Table 1.

Key performance parameters of photodetectors.

Parameters	Descriptions	Equation	Unit
Responsivity (R)	Ratio of photocurrent (Iph) through the detector to the power of input light, Poptical, where Iph is Iilluminated—Idark	$\frac{I_{illuminated}-I_{dark}}{P_{optical}}$	A/W
External quantum efficiency (EQE)	Ratio of number of charge carriers collected by the photodetector to the number of incident light. Coefficient of material and thickness of absorbing material influence the EQE.	$\frac{hc}{e\lambda }R$	-
On/off ratio	Ratio of photocurrent towards dark current. It is preferable to have high on/off ratio because it indicates that the device has better photocurrent and lower dark current which contributes to the noise in the device.	$\frac{I_{ph}}{I_{dark}}$	-
Specific detectivity ($D^{\ast })$	How weak a light the device can detect is defined by the responsivity and dark current of the photodetector. Dark current should be reduced as far as possible as it contributes to the noise in the device and also to distinguish very weak optical signal which can be expressed as follows, where $\Delta f$ is electrical bandwidth, in is noise current and A is the effective area of the detector.	$\frac{{\left(A\Delta f\right)}^{1/2}. R}{i_n}$	Jones
Response speed	Rise time and fall time of the optical signal reaction, which is from 10% to 90% and 90% to 10%, respectively, of the maximum photocurrent	-	s
Operating bandwidth	Frequency at which the output power drops by ½, that is, when photocurrent drops to 70.7%.	-	Hz
Gain (G)	Depends on carrier mobility (µ), photogenerated carriers’ lifetime ($\tau$) and applied field $\left(E\right)$. L is channel length of the device.	$\left(\mu \tau E\right)/L$	-
Noise equivalent power (NEP)	Required incident power to produce signal-to-noise (SNR) of 1 at 1 Hz bandwidth, which is also given by the noise spectral density, Sn.	$\frac{S_n}{R}$	$\raisebox{1ex}{$W$}\!\left/ \!\raisebox{-1ex}{$\surd Hz$}\right.$
Linear dynamic range (LDR)	Region of incident power within photodetector that has a linear response. It is the logarithm of the ratio between saturation power (Psat) and NEP.	$10\times {\log }_{10}\frac{P_{sat}}{NEP}$	dB