LECTURE 17 -Photodetectors
Topics to be covered Photodetectors PIN photodiode Avalanche Photodiode
Photodetectors Principle of the p-n junction Photodiode A generic photodiode.
Photodetectors Principle of the p-n junction Photodiode Variation of photon flux with distance. A physical diagram showing the depletion region. A plot of the the flux as a function of distance. There is a loss due to Fresnel reflection at the surface, followed by the decaying exponential loss due to absorption. The photon penetration depth x0 is defined as the depth at which the photon flux is reduced to e-1 of its surface value.
Photodetectors Absorption Coefficient and Photodiode Materials Absorbed Photon create Electron-Hole Pair. [ m] g 1.24 E [ ev ] g Cut-off wavelength vs. Energy bandgap Incident photons become absorbed as they travel in the semiconductor and light intensity decays exponentially with distance into the semiconductor. I x ( x) I 0 e Absorption coefficient
Absorption Coefficient Absorption coefficient α is a material property. Most of the photon absorption (63%) occurs over a distance 1/α (it is called penetration depth δ) Photon energy (ev) 5 4 3 1 2 0.9 0.8 0.7 1 108 Ge 1 107 In0.7Ga0.3As0.64P0.36 In0.53Ga0.47As Si 1 106 GaAs (m-1) InP 1 105 a-si:h 1 104 1 103 0.2 0.4 0.6 0.8 1.0 1.2 Wavelength ( m) 10 1.4 1.6 1.8
Absorption Coefficient Direct bandgap semiconductors (GaAs, InAs, InP, GaSb, InGaAs, GaAsSb), the photon absorption does not require assistant from Direct lattice vibrations. The photon is absorbed and the electron is excited directly from the VB to k CB without a change in its kvector (crystal momentum ħk), since photon momentum is very small. k k photon momentum 0 CB E CB Ec Bandgap Eg Photon Ev VB k (a) GaAs (Direct bandgap) VB Absorption coefficient α for direct bandgap semiconductors rise sharply with decreasing wavelength from λg (GaAs and InP). 11
Absorption Coefficient E Indirect bandgap semiconductors (Si and Ge), the photon absorption Indirect CB requires assistant from Ec Photon lattice vibrations (phonon). If Ev K is wave vector of lattice VB Phonon wave, then ħk represents k the momentum associated (b) Si (Indirect bandgap) with lattice vibration ħk is momentum. akphonon CB k VB phonon momentum K Bandgap, Eg k Thus the probability of photon absorption is not as high as in a direct transition and the λg is not as sharp as for direct bandgap semiconductors. 12
Photodetectors Absorption Coefficient and Photodiode Materials Photon absorption in a direct bandgap semiconductor. Photon absorption in an indirect bandgap semiconductor E E CB Direct Bandgap E C E V E g Photon Photon CB Indirect Bandgap E C E g VB VB E V Phonons k k k k
Photodetectors Quantum Efficiency and Responsivity External Quantum Efficiency Number of EHP Number of geberated and collected incidnet photons I P 0 ph e h Responsivity R Photocurrent (A) Incident Optical Power (W) I P ph 0 R e e h h c Spectral Responsivity
The pin Photodiode The pn junction photodiode has two drawbacks: Depletion layer capacitance is not sufficiently small to allow photodetection at high modulation frequencies (RC time constant limitation). SiO2 Electrode p+ i-si net end ena The pin photodiode can significantly reduce these E(x) Intrinsic layer has less doping and wider region (5 50 μm). problems. 15 n+ (a) Narrow SCL (at most a few microns) (b) long wavelengths incident photons are absorbed outside SCL low QE Electrode x
Photodetectors The pin Photodiode Reverse-biased p-i-n photodiode pin photodiode circuit pin energy-band diagram
Photodetectors The pin Photodiode Schematic diagram of pin photodiode Electrode SiO 2 p+ Electrode E(x) In contrast to pn junction built-in-field is uniform x i-si n+ E 0 W r net hu > E g E en d h + e x I ph R V out ena V r Small depletion layer capacitance gives high modulation frequencies. High Quantum efficiency.
Photodetectors The pin Photodiode A reverse biased pin photodiode is illuminated with a short wavelength photon that is absorbed very near the surface. The photogenerated electron has to diffuse to the depletion region where it is swept into the i- layer and drifted across. hu > E g p + Diffusion e h + i-si E Drift l W V r
Photodetectors The pin Photodiode p-i-n diode (a) The structure; (b) equilibrium energy band diagram; (c) energy band diagram under reverse bias.
Photodetectors Avalanche Photodiode (APD) Electrode SiO 2 I photo R Impact ionization processes resulting avalanche multiplication hu > E g n+ p e h+ p+ E E h + e r net Electrode x n + p Avalanche region e - E c E(x) h + E v Absorptio n Avalanche region x Impact of an energetic electron's kinetic energy excites VB electron to the CV.
Photodetectors Avalanche Photodiode (APD) Schematic diagram of typical Si APD. Antireflection coating Electrode SiO 2 n + p Guard ring n n + p n Avalanche breakdown p + Substrate Electrode p + Substrate Electrode Si APD structure without a guard ring More practical Si APD Breakdown voltage around periphery is higher and avalanche is confined more to illuminated region (n + p junction).
Photodetectors Heterojunction Photodiode Separate Absorption and Multiplication (SAM) APD InGaAs-InP heterostructure Separate Absorption and Multiplication APD I ph Electrode InP InP V r InGaAs R V out hu E e P and N refer to p- and n-type wider-bandgap semiconductor. E(x) P + N n n+ Avalanche region E h + Absorption region x
Photodetectors Heterojunction Photodiode Separate Absorption and Multiplication (SAM) APD E c E InP E v DE v e E c (a) Energy band diagram for a SAM heterojunction APD where there is a valence band step DE v from InGaAs to InP that slows hole entry into the InP layer. InGaAs h + E v InP E v InGaAsP grading layer InGaAs h + E v (b) An interposing grading layer (InGaAsP) with an intermediate bandgap breaks DE v and makes it easier for the hole to pass to the InP layer.
Photogenerated electron concentration exp( x) at time t = 0 v de A W B x hu > E g E h+ e i ph R V r