6Semiconductor Detectors6. Semiconductor DetectorsA large variety of semiconductor materials, structures and devices are used as photodetectors in optical receivers.The most important for communications are:pn p i n and Schottk Barrier Photodiodes•pn, p-i-n and Schottky Barrier Photodiodes•Avalanche Photodiodes•Metal-Semiconductor-Metal(MSM)PhotodiodesMetal Semiconductor Metal (MSM) Photodiodes•PhotoconductorsEqually important optical devices, but structurally completely q y p p,y p y different and not used for optical communications include:•Charge-Coupled Devices (CCDs)•CMOS Imagers•Photocathodes•Solar CellsSolar CellsOptical Absorption Optical AbsorptionOptical Absorption in Semiconductorsp p g gThe photon flux passing throughan absorbing medium isSince the carrier collectionregions are ≤1 µm, theabsorption coefficient needs tob101hi hi hbe ~104cm-1to achieve highefficiency, which only occurs fordirect bandgap materials neardi t b d t i lthe bandgap. Basically wantidentical thermal and photonidentical thermal and photonenergies for generation.Photocurrent-The Mechanism Optical absorption creates extra pairs of electrons and holesqin excess of the thermal equilibrium concentration. If this is in the depletion region, then under the built-in field, or adding to that with reverse bias, the carriers are swept out by the electric field to give a reverse current of one electron for every generated electron-hole pair. Because electrons and holes have opposite charge, they move in opposite directions and there is only one particle passing any given point, so there is current of only one electronic charge, not two.This drift of charge increases the nominal reverse current of the diode in the short circuit condition or pushes the diode into th di d i th h t i it diti h th di d i t forward bias if in the open circuit condition. The latter is the operating region for photovoltaic or solar cell operation. operating region for photovoltaic or solar cell operationOptical Responsivity andQuantum EfficiencyQuantum EfficiencyThe optical response of a photodetector is characterized byeither quantum efficiency η, or responsivity, RQuantum efficiency can be external,ηext or internal, ηint. External is theor internal External is thenumber of electrons of current perpincident photon.Internal is the number of electrons ofcurrent per absorbed photonResponsivity, R, is the photocurrent peri i i h hunit incident power (amps per watt)At 1.24µm, η = 100% corresponds to R = 1A/WPhotogeneration and Photocurrent in pn Diodesi Di dp-i-n PhotodiodepThe primary disadvantage of the pn homojunction is that with moderate doping concentrations in the conductingi h d d i i i h d i regions for low resistance, the resulting depletion regionis quite thin (e.g., 0.1-.2µm). This causes two problems:i it thi(012)Thi t bl1) low efficiency since relatively little absorption occurs in a thin depletion region (d ~ 2 ) andthin depletion region(d2α)and2) relatively high depletion capacitance, which decreases device speed (RC time constant).device speed(RC time constant)A general rule is that only carriers generated within the depletion region are efficiently and rapidly collected as photocurrent. The goal is to create a diode with a widep gdepletion regionPhotogeneration and Photocurrent in p-i-n Diodesi Di dSchottky PhotodiodeAnalogous to a p-n junction,only the built-in field is createdby surface Fermi level pinning.Photo generated electrons are Photo generated electrons areaccelerated toward to themetal-semiconductor junctionj by the built-in field and transferinto the metal, creating ahphotocurrent.Th l ilib i d l t t ti t t ti l Thermal equilibrium and electrostatics create a potential barrier, φbp for holes in p-type material or φbn for electrons in n type material With low doping analogous M i n or M i p material. With low doping, analogous M-i-n or M-i-pdiodes can be realized.Differences BetweenSchottky and pn DiodesS h k d Di dSchottky barriers behave straightforwardly as photodetectors Schottky barriers behave straightforwardly as photodetectors Photons absorbed in the depletion region near metal produce a p,j p jdrift photocurrent due to the surface field, just as in pn junction. However, the current is not controlled by recombination times and the diode cannot be used as a light emitter.No biasing configuration of a Schottky diode produces substantial N bi i fi i f S h k di d d b i l overlapping populations of electrons and holes in same place because there is no injection of carriers from the metal into the because there is no injection of carriers from the metal into the semiconductor, hence no minority carrier injection into the semiconductor.Any hole collection from the semiconductor into metal "recombines" very rapidly by non-radiative electron-electron scattering within the electron gas, hence no usable minority carrieri i hi h l h bl i i i density (holes) in metal.Diode Depletion Capacitancep pDiode capacitance determines the speed of response of optoelectronic devices e.g. RC time constantIt also determines the sensitivity of detectors since a smaller capacitance gives larger voltage swing for same number of photogenerated electronsThe diode depletion region is analogous to a dielectric, thusTh di d d l i i i l di l i hthe capacitance is viewed as a dielectric parallel plate capacitor with depletion region width w d, depletion capacitance capacitor with depletion