Semiconductor Photoconductive Detectors

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Semiconductor Photoconductive Detectors

• S W McKnight and
• C A DiMarzio

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Types of Photoconductivity

• Intrinsic photoconductors
• Absorption across primary band-gap, Eg, creates
  electron and hole photocarriers
• Extrinsic photoconductors
• Absorption from (or to) impurity site in gap
  creates photocarriers in conduction or valence
  band

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Intrinsic and Extrinsic Photoconductors
E
Ef1
1
Eg
Ef2
2
Extrinsic Photoconductor
Intrinsic Photoconductor
1. Donor level to conduction band
2. Valence band to acceptor level
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Impurities Levels in Si
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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)
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Indirect Gap Semiconductors
h?phonon
Eg
h?photon
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Direct Gap Semiconductors
E
Eg
k
h?photon
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Optical Electric Field and Power
q? (e?)1/2 (?/c) (nik)
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Optical Electric Field and Power
A x (B x C) B(AC) C(AB)
a absorption coefficient 2 ? k/c
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Absorption Coefficient for Si and GaAs
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Reflection at Front Surface
For Silicon, near 600 nm n3.95 k0.026
? R 0.35
(Can be reduced by anti-reflection coating)
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Absorption in Semiconductor
a 2 ? k / c
For Silicon near 600 nm a 4 p 0.026 / 600 x
10-9 5.44 x 105 m-1
For GaAs near 600 nm a 4.76 x 106 m-1
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Carrier Generation/Recombination
Units g e-h excitations/sec/m3
r m3/sec
1. Thermal Equilibrium
2. Direct recombination of excess carriers
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Direct Recombination of Excess Carriers
Direct recombination (low level)? dn dp ltlt no
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Photogenerated Carriers
3. Steady-state optical excitation
Neglect for dnltltno
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Differential Optical Excitation Rate
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Photoconductivity
Fp photon flux (photon/sec)
? quantum efficiency
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Hole Trapping

• Hole trapping at recombination centers
• hole is trapped
• electron trapped, completing recombination
• hole detraps to valence band

(c)
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Photoconductivity with Hole Trapping
(Steady-state)
of current-carrying photoelectrons of
trapped holes
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Photoconductive Gain
G photocurrent (electron/sec) / rate of e-h
generation
lengthl
AreaA
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Photoconductive Gain
?
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Effect of Carrier Lifetime on Detector Frequency
Response
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Photoconductor Bias Circuit
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Photoconductive Voltage
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Photoconductor Responsivity
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Responsivity Factors

• Photocarrier lifetime
• Tradeoff with response frequency
• Quantum efficiency (anti-reflection coating)
• Carrier mobility
• Detector current
• Dark resistance
• R l / s A
• Detector area Ad l w
• Sample thickness

Detector areaAd
w
t
Detector current, i
lengthl
Cross-section areaA
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Photoconductive Noise Factors

• 1/f Noise
• Contact related
• Thermal noise (Johnson noise)
• Statistical effect of thermal fluctuations
• ltIn2gt kT/R
• Generation-Recombination noise
• Statistical fluctuations in detector current
• Dark current (thermal electron-hole pairs)
• Background photogenerated carriers
• ltIn2gt Id / e

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Noise Sources
Johnson noise
G-R noise
Ep photon irradianceFp / Ad
G photoconductive gain
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Background-Limited Photoconductive Detection
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Johnson-Noise-Limited Photoconductive Detection
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Noise Sources for IR Detectors