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 Intrinsic Photoconductor Extrinsic Photoconductor E f1 E f Donor level to conduction band 2. Valence band to acceptor level EgEg

Impurities Levels in Si

Photoconductors Material E g ( max ) Material E g ( max ) Si1.1eV(i) (1.2μ)PbS0.37eV (3.3μ) GaAs1.43eV (0.87μ)InSb0.18eV (6.9μ) Ge0.67eV(i) (1.8μ)PbTe0.29eV (4.3μ) CdS2.42eV (0.51μ)Hg 0.3 Cd 0.7 Te 0.24eV (5.2μ) (77K) CdTe1.58eV (0.78μ)Hg 0.2 Cd 0.8 Te 0.083eV (15μ) (77K)

Indirect Gap Semiconductors EgEg hν photon hν phonon

Direct Gap Semiconductors EgEg hν photon k E

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 = 2 ω k / c For Silicon near 600 nm: α = 4 π / 600 x = 5.44 x 10 5 m -1 For GaAs near 600 nm: α = 4.76 x 10 6 m -1

Carrier Generation/Recombination 1. Thermal Equilibrium: 2. Direct recombination of excess carriers: Units: g = e-h excitations/sec/m 3 r = m 3 /sec

Direct Recombination of Excess Carriers Direct recombination (low level)→ δn = δp << n o

Photogenerated Carriers 3. Steady-state optical excitation: Neglect for δn<<n o

Differential Optical Excitation Rate

Photoconductivity Φ p = 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: A d = ℓ w –Sample thickness length=ℓ Cross-section area=A Detector area=A d w t Detector current, i

Photoconductive Noise Factors 1/f Noise –Contact related Thermal noise (Johnson noise) –Statistical effect of thermal fluctuations – ~ kT/R Generation-Recombination noise –Statistical fluctuations in detector current –Dark current (thermal electron-hole pairs) –Background photogenerated carriers – ~ I d / e

Noise Sources Johnson noise: G-R noise: E p = photon irradiance=Φ p / A d G = photoconductive gain

Background-Limited Photoconductive Detection

Johnson-Noise-Limited Photoconductive Detection

Noise Sources for IR Detectors