Lecture #21Turn-On TransientSlide 3Slide 4Summary of Important ConceptsSummary of Important Concepts (cont.)Slide 7Varactor DiodesTunnel DiodesTunnel Diodes (cont.)Optoelectronic DiodesSlide 12p-i-n PhotodiodesLight Emitting Diodes (LEDs)Organic LEDsEE130 Lecture 21, Slide 1 Spring 2007Lecture #21ANNOUNCEMENTS•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 allowedOUTLINE •pn Junctions:–transient response: turn-on•Summary of important pn-diode concepts•pn diode applicationsReading: Chapters 8 & 9EE130 Lecture 21, Slide 2 Spring 2007xTurn-On TransientAgain, consider a p+n diode (Qp >> Qn):ti(t)tvA(t)0pxxnqADidxdpnFor t > 0:pn(x)xnEE130 Lecture 21, Slide 3 Spring 2007•By separation of variables and integration, we have•If we assume that the build-up of stored charge occurs quasi-statically so thatthen 0for ττtQIQidtdQppFppp ptpFpeItQτ/1τ)( pkTqvpdiffusionpAeIItQ τ1τ)(/0 ptFAeIIqkTtvτ/011ln)(EE130 Lecture 21, Slide 4 Spring 2007•If p is large, then the time required to turn on the diode is approximately Q/IFjpQQQ whereEE130 Lecture 21, Slide 5 Spring 2007Summary 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 dopedEE130 Lecture 21, Slide 6 Spring 2007Summary 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 junctionEE130 Lecture 21, Slide 7 Spring 2007Summary 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 regionsEE130 Lecture 21, Slide 8 Spring 2007Varactor Diodes•Voltage-controlled capacitor–Used in oscillators and detectors (e.g. FM demodulation circuits in your radios)–Response changes by tailoring doping profile:21mnbiVVforVCrnrjEE130 Lecture 21, Slide 9 Spring 2007Tunnel Diodes•Degenerately doped such that EFp < Ev and EFn > Ec•Can achieve negative differential resistance–useful in high-speed circuits and perhaps static memoriesEE130 Lecture 21, Slide 10 Spring 2007Tunnel Diodes (cont.)EE130 Lecture 21, Slide 11 Spring 2007)(~)()1(A0NPopNPopopopkTVqLLqAgLWLqAgIIeIIOptoelectronic DiodesEE130 Lecture 21, Slide 12 Spring 20071ln0A opnLpLLLqkTIocgVVpnnnppnpEE130 Lecture 21, Slide 13 Spring 2007p-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 signalEE130 Lecture 21, Slide 14 Spring 2007Light Emitting Diodes (LEDs)•LEDs are typically made of compound semiconductors (direct bandgap)EE130 Lecture 21, Slide 15 Spring 2007Organic LEDs•Some organic materials exhibit semiconducting properties–OLEDs are attractive for low-cost, high-quality flat-panel
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