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Semiconductor photodiodes
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- 2. 2 Contents • Types ofPhotodiodes. • PN Junction Photodiodes. • PIN Photodiodes. • Avalanche Photodiodes. • Merits and Demerits. • Construction and Response.
- 3. 3 p–n Photodiode • Semiconductorphotodiodes without internal gain. • Generate a single electron–hole pair per absorbed photon. • Reverse-biased, mobile carriers are swept to majority sides under the influence of electric field. • Depletion region formed by immobile positively charged donor atoms in n-type material and immobile negatively charged acceptor atoms in p-type material.
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- 5. 5 p–n Photodiode • Widthof depletion region dependent upon doping concentrations for a given applied reverse bias. • Lower the doping, wider the depletion region. • Photons may be absorbed in both the depletion and diffusion regions. • Absorption region’s position and width depend upon energy of the incident photons and on the material.
- 6. 6 p–n Photodiode • Forweak photon absorption, absorption region may extend completely throughout the device. • Electron–hole pairs generated in both depletion and diffusion regions. • In depletion region carrier pairs separate and drift under electric field, outside this region the hole diffuses towards the depletion region to be collected.
- 7. 7 p–n Photodiode • Slowdiffusion limits photodiode response, hence photons to be absorbed in the depletion region. • Depletion region made long by decreasing the doping in n-type material. • Depletion region width is normally 1 to 3 μm, optimized for efficient detection of light at a given wavelength - silicon devices in the visible spectrum(0.4 - 0.7 μm), germanium in the near infrared (0.7 to 0.9 μm).
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- 9. 9 p–i–n Photodiode • Widerdepletion region allows longer wavelength operation, light penetrates more deeply - absorption takes place in the depletion region. • n-type material doped so lightly, can be considered intrinsic. • To make low resistance contact, a highly doped n-type (n+) layer is added.
- 10. 10 Front Illuminated p–i–nPhotodiode • Top entry device – light introduced through upper p+ layer. • Operating in 0.8 - 0.9 μm band - depletion region 20 - 50 μm for high quantum efficiency (~ 85%), fast response (< 1 ns) and low dark current (1 nA).
- 11. 11 Front Illuminated p–i–nPhotodiode • Dark current from surface leakage currents and generation–recombination currents in depletion region without illumination. • Quantum efficiency penalty from optical absorption in undepleted p+-region. • Limits to how small the device can be fabricated - light access and metallic contact on the top.
- 12. 12 Substrate Entry Technique •Substrate entry technique - smaller devices with lower capacitances (< 0.1 pF) employed. • Light enters through transparent InP substrate, device area fabricated as small as practical for bonding. Mesa structure
- 13. 13 Substrate Entry p-i-nPhotodiode • Also called as back illumination. • Mesa structure* reduces parasitic capacitances, dark currents < 1 nA. • p+-InGaAsP layer heterojunction structure (Schottky barrier) improves quantum efficiency (75 - 100 %). • Charge trapping can occur at the n−p+- InGaAs/InGaAsP interface – limits the response time of the device.
- 14. 14 Side Illuminated p-i-nPhotodiode • Light injected parallel to the junction plane. • Large absorption width (500 μm). • Sensitive at wavelength near to bandgap limit (1.09 μm) where absorption coefficient is relatively small.
- 15. 15 Planar InGaAs p–i–nPhotodiode • Incident light absorbed in low- doped n-type InGaAs layer generating carriers. • Discontinuity due to homojunction b/w n+- InP substrate & n- InGaAs absorption region reduced by including n-type InP buffer layer.
- 16. 16 Planar InGaAs p–i–nPhotodiode Charge trapping Energy Band Diagram
- 17. 17 Speed of Response •Photodiode response limited by three factors: (i) Drift time of carriers through depletion region. - speed of response limited by time taken by photogenerated carriers to drift across depletion region. - longest transit time for carriers which must traverse full depletion layer width w & drift velocity vd.
- 18. 18 Speed of Response -When depletion region field exceeds saturation value, carriers assumed to travel at constant (maximum) drift velocity. (ii) Diffusion time of carriers generated outside depletion region. - time taken for carriers to diffuse a distance d, Dc - minority carrier diffusion coefficient.
