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Types of Semiconductor Detectors

Types of Semiconductor Detectors. S W McKnight and C A DiMarzio. Outline. Bolometers Photoconductive detectors Photovoltaic detectors. Bolometers. Incident Radiation. Absorbing film. Semiconductor Bolometer. Electric leads/ Heat sinks. Cryogenically cooled ~ 4.2 K.

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Types of Semiconductor Detectors

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  1. Types of Semiconductor Detectors S W McKnight and C A DiMarzio

  2. Outline • Bolometers • Photoconductive detectors • Photovoltaic detectors

  3. Bolometers Incident Radiation Absorbing film Semiconductor Bolometer Electric leads/ Heat sinks Cryogenically cooled ~ 4.2 K

  4. Semiconductor Bolometer E k Ef Eb Impurity level Binding Energy (Eb) ~ 50 meV

  5. Effect of ΔT on Si Bolometer Conductivity Ambient temperature = 4.2K → kT=0.362 meV Temperature = 4.3K → kT=0.371 meV

  6. Photoconductive Detectors E k Eg

  7. Conductivity Conductivity: σ = n e μ e=electron charge (1.6x10-19 C) =Electron scattering time (average time between scattering events) Mobility:

  8. Photoconductivity Dark current: σd = no e μn + po e μp Photocurrent: σph = Δn e μn + Δp e μp Δn =Δp = photo-induced carrier density (m-3) = Nphη / V Nph = incident photon flux (s-1) η = quantum efficiency = carrier recombination time V = sample volume

  9. Photoconductivity Recombination in n-type material: Steady-state solution: Quantum efficiency: η = (1-R) Pe-h Pe-h = probability of absorption creating electron-hole pair

  10. Photoconductors

  11. HgxCd1-xTe Band Gap Eg=-0.302+1.93x+ 5.35x10-4 T(1-2x) -0.810x2 + 0.823x3 Eg= -0.26eV Eg= 1.6eV HgTe “Zero-gap” (inverted bands) CdTe

  12. Photovoltaic Detectors • P-N junction detector • Incident light creates voltage • Same mechanism as solar cell

  13. P-N Junction E Donor Levels Ef electrons Eg “holes” Ef Acceptor Levels x Doped Semiconductor (p-type) Doped Semiconductor (n-type)

  14. P-N Junction - + E electrons “holes” Ef x

  15. P-N Junction E + - electrons Ef “holes” x Depletion Region

  16. P-N Junction E Ec electrons Ef “holes” Ev - + x Depletion Region

  17. P-N Junction Currents Jdiffusion E Jdrift Ec Junction “built-in” voltage Vo Ef Ev - + x Depletion Region

  18. P-N Junction Currents N-doped material: n≈Nd (# of donors)

  19. P-N Junction Currents (No Bias Voltage) Jdrift = A np = -Jo Jdiffusion = B e-Vo/kT JTotal = -Jo + B e-Vo/kT = 0 (equilibrium) → B= Jo e+Vo/kT

  20. Biased P-N Junction Jdiffusion E Jdrift Ec Vo-Va Va Ef Ev Depletion Region Va x

  21. P-N Junction Currents (Bias Voltage=Va) Jdrift = A np = Jo Jdiffusion = B e-(Vo-Va)/kT JTotal = -Jo + B e-(Vo-Va)/kT B= Jo e+Vo/kT → JTotal = Jo (eVa/kT -1 )

  22. 0.09 0.08 0.07 0.06 0.05 Junction Current (Amps) 0.04 0.03 0.02 0.01 0 -0.01 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 V (volts) a P-N Junction Current IJunction Io=A Jo = 0.005 A - + VJunction

  23. Photovoltaic Detection Jdiffusion E Jdrift Ec Junction “built-in” voltage Vo Ef Ev - + Depletion Region x

  24. Photovoltaic Detection • Absorption in depletion region creates electrons/hole pairs • Built in electric field accelerates electrons and holes toward neutral region • Photocurrent adds Iph= ηe Nph to drift current

  25. P-N Junction Photocurrent 0.06 Io=A Jo = 0.005 A 0.04 Iph=A Jph = 0.02 A Junction Current (Amps) 0.02 0 -0.02 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 V (volts) a

  26. Photovoltaic Sensing Circuit + Vph -

  27. Photoconductive Sensing Circuit Iph - + Vd

  28. Photoconductive Detection E Jdrift Ec Vo+ Vd Ev Ef - + Depletion Region x

  29. Avalanche Photodetection E Jdrift Ec Vo+ Vd Ev Ef Depletion Region x

  30. Avalanche Photodiode • Large reverse bias on junction • Photoelectrons create electron/hole pairs in depletion region • Electron and holes can create more electron/hole pairs • Device has gain (like PMT)

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