Types of Semiconductor Detectors S W McKnight and C A DiMarzio

Outline Bolometers Photoconductive detectors Photovoltaic detectors

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

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

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

Photoconductive Detectors k Eg

Conductivity Conductivity: σ = n e μ Material μn (cm2/V-s) μp e=electron charge (1.6x10-19 C) =Electron scattering time (average time between scattering events) Mobility: Material μn (cm2/V-s) μp (cm2/V-s) Si 1350 480 CdS 250 15 GaAs 8500 400 InSb 100,000 1700 GaAs (77K) 200,000 10,000

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) = Nph η / V Nph = incident photon flux (s-1) η = quantum efficiency = carrier recombination time V = sample volume

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

Photoconductors Material Eg (max) Si 1.1eV(i) (1.2μ) PbS GaAs 1.43eV (0.87μ) InSb 0.18eV (6.9μ) Ge 0.67eV(i) (1.8μ) PbTe 0.29eV (4.3μ) CdS 2.42eV (0.51μ) Hg0.3Cd0.7Te 0.24eV (5.2μ) (77K) CdTe 1.58eV (0.78μ) Hg0.2Cd0.8 Te 0.083eV (15μ) (77K)

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

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

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

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

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

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

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

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

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

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

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 )

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

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

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

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 -0.02 -1 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1 V (volts) a

Photovoltaic Sensing Circuit + Vph -

Photoconductive Sensing Circuit Iph - + Vd

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

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

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)