- 19. 19 Speed of Response •Time constant incurred by capacitance of the photodiode with its load. - voltage-dependent capacitance due to variation in stored charge at the junction. - junction capacitance εs - permittivity of semiconductor material, A - diode junction area, w - depletion layer width. - Photodiode capacitance = junction capacitance + capacitance of leads and packaging.
- 20. 20 Bandwidth Limitation • Devicebandwidth limited by drift time of carriers through depletion region tdrift. • With no gain mechanism present within the device structure, maximum possible quantum efficiency is 100%. • Equivalent to ultimate gain–bandwidth product.
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- 22. 22 Photodiode Response • Toobtain high quantum efficiency, width of depletion layer to be far greater than reciprocal of absorption coefficient for the material used to fabricate the detector. • Most of the incident light absorbed, negligible diffusion outside depletion region. • Detector capacitance larger - speed of response limited by the RC time constant.
- 23. 23 Photodiode Response • Outputpulse displays a long tail due to diffusion component to input optical pulse. • Devices with very thin depletion layers tend to exhibit distinctive fast response and slow response components to their output pulses. • To reduce RC time constant limitation, use traveling-wave photodiode - absorption and carrier drift regions positioned orthogonally to each other.
- 24. 24 Noise – Darkcurrent • Photogenerated current due to background radiation entering the device - dark current with no intended optical signal present. • Dark current minimized through use of high- quality, defect-free material - reduces number of carriers generated in depletion region and those which diffuse into this layer from the p−- and n+- regions. • Surface currents minimized by careful fabrication & surface passivation, surface state & impurity ion concentrations reduced.
- 25. 25 Avalanche Photodiodes • Semiconductorphotodiodes with internal gain.
- 26. 26 Avalanche Photodiodes • Mostof the photons absorbed and primary carrier pairs generated in depletion region. • Impact ionization - high-field region in which holes and electrons acquire sufficient energy to excite new electron–hole pairs – avalanche breakdown. • High reverse bias (50 to 400 V) creates new carriers by impact ionization, can produce additional carriers by the same mechanism.
- 27. 27 Avalanche Photodiodes • Deviceswhich operate at much lower bias voltages (15 to 25 V) are available. • Carrier multiplication factors as great as 104 obtained using defect-free materials to ensure uniformity of carrier multiplication over entire photosensitive area. • Excessive leakage at the junction edges eliminated using a guard ring structure.
- 28. 28 Avalanche Photodiodes • Silicon,germanium and InGaAs APDs available. • Carriers generated in undepleted material are collected somewhat slowly by diffusion process – produces a long ‘diffusion tail’ on a short optical pulse. • When APD is fully depleted by employing electric fields in excess of 104 V/m, all carriers drift at saturation-limited velocities.
- 29. 29 Avalanche Photodiodes • Responsetime limited by (i). Carrier transit time across absorption region. (ii). Carrier time to perform avalanche multiplication. (iii). RC time constant due to diode junction capacitance and its load.
- 30. 30 Avalanche Photodiodes • Lowgain - transit time & RC effects dominate, gives definitive response time, constant bandwidth. • High gain - avalanche build up time dominates, device bandwidth decreases with increasing gain. • Constant gain–bandwidth product, fast rise time (150 - 200 ps), fall time (≥ 1ns) dictated by transit time of holes traveling at slower speed.
- 31. 31 Avalanche Photodiodes withGuard Ring Eliminates excessive leakage at the junction edges. Silicon Germanium Sensitive, fast APDs
- 32. 32 APD Advantages • Detectionof very low light levels - gain. • Multiplication factor is a measure of the internal gain, (I - total output current, Ip - primary photocurrent, ) • Increase in sensitivity of 5 to 15 dB over p–i–n photodiodes. • Wider dynamic range. • As single-photon-counting avalanche detectors.
- 33. 33 APD Drawbacks • Fabricationdifficult, complex structure, increased cost. • High bias voltages required, wave- length dependent. • Variation of gain (multiplication factor) with temperature.
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