Responsivity Calculator

Calculate spectral responsivity from TMM-based quantum efficiency. Compare per-channel responsivity against the ideal silicon photodiode response.

Spectral Responsivity Calculator

Convert QE spectrum to spectral responsivity R(λ) = QE × qλ/(hc). Compare R/G/B channels with ideal Si photodiode.

Si Thickness: 3.0 μm

CF Bandwidth: 100 nm

Peak R (Red)

0.324 A/W

Peak R (Green)

0.347 A/W

Peak R (Blue)

0.246 A/W

Model scope

Use this browser tool for intuition, relative trends, and design-space exploration. Its local simplified model is not a substitute for RCWA/FDTD sign-off, silicon calibration, or vendor process data.

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Noise, SNR, and Dynamic Range · Quantum Efficiency

Physics Notes

Quantum Efficiency to Responsivity

Plain-English Intuition

QE tells you what fraction of photons turn into electrons — a number between 0 and 1. But circuit designers usually need current per watt of optical power (A/W) instead. Responsivity converts between the two using the energy of one photon, and it naturally grows with wavelength: longer-wavelength photons carry less energy each, so the same QE produces more current per watt.

Spectral responsivity converts optical power at a wavelength into photocurrent, using the photon energy and quantum efficiency.

Assumptions

Responsivity converts photons-to-electrons efficiency into current per optical watt at a single wavelength.
One collected electron is assumed per successful photon event; avalanche gain, multiplication, and circuit bandwidth are omitted.
Broadband response requires spectral integration over source power, not a single wavelength point.

Outputs

Photon energy, QE-to-A/W conversion, photocurrent for optical power, and wavelength dependence of responsivity.
A bridge between optical QE simulations and electrical current or photodiode measurement units.

Validation Example

At fixed QE, responsivity should increase linearly with wavelength because each photon carries less energy.
Setting optical power to zero should produce zero photocurrent regardless of QE.

Core Equations

Photon energy

$$E_{\text{ph}} = \frac{hc}{\lambda}$$

$h$: Planck constant
$c$: Speed of light

Longer wavelengths carry less energy per photon.

Responsivity

$$\mathcal{R}(\lambda) = \frac{QE(\lambda) \cdot q \lambda}{hc}$$

$\mathcal{R}$: Responsivity (A/W)
$q$: Elementary charge

With lambda in micrometers, R ~= QE*lambda/1.2398 A/W.

Photocurrent

$$I_{\text{ph}} = \mathcal{R}(\lambda) \cdot P_{\text{opt}}$$

$I_{\text{ph}}$: Photocurrent
$P_{\text{opt}}$: Optical power

Responsivity links optical simulation to electrical current.

Model Interpretation

The same QE gives higher A/W at longer wavelengths until silicon absorption falls.
Responsivity is not color accuracy; it is a power-to-current metric.
Measured responsivity includes optics, fill factor, and collection efficiency.

QE Versus A/W

QE counts electrons per photon; responsivity counts amperes per watt.
Because $E_{\text{ph}}=hc/\lambda$, the same photon conversion efficiency produces more current per watt at longer wavelength.
Responsivity can rise with wavelength even when photon absorption is not improving.

Measurement Use

Use monochromatic calibrated optical power to measure spectral responsivity.
Subtract dark current and verify linearity before converting photocurrent to responsivity.
Compare measured $\mathcal{R}(\lambda)$ with optical QE only after accounting for fill factor and collection efficiency.

Known Missing Physics

The conversion assumes one collected electron per successful photon event and omits avalanche gain or multiplication.
It does not include bandwidth, capacitance, transimpedance gain, or readout circuit limitations.
Broadband responsivity requires spectral integration over the source spectrum, not a single-wavelength value.