Lecture 21 ANNOUNCEMENTS No coffee hour today Quiz
- Slides: 15
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 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 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 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 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 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 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 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 • 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
Optoelectronic Diodes 11 Spring 2007 EE 130 Lecture 21, Slide 11
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 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) 14 Spring 2007 EE 130 Lecture 21, Slide 14
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
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