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
E
Intrinsic and Extrinsic
Photoconductors
Ef1
1
Eg
2
Intrinsic
Photoconductor
Ef2
Extrinsic
Photoconductor
1. Donor level to conduction band
2. Valence band to acceptor level
Impurities Levels in Si
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
Indirect Gap Semiconductors
hνphonon
hνphoton
Eg
Direct Gap Semiconductors
E
hνphoton
Eg
k
Optical Constants of Silicon
8
Optical Constants (n, k)
7
6
5
n
k
k*1000
4
3
2
1
0
0
200
400
600
Wavelength (nm)
800
1000
1200
GaAs Optical Constants
6
5
n, k
4
n
3
k
100*k
2
1
0
0
200
400
600
Wavelength (nm)
800
1000
1200
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 Semiconductor
α=2ωk/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
1
0.9
Optical Power
0.8
In(z)=Io e- z
0.7
0.6
0.5
Si
0.4
0.3
GaAs
0.2
0.1
0
0
1
2
3
4
5
6
Z (microns)
7
8
9
10
Carrier Generation/Recombination
Units: g = e-h excitations/sec/m3
r = m3/sec
1. Thermal Equilibrium:
2. Direct recombination of excess carriers:
Direct Recombination of Excess
Carriers
Direct recombination (low level)→ δn = δp << no
Photogenerated Carriers
3. Steady-state optical excitation:
Neglect for δn<<no
Differential Optical Excitation Rate
Photoconductivity
length=l
Area=A
Φp = photon flux (photon/sec)
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
(Steady-state)
# of current-carrying photoelectrons = # of trapped holes
Photoconductive Gain
G = photocurrent (electron/sec) / rate of e-h generation
length=l
Area=A
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)
Detector
area=A
• Carrier mobility
• Detector current
Detector
current, i
• Dark resistance
d
– R= ℓ / σ A
– Detector area: Ad = ℓ w
– Sample thickness
w
t
length=ℓ
Cross-section
area=A
Photoconductive Noise Factors
• 1/f Noise
– Contact related
• Thermal noise (Johnson noise)
– Statistical effect of thermal fluctuations
– <In2> ~ kT/R
• Generation-Recombination noise
– Statistical fluctuations in detector current
– Dark current (thermal electron-hole pairs)
– Background photogenerated carriers
– <In2> ~ 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