6.973 Semiconductor OptoelectronicsLecture 6: Diodes Under IlluminationRajeev J. RamElectrical EngineeringMassachusetts Institute of TechnologyReading:• Chuang, 14.2 & 14.3Outline:• Illumination of p-N Diodes• Capacitance of Diodes• Introduction to p-i-n Diodes for PhotodetectorsHeterojunctionHeterojunctionDiodeDiodekinkVo= built-in potentialBand Diagram for DiodeBand Diagram for DiodeNon-equilibrium VA=0No recombination in depletion regionQuasiQuasi--equilibrium Transportequilibrium TransportContinuity Equation:Electrostatics with slowly varying potential:For small deviations from equilibrium, current will flow if the chemical potential is not constant:BoltzmannFor now, µnis just some constant of proportionalityDiode LawDiode Law-2 101602 10164 10166 10168 10161 10171.2 1017-0.5 0 0.5 1 1.5 22.5Carrier Density (cm-3)Positions (microns)Neglecting recombination in the active region…Diagnostic ProblemsDiagnostic ProblemsSimWindowsSimWindowsSoftwareSoftwareSelf-consistent solution of modifieddrift-diffusion & Poisson’s Equation…Diode Under IlluminationDiode Under Illuminationlightpp--typetypeGaAsGaAsNN--typetypeAlGaAsAlGaAsdiffusionDiode Under IlluminationDiode Under IlluminationContinuity Equation:Under bias (w/o illumination):Under bias (w/ illumination):Diode Under Uniform IlluminationDiode Under Uniform IlluminationP-side:N-side:Diode Under IlluminationDiode Under IlluminationdiffusionLimitations of Limitations of pp--nnJunction Junction PhotodetectorsPhotodetectorslightpp--typetypeGaAsGaAsNN--typetypeAlGaAsAlGaAsdiffusionTypical material:GaAsGaAsMaterials PerformanceMaterials PerformancePure n-type material (no~ 1014cm-3)The longestlifetime of holes τp~3·10-6sDiffusion length Lp= (Dp·τp)1/2Lp~30-50 µm.Pure p-type materialWithout trapsThe longestlifetime of electrons τn~ 5·10-9sDiffusion length Ln= (Dn·τn)1/2Ln~10 µmHigh injection level (filled traps)The longest lifetime of electrons τ ~2.5·10-7sDiffusion length LnLn~ 70 µmCapacitance of Capacitance of HomojunctionHomojunctionDiodesDiodesHomojunction• Wider depletion region gives lower capacitance, faster devicesIntroduction to Introduction to pp--ii--nnDiodesDiodespp--typetypeintrinsicintrinsicnn--typetype• Same built-in potential as p-n junction• Wider depletion width, so more absorption in high-field region• Lower peak electric field than p-n
View Full Document
Unlocking...