1. Lecture #21 ANNOUNCEMENTS • No coffee hour today • Quiz #3 on Friday (March 9) • Material of HW #5 & #6 (Lectures 13-17) • Closed book, no calculators; 3 pages of notes allowed OUTLINE • pn Junctions: • transient response: turn-on • Summary of important pn-diode concepts • pn diode applications Reading: Chapters 8 & 9 EE130 Lecture 21, Slide 1

2. Turn-On Transient • Again, consider a p+n diode (Qp >> Qn): i(t) Dpn(x) t vA(t) x xn t For t > 0: EE130 Lecture 21, Slide 2

3. By separation of variables and integration, we have • If we assume that the build-up of stored charge occurs quasi-statically so that then EE130 Lecture 21, Slide 3

4. If tp is large, then the time required to turn on the diode is approximately DQ/IF EE130 Lecture 21, Slide 4

5. Summary of Important Concepts • Under forward bias, minority carriers are injected into the quasi-neutral regions of the diode • Current flowing across junction is comprised of hole and electron components • In order for one of these components to be dominant, the junction must be asymmetrically doped EE130 Lecture 21, Slide 5

6. Summary of Important Concepts (cont.) • The ideal diode equation stipulates the relationship between JN(-xp) and JP(xn) • If holes are forced to flow across a forward-biased junction, then electrons must also be injected across the junction EE130 Lecture 21, Slide 6

7. Summary of Important Concepts (cont.) • Under reverse bias, minority carriers are collected into the quasi-neutral regions of the diode • Minority carriers within a diffusion length of the depletion region will diffuse into the depletion region and then be swept across the junction by the electric field. • Current flowing in a reverse-biased diode depends on the rate at which minority carriers are supplied in the quasi-neutral regions EE130 Lecture 21, Slide 7

8. Varactor Diodes • Voltage-controlled capacitor • Used in oscillators and detectors (e.g. FM demodulation circuits in your radios) • Response changes by tailoring doping profile: EE130 Lecture 21, Slide 8

9. Tunnel Diodes • Degenerately doped such that EFp < Ev and EFn > Ec • Can achieve negative differential resistance • useful in high-speed circuits and perhaps static memories EE130 Lecture 21, Slide 9

10. Tunnel Diodes (cont.) EE130 Lecture 21, Slide 10

11. Optoelectronic Diodes EE130 Lecture 21, Slide 11

12. EE130 Lecture 21, Slide 12

13. p-i-n Photodiodes • W  Wi-region, so most carriers are generated in the depletion region faster response time (~10 GHz operation) • Operate near avalanche to amplify signal EE130 Lecture 21, Slide 13

14. Light Emitting Diodes (LEDs) • LEDs are typically made of compound semiconductors (direct bandgap) EE130 Lecture 21, Slide 14

15. Organic LEDs • Some organic materials exhibit semiconducting properties • OLEDs are attractive for low-cost, high-quality flat-panel displays EE130 Lecture 21, Slide 15