Photodetector Basics

Transcription

1 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

2 Photon energy (ev) Ge In 0.7 Ga 0.3 As 0.64 P Sharp decrease in α In 0.53 Ga 0.47 As Si for λ>e g GaAs α (m -1 ) InP a-si:h - Photodetection t ti for indirect bandgap materials? Wavelength (μm)

3 - 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)

4 Photodetection efficiency R (Responsivity) = I P I q η (Quantum Efficiency) = P h ν q λμ [ m] R = η = η 0.3 hν Responsivity y( (A/W) Ideal Photodiode QE = 100% ( η = 1) Si Photodiode λ g Wavelength (nm)

5 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

6 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

7 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

8 d Photoconductors: I n, p + v - w - Very easy to make - arge gain - But slow (speed limited by τ) and significant dark currents

9 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

10 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

11 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

12 CMOS Image Sensor

13 MSM (Metal-Semiconductor Metal) PD Carrier collection by lateral E-field provided by Schottky contacts Not very efficient but can be very fast

14 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 +

15 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.