Types of Semiconductor Detectors

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 ?T on Si Bolometer Conductivity
Ambient temperature 4.2K ? kT0.362 meV
Temperature 4.3K ? kT0.371 meV
6
Photoconductive Detectors
E
k
Eg
7
Conductivity
Conductivity s n e µ
eelectron 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 sd no e µn po e µp
Photocurrent sph ?n e µn ?p e µp
?n ?p photo-induced carrier density (m-3)
Nph ? / V
Nph incident photon flux (s-1)
? quantum efficiency
carrier recombination time
V sample volume
9
Photoconductivity
Recombination in n-type material
Steady-state solution
Quantum efficiency
? (1-R) Pe-h
Pe-h probability of absorption creating
electron-hole pair
10
Photoconductors
Material Eg (?max) Material Eg (?max)
Si 1.1eV(i) (1.2µ) PbS 0.37eV (3.3µ)
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-0.3021.93x 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

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

-
E
electrons
holes
Ef
x
15
P-N Junction
E

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

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

-
x
Depletion Region
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P-N Junction Currents
N-doped material nNd ( of donors)
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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 eVo/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 VoltageVa)
Jdrift A np Jo
Jdiffusion B e-(Vo-Va)/kT
JTotal -Jo B e-(Vo-Va)/kT
B Jo eVo/kT
? JTotal Jo (eVa/kT -1 )
22
P-N Junction Current
IJunction
IoA 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 ? e Nph to drift current

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P-N Junction Photocurrent
0.06
IoA Jo 0.005 A
0.04
IphA 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)