 t ts t 13

Storage Delay Time ts • ts is the primary “figure of merit” used to characterize the transient response of pn junction diodes • By separation of variables and integration from t = 0+ to t = ts, noting that and making the approximation We conclude that Spring 2007 EE 130 Lecture 20, Slide 14 14