Download 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
Cryogenically cooled ~ 4.2 K
Electric leads/
Heat sinks
Semiconductor Bolometer
E
Ef
k
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
E
Eg
k
Conductivity
Conductivity: σ = n e μ
e=electron charge (1.6x10-19 C)
=Electron scattering time (average
time between scattering events)
Mobility:
Si
μn (cm2/Vμp
s)
(cm2/V-s)
1350
480
CdS
250
15
GaAs
8500
400
InSb
100,000
1700
Material
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)
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)
CdTe
1.58eV (0.78μ)
0.24eV (5.2μ)
(77K)
0.083eV (15μ)
(77K)
Hg0.3Cd0.7
Te
Hg0.2Cd0.8
Te
HgxCd1-xTe Band Gap
Eg=-0.302+1.93x+
5.35x10-4 T(1-2x) 0.810x2 + 0.823x3
Eg=
-0.26eV
HgTe
“Zero-gap”
(inverted bands)
Eg=
1.6eV
CdTe
Photovoltaic Detectors
• P-N junction detector
• Incident light creates voltage
• Same mechanism as solar cell
P-N Junction
E
Donor Levels
Ef
Eg
Ef
electrons
“holes”
Acceptor Levels
x
Doped Semiconductor (p-type)
Doped Semiconductor (n-type)
P-N Junction
- +
E
Ef
electrons
“holes”
x
P-N Junction
E
- +
electrons
“holes”
Ef
Depletion
Region
x
P-N Junction
E
Ec
Ef
electrons
“holes”
Ev
- +
Depletion
Region
x
P-N Junction Currents
E
Jdrift
Jdiffusion
Ec
Vo
Junction “built-in”
voltage
Ef
Ev
- +
Depletion
Region
x
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
E
Jdrift
Ec
Vo-Va
Va
Ef
Ev
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
Junction Current (Amps)
0.09
0.08
0.07
0.06
IJunction
Io=A Jo = 0.005 A
-
+V
0.05
Junction
0.04
0.03
0.02
0.01
0
-0.01
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
Va (volts)
0.4
0.6
0.8
1
Photovoltaic Detection
E
Jdrift
Jdiffusion
Ec
Vo
Junction “built-in”
voltage
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
Junction Current (Amps)
P-N Junction Photocurrent
0.06
0.04
Io=A Jo = 0.005 A
Iph=A Jph = 0.02 A
0.02
0
-0.02
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
Va (volts)
0.4
0.6
0.8
1
Photovoltaic Sensing Circuit
+
Vph
-
Photoconductive Sensing Circuit
Iph
-
+
Vd
Photoconductive Detection
E
Jdrift
Ec
Vo+ Vd
Ev
Ef
- +
Depletion
Region
x
Avalanche Photodetection
E
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)