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What is Silicon photonics?

Technology for integrating optical circuits such as waveguides, modulators and photodetectors on silicon. It uses the materials and processes of CMOS fabs, enabling volume production on 200 mm or 300 mm wafers. Because silicon emits light poorly, lasers are added using compound semiconductors.

Japanese name: シリコンフォトニクス

Silicon photonics appears in 7 practice questions across 4 topics, including Silicon photonics, Compound semiconductors, Semiconductor physics.

Silicon waveguide cross-sections (strip and rib) SiO₂ (n ≈ 1.45) Si (n ≈ 3.5) Optical mode SiO₂ cladding Buried oxide (BOX) Si substrate Width ~500 nm t = 220 nm SiO₂ cladding Buried oxide (BOX) Si substrate Width ~500 nm Slab Strip waveguide Rib waveguide Strong confinement, tight bends Slab connects to electrodes
Silicon waveguide cross-sections (strip and rib) · View in diagrams

Field: Silicon photonics

Try it: questions about Silicon photonics

In silicon photonics, why are compound semiconductors combined with silicon for the laser light source?

  1. ABecause silicon does not transmit light at all
  2. BBecause compound semiconductors are always cheaper
  3. CBecause silicon has an indirect band gap and cannot emit light efficiently
Show answer and explanation

Answer: C. Because silicon has an indirect band gap and cannot emit light efficiently

Silicon photonics builds waveguides, modulators and germanium photodetectors precisely and in volume in CMOS fabs, but silicon's indirect gap rules out an efficient light source. InP-based lasers are therefore bonded onto silicon wafers (heterogeneous integration) or attached as separate chips. Driven by optical interconnects for AI data centers, co-packaged optics and related technologies are advancing rapidly.

What is the main role of the buried oxide (BOX) layer in the SOI wafers used in silicon photonics?

  1. AIt acts as the absorption layer of a photodetector beneath the waveguide, absorbing telecom-wavelength light and converting it into current
  2. BIt acts as a light-emitting layer that generates light in the top silicon layer
  3. CIt serves as the lower cladding of the waveguide, preventing light from leaking into the silicon substrate
Show answer and explanation

Answer: C. It serves as the lower cladding of the waveguide, preventing light from leaking into the silicon substrate

An SOI (Silicon on Insulator) wafer consists of a silicon substrate topped with a buried oxide (BOX) and then a thin single-crystal silicon layer. Waveguides are formed by patterning the top silicon layer (220 nm thick is common), and the low-index BOX serves as the lower cladding. If the BOX is too thin, light extending outside the waveguide (the evanescent field) leaks into the high-index silicon substrate, so a thick BOX of about 2–3 µm is commonly used. Oxide has a large band gap and neither absorbs nor emits light.

Silicon photonics in the explanations

Questions that mention Silicon photonics (7)

  1. In silicon photonics, why are compound semiconductors combined with silicon for the laser light source?
  2. What is the main role of the buried oxide (BOX) layer in the SOI wafers used in silicon photonics?
  3. Silicon photonics sometimes combines silicon nitride (SiN) waveguides with silicon waveguides. Which correctly describes SiN waveguides?
  4. What is the main reason silicon photonics is said to be well suited to mass production in semiconductor fabs?
  5. In an indirect-gap semiconductor such as silicon, why is the probability of electron-hole recombination with light emission low?
  6. Which material is suitable for photodiodes that receive light in the 1.55 µm band used in optical fiber communication?
  7. Which correctly describes wavelength division multiplexing (WDM)?

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Related terms

InP (indium phosphide) Waveguide Band gap CMOS CPO (co-packaged optics) Refractive index Dry etching Ion implantation Selective epitaxial growth GaN (gallium nitride)

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