11DiodesRectifiers, Zener diodeslight emitting diodes, laser diodesphotodiodes, optocouplersPrepared by Scott RobertsonFall 2007Physics 33302Impurity-doped semiconductors• Semiconductors (Ge, Si) have four valence electrons.• P and As have 5 valence electrons.These are electron donors and make n-type silicon. • B, Al, In have 3 valence electrons. These make “holes” or p-type silicon.Reading: D & H ch. 5; H & H23p-n junctions (diodes) are “one-way” for currentn-typep-type------------+ + + + + + + + + + + + If the p side is made positive, holes flow toward junction from the p side and electrons flow toward the junction from the n side. Current flows from one side to the other.If the n side is made positive, electrons are drawn away from the junction and holes are drawn away from the junction and NO current flows. AnodeCathode“the cartoon”“the symbol”4How a hole moves35I-V characteristics of diodes• With negative bias, diodes pass only leakage current. • With positive bias, the diode current increases exponentially with V. • The “knee” for Si is near 0.6 V.• Peak inverse voltage (PIV) is the maximum VLog I0.6 VIleak=1 μA maybe“PIV”breakdown occurs& damageVery highresistanceVery lowresistance6Application #1: radio receiver1N914 high frequency, small signal diodechanges 150 kHz AC to low frequency DCAntenna picks up radio frequency EarphonesEarphones cannot respond to 150 kHz AM radio frequency, but they do respond to the amplitude modulation. LC “tuner”47Application #2a: AC to DC115 VAC12 VAC (RMS) transformer+15 VDC outVouttC charges brieflyC dischargesVoltage has “ripple”16.7 ms“Half-wave rectifier”With 1N4002 diode100 peak inverse volts, 1 AAC input-15 VDC outoptional8Application #2b: AC to DC115 VACtransformerDC outVt8.3 ms “ripple”“full-wave rectifier”4 - diode “bridge” can be purchased as one unit-+For low ripple, 1/Cω should be much less than Rload.59Positive peak voltage detector+−VoutVAVB+−VAVBFollowerOutput goes positive so that VA= VB. Capacitor “remembers.”VoutVinWhy is this not practical?How can you fix it? 10Zener diodes are voltage regulatorsVIIleakBreakdown occursZener voltageVunregulatedVregulatedZener diodes are available for 3.3 V, 3.6 V, 3.9 V and up to 33 V. Do not exceed their power rating (< 1 watt usually) Iin–IoutZener power = (Iin-Iout)*VzenerIin= (Vunreg-Vreg)/RIoutSymbol has a Z in it611Light emitting diodesWikipedia photosThe longest wire is the anode.CathodeAnode12Light emitting diodes• Electron meets hole → recombination energy• Si has “indirect bandgap”Recombination in Si gives heat, not light• GaAs (a semiconductor) has a “direct band gap”and there is recombination light.Usually an n-type substrate with p-layer on top.• More exotic materials with different band gapsgive a wider range of wavelengths: GaN, InGaAs, GaAsP, InGaN, AlGaInP, etc.713The LED is biased forwardand the current should be limited by a resistor• Red is least expensive(brake lights, stop lights)• Blue is more expensive• UV is very very expensivebelow 250 nm. 470 Ω typ.5 V 10 mAtyp.Some LEDs are packaged with a resistor inside. Symbol shows light out14“White” LEDs use a Ce:YAG phosphorto absorb blue and emit yellow:blue + yellow is perceived as whiteWikipedia imageYAG = yttrium aluminum garnet815Color vs. Potential Difference @ 20 mAInfrared 1.6 V Red 1.8 V to 2.1 V Orange 2.2 V Yellow 2.4 V Green 2.6 V Blue 3.0 V-3.5 V Ultraviolet 3.5 VEnergy conservation: You can’t get a 3 eV blue photon from a 2 volt electron!Short wavelength LEDs require more bias voltage. Ref: WikipediaLEDs are an alternative Zener diodes at low voltages.16Applications• TV remotes (far IR)• Calculator displays• Red only: Brake lights, stop lights• White: bike lights, flashlights, headlamps• Power is limited to <10 W per diode (2007)• High efficiency lighting115 lumen/watt (same as fluorescents)tungsten bulbs are only 15 lumens/watt917181019Marketed as an auto interior light. 20State-of-the artUV LED~$300Light output is not stated.1121Laser diodes• Similar to LEDs, but there is a cavity • Applications– Fiber optic communications– Research, Bose-Einstein condensates– Laser pointers22Laser diodes have a built-in photodetector so that negative feedback can be used to set the output level.1223Photodiodesare photon detectorsPhotons in → current out(like a solar cell)24Simple photodiode circuitsRVoutWhy do we have –Vbias?Electron-hole pairs created in the junction move away from the junction and make current where there wasn’t any. These can be made cheaply from Si. RVout-Vbias↑ Holes↓ Electrons1325Better photodiode circuit+−VoutVBVA-VbiasRVA= VB= 0, So the op amp input is a “virtual ground” (zero volts for any current).If the op amp input impedance is nearly zero, then the RC time constant for the photodiode is very small, and the response is very fast. Zinput= 026Photodiode specifications:• Output, often Amps/watt• Quantum efficiency (electrons per photon)as a function of wavelength• Dark current (the noise) • Rise time (depends on load resistor)• Fall time (depends on load resistor)• Junction capacitance (depends on Vbias)1427281529OptocouplerApplications: to transfer information to a circuit “off ground”or to transfer information without electrostatic pickup Fiber optic cableDual in-line