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

2. Outline
Bolometers
Photoconductive detectors
Photovoltaic detectors

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

4. Semiconductor Bolometerk 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 Detectorsk Eg

7. 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

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)
= Nph η / 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 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)

11. HgxCd1-xTe Band Gap
Eg= x+ 5.35x10-4 T(1-2x) x x3
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 Ef Eg Ef x Donor Levels electrons “holes”
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 Jdrift Vo E 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 Jdrift Vo-Va Va Va E Ec Ef Ev x
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. 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 = 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= η e Nph to drift current

25. P-N Junction Photocurrent
0.06
Io=A Jo = 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

26. Photovoltaic Sensing Circuit+
Vph -

27. Photoconductive Sensing Circuit
Iph - + Vd

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

29. Avalanche Photodetection
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)