Download - Semiconductor Photoconductive Detectors
Semiconductor Photoconductive Detectors
S W McKnight and
C A DiMarzio
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
Intrinsic and Extrinsic PhotoconductorsE
Intrinsic Photoconductor
Extrinsic Photoconductor
Ef1
Ef2
1
2
1. Donor level to conduction band
2. Valence band to acceptor level
Eg
Impurities Levels in Si
PhotoconductorsMaterial 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)
Indirect Gap Semiconductors
Eghνphoton
hνphonon
Direct Gap Semiconductors
Eghνphotonk
E
Optical Constants of Silicon
0
1
2
3
4
5
6
7
8
0 200 400 600 800 1000 1200
Wavelength (nm)
Op
tic
al
Co
ns
tan
ts (
n,
k)
n
k
k*1000
GaAs Optical Constants
0
1
2
3
4
5
6
0 200 400 600 800 1000 1200
Wavelength (nm)
n, k
n
k
100*k
Optical Electric Field and Power
q=ω (ε)1/2 = (ω/c) (n+ik)
Optical Electric Field and Power
A x (B x C) = B(A·C) – C(A·B)
α = absorption coefficient = 2 ω k/c
Absorption Coefficient for Si and GaAs
Reflection at Front Surface
For Silicon, near 600 nm: n=3.95 k=0.026
→ R = 0.35
(Can be reduced by anti-reflection coating)
Absorption in Semiconductorα = 2 ω k / c
For Silicon near 600 nm: α = 4 π 0.026 / 600 x 10-9 = 5.44 x 105 m-1
For GaAs near 600 nm: α = 4.76 x 106 m-1
0 1 2 3 4 5 6 7 8 9 100
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Z (microns)
Op
tica
l Po
we
r In(z)=Io e- z
Si
GaAs
Carrier Generation/Recombination
1. Thermal Equilibrium:
2. Direct recombination of excess carriers:
Units: g = e-h excitations/sec/m3
r = m3/sec
Direct Recombination of Excess Carriers
Direct recombination (low level)→ δn = δp << no
Photogenerated Carriers3. Steady-state optical excitation:
Neglect for δn<<no
Differential Optical Excitation Rate
Photoconductivity
Φp = photon flux (photon/sec)
Area=A
length=l
η = quantum efficiency
Hole Trapping
Hole trapping at recombination centers:
a. hole is trapped
b. electron trapped, completing recombination
c. hole detraps to valence band
(c)
Photoconductivity with Hole Trapping
# of current-carrying photoelectrons = # of trapped holes
(Steady-state)
Photoconductive Gain
G = photocurrent (electron/sec) / rate of e-h generation
Area=A
length=l
Photoconductive Gain
→
Effect of Carrier Lifetime on Detector Frequency Response
Photoconductor Bias Circuit
Photoconductive Voltage
Photoconductor Responsivity
Responsivity Factors• Photocarrier lifetime
– Tradeoff with response frequency
• Quantum efficiency (anti-reflection coating)
• Carrier mobility• Detector current• Dark resistance
– R= ℓ / σ A– Detector area: Ad = ℓ w– Sample thickness
length=ℓ
Cross-section area=A
Detector area=Ad w
tDetector current, i
Photoconductive Noise Factors• 1/f Noise
– Contact related
• Thermal noise (Johnson noise)– Statistical effect of thermal fluctuations– <In
2> ~ kT/R
• Generation-Recombination noise– Statistical fluctuations in detector current– Dark current (thermal electron-hole pairs)– Background photogenerated carriers– <In
2> ~ Id / e
Noise Sources
Johnson noise:
G-R noise:
Ep = photon irradiance=Φp / Ad
G = photoconductive gain
Background-Limited Photoconductive Detection
Johnson-Noise-Limited Photoconductive Detection
Noise Sources for IR Detectors