Silicon Absorption Depth
Visualize how photon absorption depth in silicon varies with wavelength using the Beer-Lambert law. Understand why pixel thickness is a critical design parameter for quantum efficiency.
Silicon Absorption Depth Visualizer
Explore how light penetration depth in silicon depends on wavelength using Beer-Lambert law.
Absorption coeff. alpha
6,397 cm-1
Penetration depth delta
1.563 um
Absorbed in PD
72.2%
Absorbed in full Si
85.3%
Physics
The intensity of light propagating through silicon decays exponentially according to the Beer-Lambert law:
I(z) = I₀ exp(-αz)
where α = 4πk/λ is the absorption coefficient, computed from the imaginary part of silicon's complex refractive index (n + ik).
Wavelength Dependence
- Blue (400-500 nm): α is very large, so photons are absorbed within the first ~0.5 um. Even thin silicon captures blue light efficiently.
- Green (500-600 nm): Moderate absorption depth (~1-2 um). Standard BSI pixel thicknesses capture most green photons.
- Red/NIR (600-1000 nm): α drops steeply near the silicon bandgap (1.12 eV, ~1100 nm). Absorption depths exceed 5-10 um, making thicker silicon essential for red QE.
Design Implication
Increasing silicon thickness from 3 um to 6 um has negligible impact on blue QE but can improve NIR QE by 2-3x. This is a key motivation for deep-trench BSI architectures.