Glossary entry

Dark current (I_d)

The current flowing through a photodetector in the absence of any incident light. Sets the noise floor and minimum detectable signal.

Dark current is the current flowing through a reverse-biased photodetector when no light reaches the active area. It originates from thermally generated carriers in the depletion region, surface leakage along the diode periphery, and (in APDs) avalanche-multiplied versions of these.

Components:

Bulk dark current: thermal generation–recombination in the depletion region, scales with depletion volume and decreases exponentially with bandgap
Surface leakage: along the diode mesa or junction periphery, depends on passivation quality; typically the dominant component at low temperature
Diffusion dark current: minority carriers diffusing into the depletion region; usually negligible compared to bulk generation
Tunneling dark current: significant in high-bias APDs and small-bandgap materials (Ge, InAs)

Temperature dependence: bulk dark current scales as $I_d \propto T^{3/2} \exp(-E_g / 2kT)$ for generation-limited devices. Cooling by 7 K typically halves dark current for InGaAs detectors. Cryogenic IR detectors exploit this dependence aggressively.

Typical room-temperature dark current values:

Detector	Bias	Dark current
Si PIN (large area, telecom)	$-5$ V	0.1 – 1 nA
InGaAs PIN (50 μm, telecom)	$-5$ V	0.5 – 5 nA
InGaAs PIN (high speed, small)	$-5$ V	10 – 100 pA
Ge PIN	$-5$ V	100 – 1000 nA
InGaAs APD ( $M = 10$ )	near breakdown	5 – 50 nA
InGaAs APD ( $M = 100$ , single-photon)	$\sim$ breakdown	varies, $\sim$ kHz – MHz dark count
Ge-on-Si waveguide PD	$-2$ V	10 – 200 nA
HgCdTe (LWIR, cooled)	low	$\sim$ pA

Dark current produces:

Shot noise: $\sigma_I^2 = 2 e I_d B$ , where $B$ is the electronic bandwidth
DC offset: must be subtracted in low-light measurements (typically via lock-in or chopped reference)
APD gain noise floor: dark current is also multiplied by the avalanche gain, so APD operating points must balance signal gain against noise gain

For sensitive measurements (single-photon counting, weak fluorescence, short-pulse detection), dark current is the dominant noise source and drives the choice of detector area, bias, and operating temperature.