region width depletion capacitance is, C j13, C j 1.15 x 10 F/cm 0.1fF/µm for w d 1 µm, and,for~1µm,and,ε~13,~1.15x10-82=0.1fF/µm2rFrequency Limitations Frequency LimitationsThe diode frequency response is limited by two factors: The diode frequency response is limited by two factors: 1) RC charging time, which for a 50 W load is ~100 GHz for a 50 µm diameter diode.GHz for a50µm diameter diode2) Transit time, t = w d/v sat which is ~ 10-11sec or 100 GHz for a 1 µm depletion width diode.GH f1d l i id h di dActual diode parameters are seldom the frequency limitation, but parasitics associated with bonding and interconnects. Integration with the amplifier is key to high frequency receiver performance.Avalanche PhotodiodesAt low voltages, the maximum quantum efficiency in a diodeis a current of one electron per absorbed photon (100%).However, with increasing reversebias voltage and at a higher electricfield, it becomes possible for anelectron (or hole) to be acceleratedelectron(or hole)to be acceleratedso that its kinetic energy exceedsg p gythe bandgap energy and it cancreate an additional electron-holepair through impact ionization--theinverse of Auger recombinationi f A bi i(both are 3 particle processes). Sucha process must exist from detaileda process must exist from detailedbalance in thermodynamics.() Avalanche Photodiodes (2)It is possible to collect more than one electron of photocurrent per absorbed photon with very high bias across the depletion per absorbed photon with very high bias across the depletion region--each photoelectron (hole) generates additional electrons (and holes) when the electron energy exceeds the electrons(and holes)when the electron energy exceeds the bandgap energy in an exponential growing process called avalanche gain or multiplication. avalanche"gain"or"multiplication"Impact ionization CoefficientsWe describe the impact ionization process through impact ionization coefficients (or rates), αn and αpp g pThese represent the strength of the processes for electrons and holes, respectively.1/αn will correspond to the average distance for which an will correspond to the average distance for which an electron is accelerated before it creates an electron-hole pair by impact ionization, and similarly for 1/p for holesb impact ioni ation and similarl for1/αfor holesp() Impact ionization Coefficients (2)αn and αp are proportional to exp (-C/E), where C is a constant for a particular material and carrier type, and E is the electric for a particular material and carrier type and E is the electric field. For electric fields ~ 3 x 105V/ cm, for example, theµdistance between ionization events is ~ 1 µm in GaAs, which means relatively thick avalanche regions arerequired to achieve evenmoderate avalanched t l hgains, e.g., ~3-4microns. This results inmicrons This results inquite high bias voltages,~ 100 V, not a desiredrange for CMOSsystems architecturesand slower deviced l d iresponse (~20 GHz).Multiplication Noise and Gain Bandwidth ProductImpact ionizationby a single carrier(electrons) G = 8in this case, andτt = w/ve-sat+ w/v h-satI t i i ti b Impact ionization by both electrons and holes1<G<but holes 1< G < ∞ but w/v e-sat< τt< ∞p p Avalanche photodiode problemsProblems -"excess noise" and non-uniform avalanche multiplication occur when both carriers can initiate impact ionization events occur when both carriers can initiate impact ionization events Three consequences1) We have much larger variability in the avalanche gain (literally anything from 1 to ∞), causing an additional source of variability in the resulting detected signal because the gain process is now a sum of (a) an average of M successive electron impact ionizations, plus of(a)an average of M successive electron impact ionizations plus (b) (an average of) M+1 electron impact ionizations and 1 hole impact ionization (the initial electron creates a hole that creates an electron that starts the process all over again), plus ... .l t th t t t th ll i)l2) Overall response is slowed down because electrons generate holes, which generate electrons, which generate holes, etc.,g,g,3) We can have a "run-away" process where the avalanche gain becomes infinite, with electrons impact ionizing to give holes that impact ionize to give electrons that impact ionize to give holes, and impact ionize to give electrons that impact ionize to give holes and so on.Solutions to Avalanche Photodiode ProblemsThese problems become worse as αn and αp, become closer to one another, which is unfortunately the case for most III-V materials. The ratio between αn and αp is large ONLY in silicon, but silicon does not absorb beyond 1.1µm. One solution is to make optical absorption and avalanche gain regions out of different materialsImpact-Ionization Impact-Ionization Engineered APDshotodiode Noise AvalancheMetal-semiconductor-metal (MSM) photodiodeForm two Schottky diodes close to one another on the same Form two Schottky diodes close to one another on the same (doped) semiconductor surface. Bias the resulting structure with some d.c.voltage and one of the diodes becomes reversegbiased, forming a depletion region that will tend to sweep out photocarriers. The other diode becomes forward biased, allowing the collected photocurrent to flow out just as if we had ll i h ll d h fl j if h d formed an Ohmic contactDesirable to keep theDesirable to keep thedistance between thetwo metal electrodessmall