Photodetection: Absorption => Current Generation hυ Currents Materials for photodetection: t ti E g <h hν Various methods for generating currents with photo-generated carriers: photoconductors, photodiodes, avalanche photodiodes
1 10 8 5 4 3 2 1 Photon energy (ev) 0.9 0.8 0.7 Ge 1 10 7 In 0.7 Ga 0.3 As 0.64 P 0.36 - Sharp decrease in α In 0.53 Ga 0.47 As Si for λ>e 1 10 6 g GaAs α (m -1 ) InP 1 10 5 a-si:h - Photodetection t ti for indirect bandgap materials? 1 10 4 1 10 3 1.0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 Wavelength (μm)
- Photodetection for indirect bandgap materials? E E CB E c CB Indirect Bandgap, E g Direct Bandgap E g Photon E v Photon E c VB VB E v Phonon k k k k (a) GaAs (Direct bandgap) (b) Si (Indirect bandgap)
Photodetection efficiency R (Responsivity) = I P I q η (Quantum Efficiency) = P h ν q λμ [ m] R = η = η 0.3 hν 124 1.24 Responsivity y( (A/W) 1 0.9 0.8 0.7 0.6 05 0.5 0.4 0.8 Ideal Photodiode QE = 100% ( η = 1) 0.2 0.1 0 Si Photodiode λ g 0 200 400 600 800 1000 1200 Wavelength (nm)
Photoconductor Without light, w d Conductivity: σ = q μ n + q μ p ( μ : electron, hole mobility) eh, V J = σ E and I = wd σ e h n 0, p 0 With light, I + v - n= n +Δ n, p = p +Δp 0 0 σ +Δ σ = q μ ( n +Δ n ) + q μ ( p +Δ p ) e V V Δ I = wd Δσ = wd ( qμeδ n+ qμhδp) h 0
With light, n, p w d V Δ I = wdδ σ = wd qμeδ n+ qμhδp ( ) V + v - I P τ Since Δ n=δ p = ηint and assuming Δn, Δp are uniform, hν wd V P τ V P τ Δ I = wdδ σ = wd q( μe + μh) ηint = q( μe + μh) ηint V 2 hν wd hν ΔI q τ R = (assuming dark current is small) = ( μ e + μ h ) η int 2 P hν τ η = ( μe + μh) ηint = G η 2 int
h+ e Photoconductor I ph τ Gain: G = ( μe + μh) V 2 τ τ τ Assuming μe >> μh, G = μe V = = 2 2 τ e μ V τ e = = = ; time for travelling distance V μ e e μ E v τ >> τ e ==> electrons circulate many time before recombination e
d Photoconductors: I n, p + v - w - Very easy to make - arge gain - But slow (speed limited by τ) and significant dark currents
photodiode Faster, dark-current-free photodetectors? p n B - h + As + e PN junction in reverse bias -No significant current flow=> small dark currents E (x) W p M 0 W n x - Photo-generated carriers are removed by built-in field in depletion region (space charge region) E o
P N Photo-generated t carriers drift into P (holes) and N (electrons) regions creating currents. I = η int P q hν E (x) W p 0 W n x M p One photon creates electron and hole Problem: depletion region is very thin (< 1 μm) ) E o η int is very small. => >Use PIN structure t
SiO 2 Electrode p + Electrode (a) i-si n + ρ net en d (b) x PIN Photodiode en a E(x) (c) x E o W hυ > E g (d ) E h + e I ph R V out V r
CMOS Image Sensor
MSM (Metal-Semiconductor Metal) PD Carrier collection by lateral E-field provided by Schottky contacts Not very efficient but can be very fast
Avalanche Photodiode (APD): PIN PD + Gain (avalanche: a large mass of snow, ice, earth, rock, or other material in swift motion down a mountainside) id Achieve gain by multiplying electrons and/or holes. Impact Ionization: Under high E-field, electrons and holes can have sufficiently high kinetic energies breaking bonds and creating new e-h pairs. E E e h + E c e E v n + p Avalanche region š h +
In real APD, care is taken so that only one type of carrier (either electron or hole) causes impact ionization. APD has limited application for optical communication since high-performance EDFA is easily available APD is very useful for optical interconnect applications.