Spring 2007EE130 Lecture 21, Slide 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

Spring 2007EE130 Lecture 21, Slide 2 x Turn-On Transient Again, consider a p + n diode (Q p >> Q n ): t i(t)i(t) t vA(t)vA(t) For t > 0: pn(x)pn(x) xnxn

Spring 2007EE130 Lecture 21, Slide 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

Spring 2007EE130 Lecture 21, Slide 4 If  p is large, then the time required to turn on the diode is approximately  Q/I F

Spring 2007EE130 Lecture 21, Slide 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

Spring 2007EE130 Lecture 21, Slide 6 Summary of Important Concepts (cont.) The ideal diode equation stipulates the relationship between J N (-x p ) and J P (x n )  If holes are forced to flow across a forward-biased junction, then electrons must also be injected across the junction

Spring 2007EE130 Lecture 21, Slide 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

Spring 2007EE130 Lecture 21, Slide 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:

Spring 2007EE130 Lecture 21, Slide 9 Tunnel Diodes Degenerately doped such that E Fp E c Can achieve negative differential resistance –useful in high-speed circuits and perhaps static memories

Spring 2007EE130 Lecture 21, Slide 10 Tunnel Diodes (cont.)

Spring 2007EE130 Lecture 21, Slide 11 Optoelectronic Diodes

Spring 2007EE130 Lecture 21, Slide 12

Spring 2007EE130 Lecture 21, Slide 13 p-i-n Photodiodes W  W i-region, so most carriers are generated in the depletion region  faster response time (~10 GHz operation) Operate near avalanche to amplify signal

Spring 2007EE130 Lecture 21, Slide 14 Light Emitting Diodes (LEDs) LEDs are typically made of compound semiconductors (direct bandgap)

Spring 2007EE130 Lecture 21, Slide 15 Organic LEDs Some organic materials exhibit semiconducting properties –OLEDs are attractive for low-cost, high-quality flat- panel displays