Back-Illuminated vs Front-Illuminated
Back-illuminated sensors bypass gate structures for higher QE and better blue/UV response: QE ratio and SNR gain in the shot- and read-noise limits.
Effective FI QE
13.0%
Effective BI QE
25.0%
QE Improvement
1.92×
SNR Improvement
1.39×
2.84 dB (20·log₁₀)
QE_eff = min(1, QE·microlens gain)·CFA; SNR gain = √(QE ratio) shot-limited, QE ratio read-limited
SNR is an amplitude ratio: dB = 20·log₁₀(SNR gain) (QE ratio 4 → 2× → 6.02 dB shot-limited)
Both sensors share the same CFA and microlens; the BI advantage is the removal of the gate and metal stack
The spectra are schematic: a Gaussian shape about 550 nm, with a 70 % front-side loss below 400 nm
Model and references
Illustrative. A simplified model that shows trends and orders of magnitude. Don't use its numbers for design. About the tiers
The peak QEs are inputs; effective QE is min(1, QE·microlens gain)·CFA transmission. The SNR gain is √(QE ratio) when photon shot noise dominates and the QE ratio when read noise dominates, an amplitude ratio quoted as 20·log₁₀. The spectra are a schematic Gaussian about 550 nm with a 70 % front-side loss below 400 nm, not a modelled or measured response.
- Janesick J. R. (2007). Photon Transfer. SPIE Press. Read-noise, shot-noise and dark-noise regimes; total noise as the quadrature sum. doi:10.1117/3.725073