1. Spring 2007EE130 Lecture 20, Slide 1 Lecture #20 OUTLINE pn Junctions (cont’d) – small-signal model – transient response turn-off Reading: Chapters 7, 8

2. Spring 2007EE130 Lecture 20, Slide 2 Small-Signal Model of the Diode CR=1/G vava i Small-signal conductance : Small signal equivalent circuit: + 

3. Spring 2007EE130 Lecture 20, Slide 3 Review: Charge Storage in pn-Diode

4. Spring 2007EE130 Lecture 20, Slide 4 2 types of capacitance associated with a pn junction: 1.C J depletion capacitance (due to variation of depletion charge) 2.C D diffusion capacitance (due to variation of stored minority charge in the quasi-neutral regions) For a one-sided p + n junction, Q P >> Q N so Q = Q P + Q N  Q P :

5. Spring 2007EE130 Lecture 20, Slide 5 What are three ways to reduce C dep ? Depletion Capacitance np W “insulator”conductor

6. Spring 2007EE130 Lecture 20, Slide 6 Total pn-Junction Capacitance C D dominates at moderate to high forward biases C dep dominates at low forward biases, reverse biases R=1/G vava i + 

7. Spring 2007EE130 Lecture 20, Slide 7 C J -vs.-V A (Reverse Bias)

8. Spring 2007EE130 Lecture 20, Slide 8 If the slope of the (1/C dep ) 2 vs. V A characteristic is -2x10 23 F -2 V -1, the intercept is 0.84V, and A is 1  m 2, find the lighter and heavier doping concentrations N l and N h. Solution: Example

9. Spring 2007EE130 Lecture 20, Slide 9 Summary: Small Signal Model Depletion capacitance Diffusion capacitance Conductance

10. Spring 2007EE130 Lecture 20, Slide 10 Transient Response of pn Diode Suppose a pn-diode is forward biased, then suddenly turned off at time t = 0. Because of C D, 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.

11. Spring 2007EE130 Lecture 20, Slide 11 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

12. Spring 2007EE130 Lecture 20, Slide 12 Decay of Stored Charge Consider a p + n diode (Q p >> Q n ): t i(t)i(t) t vA(t)vA(t) tsts tsts For t > 0: pn(x)pn(x)

13. Spring 2007EE130 Lecture 20, Slide 13 Examples (qualitative) t i(t)i(t) tsts Increase I F t i(t)i(t) tsts Increase I R t i(t)i(t) tsts Decrease  p

14. Spring 2007EE130 Lecture 20, Slide 14 Storage Delay Time t s t s 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 = t s, noting that and making the approximation We conclude that