to achieve highspeed, which leads tothe choice of anth h i finterdigitated structureMetal-semiconductor-metal (MSM) photodiode (2)MSMs have several attractive featuresy p g yp q,• only one doping type semiconductor is required,• only one kind of semiconductor is required• fabrication of interdigitated MSM photodetectors is quite •fabrication of interdigitated MSM photodetectors is quite simple and compatible with integrated circuit processing, which allows processes with fine (e.g., 1µm wide) lines required for ll ith fi(1id)li i d f dense interdigitation and high speed• devices can also have very low capacitance for high speedCCD(charge coupled device) CCD (charge coupled device) Common form of photodetector arraysand readout method employed in mostand reado t method emplo ed in mostsmall camcorder TV cameras.Concept is reminiscent of theConcept is reminiscent of theSchottky photodiode, exceptthere is the additional presenceof the high-bandgap oxide as acurrent blocking insulator.Band edges for the CCD cell (a)Band edges for the CCD cell(a)with initial bias but no photoinjectedcharge, (b) with bias,after photogenerated electrons have moved in the depletion region towards the positive electrode, (c) the thermal equilibrium situation that would exist with bias after any excess charge densities had leaked awayg()Charge coupled device (2)Pockets of charge moved serially out of the structure by "bucketbrigade" method like the example three-phase clocking methodThis can be donewith 2-phaseith2hclocking if there isan asymmetry inan asymmetry inthe oxide thicknessunder the gatesCMOS image sensorsgAdvanced CMOS technology has led to an effort to utilize it directly for image sensors (Moore’s Law in action)directly for image sensors(Moore’s Law in action)If it can be done without modifying the CMOS process, it leads to low cost image sensors. These can also be combined withlow cost image sensors These can also be combined with silicon digital (and analog) electronic signal processingCMOS image sensors generally employ silicon photodiodes or CMOS image sensors generally employ silicon photodiodes or variants in which the charge is created directly in a polysilicon gtransistor gateThe photogenerated charge is read out by sequentially turningon switches to read out the charge or voltage on each photodetector in turn rather than by the "bucket brigade" analog shift register of the CCD.In general, less expensive, but lower resolution.PhotoconductorsA piece of semiconductor material with two Ohmic contacts, and a voltage is applied between them. The semiconductor is mostlt i li d b t th Th i d t i t likely doped and thus conducting, hence there is some current flowing even without light shining on the material(a dark current) flowing even without light shining on the material (a dark current) If we shine light on the material,electron-hole pairs will beelectron-hole pairs will begenerated and the carrierconcentration is increased inconcentration is increased inthe material, thus theyconductivity of the materialincreases, giving larger currentPhotoconductors (2)Photoconductors differ from the photodiodes in several important ways y1) Current is carried both by minority and majority carriers in the photoconductor2) Current continues flowing in the photoconductor until all the excess electrons and holes recombine, but the majority carriers do not recombine at the electrodes. For every electron in ndoped material that leaves the structure by passing into thecontact region, another electron is injected at the other contact to maintain charge neutralityMinority carriers (holes in this example) do recombine when they Mi it i(h l i thi l)d bi h th reach the electrode. Thus the time to turn off photoconduction is governed by either by minority carrier lifetime inside the material governed by either by minority carrier lifetime inside the material or transit time of the minority carriers to the electrodesg Photoconductive gainIf the majority carrier transit time is long compared to the effective minority carrier lifetime (transit or bulk, whichever is )y y y p shorter), then an electron may effectively make many passes through the material before recombining.,p yAs a result, it is possible to have many electrons of current flow through the structure for one absorbed photon, i.e., quantum efficiency greater than one, a phenomenon known asy g,pphotoconductive gainPhotoconductors (3)Less desirable features1) Can be relatively slow; unless they are made very small, transittimes can be longtimes can be long2) Use of photoconductive gain occurs also at the expense ofspeed s ce e jo y c e us s esse y g pc es speed since the majority carrier must transit essentially g timesthrough the structure for a photoconductive gain of g pc.3) Dark current can contribute significantly to noise and it isdifficult to detect a small photocurrent in the presence of a largerdark current.Can make a photoconductor fast by arranging for a very short Can make a photoconductor fast by arranging for a very short minority carrier lifetime in the material, but then, the responsivityy p gcan be low because only a corresponding fraction of an electron’sworth of current flows through the circuit.Very fast photoconductors can be made by “killing” the lifetimeand this is used to make very fast "switches" triggered by short(femtosec) laser pulses。
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