Lecture 21 ANNOUNCEMENTS No coffee hour today Quiz

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Lecture #21 ANNOUNCEMENTS • No coffee hour today • Quiz #3 on Friday (March

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 1 Spring 2007 EE 130 Lecture 21, Slide 1

Turn-On Transient Again, consider a p+n diode (Qp >> Qn): i(t) Dpn(x) t xn

Turn-On Transient Again, consider a p+n diode (Qp >> Qn): i(t) Dpn(x) t xn x v. A(t) t For t > 0: 2 Spring 2007 EE 130 Lecture 21, Slide 2

 • By separation of variables and integration, we have • If we assume

• By separation of variables and integration, we have • If we assume that the build-up of stored charge occurs quasi-statically so that then 3 Spring 2007 EE 130 Lecture 21, Slide 3

 • If tp is large, then the time required to turn on the

• If tp is large, then the time required to turn on the diode is approximately DQ/IF 4 Spring 2007 EE 130 Lecture 21, Slide 4

Summary of Important Concepts • Under forward bias, minority carriers are injected into the

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 5 Spring 2007 EE 130 Lecture 21, Slide 5

Summary of Important Concepts (cont. ) • The ideal diode equation stipulates the relationship

Summary of Important Concepts (cont. ) • The ideal diode equation stipulates the relationship between JN(-xp) and JP(xn) Ø If holes are forced to flow across a forward-biased junction, then electrons must also be injected across the junction 6 Spring 2007 EE 130 Lecture 21, Slide 6

Summary of Important Concepts (cont. ) • Under reverse bias, minority carriers are collected

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 7 Spring 2007 EE 130 Lecture 21, Slide 7

Varactor Diodes • Voltage-controlled capacitor – Used in oscillators and detectors (e. g. FM

Varactor Diodes • Voltage-controlled capacitor – Used in oscillators and detectors (e. g. FM demodulation circuits in your radios) – Response changes by tailoring doping profile: 8 Spring 2007 EE 130 Lecture 21, Slide 8

Tunnel Diodes • Degenerately doped such that EFp < Ev and EFn > Ec

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 9 Spring 2007 EE 130 Lecture 21, Slide 9

Tunnel Diodes (cont. ) 10 Spring 2007 EE 130 Lecture 21, Slide 10

Tunnel Diodes (cont. ) 10 Spring 2007 EE 130 Lecture 21, Slide 10

Optoelectronic Diodes 11 Spring 2007 EE 130 Lecture 21, Slide 11

Optoelectronic Diodes 11 Spring 2007 EE 130 Lecture 21, Slide 11

12 Spring 2007 EE 130 Lecture 21, Slide 12

12 Spring 2007 EE 130 Lecture 21, Slide 12

p-i-n Photodiodes • W Wi-region, so most carriers are generated in the depletion region

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 13 Spring 2007 EE 130 Lecture 21, Slide 13

Light Emitting Diodes (LEDs) • LEDs are typically made of compound semiconductors (direct bandgap)

Light Emitting Diodes (LEDs) • LEDs are typically made of compound semiconductors (direct bandgap) 14 Spring 2007 EE 130 Lecture 21, Slide 14

Organic LEDs • Some organic materials exhibit semiconducting properties – OLEDs are attractive for

Organic LEDs • Some organic materials exhibit semiconducting properties – OLEDs are attractive for low-cost, high-quality flatpanel displays 15 Spring 2007 EE 130 Lecture 21, Slide 15