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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”
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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 Photoconductors E Ef1 1 Eg 2 Ef2 Extrinsic Photoconductor Intrinsic Photoconductor 1. Donor level to conduction band 2. Valence band to acceptor level
Indirect Gap Semiconductors hνphonon Eg hνphoton
Direct Gap Semiconductors E Eg k hνphoton
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
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 = 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
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
length=l Area=A Photoconductivity Φp = photon flux (photon/sec) η = quantum efficiency
Hole Trapping • Hole trapping at recombination centers: • hole is trapped • electron trapped, completing recombination • 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
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 Detector area=Ad w t Detector current, i 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