Optical Fiber Communication Lecture 11 Detectors

Transcription

1 Optical Fiber Communication Lecture 11 Detectors Warriors of the Net

2 Detector Technologies MSM (Metal Semiconductor Metal) PIN Layer Structure Semiinsulating GaAs Contact InGaAsP p 5x10 18 Absorption InGaAs n- 5x10 14 Contact InP n 1x10 19 Features Simple, Planar, Low Capacitance Low Quantum Efficiency Trade-off Between Quantum efficiency and Speed APD Contact InP p 1x10 18 Gain-Bandwidth: Multiplication InP n 5x GHz Transition InGaAsP n 1x10 16 Low Noise Absorption InGaAs n 5x10 14 Difficult to make Contact InP n 1x10 18 Complex Substrate InP Semi insulating Waveguide Absorption Layer Guide Layers High efficiency High speed Difficult to couple into Key: Absorption Layer Contact layers

3 Photo Detection Principles Device Layer Structure Bias voltage usually needed to fully deplete the intrinsic I region for high speed operation Band Diagram showing carrier movement in E-field Light intensity as a function of distance below the surface Carriers absorbed here must diffuse to the intrinsic layer before they recombine if they are to contribute to the photocurrent. Slow diffusion can lead to slow tails in the temporal response.

4 Current-Voltage Characteristic for a Photodiode

5 Characteristics of Photodetectors Internal Quantum Efficiency External Quantum efficiency Number of Collected electrons i 1 e Number of Photons *Entering* detector W Number of Collected electrons iph / q 1 e Rp 1 e Number of Photons *Incident* on detector P / h o W Responsivity Photo Current (Amps) iph q R 1 R p 1 e Incident Optical Power (Watts) P h o W Photocurrent i ph P q o h Fraction Transmitted into Detector W 1R p 1e RP o Incident Photon Flux (#/sec) Fraction absorbed in detection region

6 Responsivity Output current per unit incident light power; typically 0.5 A/W R h e M

7 Photodiode Responsivity

8 Detector Sensitivity vs. Wavelength Absorption coefficient vs. Wavelength for several materials (Bowers 1987) Photodiode Responsivity vs. Wavelength for various materials (Albrecht et al 1986)

9 PIN photodiodes Energy-band diagram p-n junction Electrical Circuit

10 Basic PIN Photodiode Structure Rear Illuminated Photodiode Front Illuminated Photodiode

11 PIN Diode Structures Diffused Type (Makiuchi et al. 1990) Diffused Type (Dupis et al 1986) Etched Mesa Structure (Wey et al. 1991) Diffused structures tend to have lower dark current than mesa etched structures although they are more difficult to integrate with electronic devices because an additional high temperature processing step is required.

12 Avalanche Photodiodes (APDs) High resistivity p-doped layer increases electric field across absorbing region High-energy electron-hole pairs ionize other sites to multiply the current Leads to greater sensitivity

13 APD Detectors Signal Current q is M P h APD Structure and field distribution (Albrecht 1986)

14 APDs Continued

15 Detector Equivalent Circuits R d I ph I d C d PIN R d I ph I d I n C d APD I ph =Photocurrent generated by detector C d =Detector Capacitance I d =Dark Current I n =Multiplied noise current in APD R d =Bulk and contact resistance

16 MSM Detectors Simple to fabricate Quantum efficiency: Medium Problem: Shadowing of absorption region by contacts Capacitance: Low Light Simplest Version Schottky barrier gate metal Semi insulating GaAs Bandwidth: High Can be increased by thinning absorption layer and backing with a non absorbing material. Electrodes must be moved closer to reduce transit time. Compatible with standard electronic processes GaAs FETS and HEMTs InGaAs/InAlAs/InP HEMTs Non absorbing substrate To increase speed decrease electrode spacing and absorption depth Absorption layer E Field penetrates for ~ electrode spacing into material

17 Waveguide Photodetectors Waveguide detectors are suited for very high bandwidth applications Overcomes low absorption limitations Eliminates carrier generation in field free regions Decouples transit time from quantum efficiency Low capacitance More difficult optical coupling (Bowers IEEE 1987)

18 Carrier transit time Transit time is a function of depletion width and carrier drift velocity t d = w/v d

19 Detector Capacitance P x p x n N Capacitance must be minimized for high sensitivity (low noise) and for high speed operation Minimize by using the smallest light collecting area consistent with efficient collection of the incident light p-n junction C A w xp xn W For a uniformly doped junction Minimize by putting low doped I region between the P and N doped regions to increase W, the depletion width W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed. C A 2 W 2q Nd Vo Vbi 2(Vo Vbi) qnd 1/ 2 1/ 2 Where: =permitivity q=electron charge Nd=Active dopant density Vo=Applied voltage V bi=built in potential A=Junction area

20 Bandwidth limit C= 0 K A/w where K is dielectric constant, A is area, w is depletion width, and 0 is the permittivity of free space (8.85 pf/m) B = 1/2RC

21 PIN Bandwidth and Efficiency Tradeoff Transit time =W/v sat v sat =saturation velocity=2x10 7 cm/s R-C Limitation RC R in A W Responsivity Diffusion 1 p 1 R q R e h W =4 ns/µm (slow)

22 Dark Current Surface Leakage Bulk Leakage Surface Leakage Ohmic Conduction Generation-recombination via surface states Bulk Leakage Diffusion Generation-Recombination Tunneling Usually not a significant noise source at high bandwidths for PIN Structures High dark current can indicate poor potential reliability In APDs its multiplication can be significant

23 Signal to Noise Ratio i p = average signal photocurrent level based on modulation index m where L B L D p p R TB k B qi B M F M I I q M i N S / p p I m i

24 Optimum value of M M x 2 2qI L 4kBT / opt xqi p I D where F(M) = M x and m=1 R L

25 Noise Equivalent Power (NEP) Signal power where S/N=1 Units are W/Hz 1/2 h 4kT NEP 2eI M x D 2 e M R L

26 Typical Characteristics of P-I-N and Avalanche photodiodes

27 Comparisons PIN gives higher bandwidth and bit rate APD gives higher sensitivity Si works only up to 1100 nm; InGaAs up to 1700, Ge up to 1800 InGaAs has higher for PIN, but Ge has higher M for APD InGaAs has lower dark current