Title: Semiconductor Photoconductive Detectors
1Semiconductor Photoconductive Detectors
- S W McKnight and
- C A DiMarzio
2Types 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
3Intrinsic 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
4Impurities Levels in Si
5Photoconductors
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)
6Indirect Gap Semiconductors
h?phonon
Eg
h?photon
7Direct Gap Semiconductors
E
Eg
k
h?photon
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10Optical Electric Field and Power
q? (e?)1/2 (?/c) (nik)
11Optical Electric Field and Power
A x (B x C) B(AC) C(AB)
a absorption coefficient 2 ? k/c
12Absorption Coefficient for Si and GaAs
13Reflection at Front Surface
For Silicon, near 600 nm n3.95 k0.026
? R 0.35
(Can be reduced by anti-reflection coating)
14Absorption 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
15Carrier Generation/Recombination
Units g e-h excitations/sec/m3
r m3/sec
1. Thermal Equilibrium
2. Direct recombination of excess carriers
16Direct Recombination of Excess Carriers
Direct recombination (low level)? dn dp ltlt no
17Photogenerated Carriers
3. Steady-state optical excitation
Neglect for dnltltno
18Differential Optical Excitation Rate
19Photoconductivity
Fp photon flux (photon/sec)
? quantum efficiency
20Hole Trapping
- Hole trapping at recombination centers
- hole is trapped
- electron trapped, completing recombination
- hole detraps to valence band
(c)
21Photoconductivity with Hole Trapping
(Steady-state)
of current-carrying photoelectrons of
trapped holes
22Photoconductive Gain
G photocurrent (electron/sec) / rate of e-h
generation
lengthl
AreaA
23Photoconductive Gain
?
24Effect of Carrier Lifetime on Detector Frequency
Response
25Photoconductor Bias Circuit
26Photoconductive Voltage
27Photoconductor Responsivity
28Responsivity 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
29Photoconductive 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
30Noise Sources
Johnson noise
G-R noise
Ep photon irradianceFp / Ad
G photoconductive gain
31Background-Limited Photoconductive Detection
32Johnson-Noise-Limited Photoconductive Detection
33Noise Sources for IR Detectors