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Dark Current

Definition: a current from a photodetector which occurs even in the absence of a light input

German: Dunkelstrom

Category: physical foundations


Cite the article using its DOI: https://doi.org/10.61835/inc

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Most photodetectors such as photodiodes, phototransistors, CCD sensors and phototubes produce a signal current which is approximately proportional to the incident optical power. However, even in the absence of any light input, there is often some tiny amount of DC current, which one calls the dark current. An additional fluctuating current with zero mean value, caused by thermal fluctuations, is usually not called a dark current.

Depending on the photodetector device and the conditions, the dark current can have very different magnitudes – sometimes well below 1 nA, in other cases many orders of magnitude more.

For many applications, the dark current is totally negligible, but in some cases it matters – for example, when extremely small optical powers need to be detected. One may in principle subtract the dark current from the obtained signal either with analog electronics or with software, but that works only to a limited extent because the dark current can be substantially temperature-dependent (see below), and it also exhibits shot noise.

Origins of Dark Current

The dark current of a photodetector can have different origins.

Dark Current in Photodetectors with Internal Photoelectric Effect

In photodiodes and other detectors with some p–n or p–i–n junction, it is often caused by thermal excitation (generation) of carriers – not necessarily directly from valence to conduction band, but possibly through defect states related to crystal defects or impurities (and in that case of course with a lower activation energy). The rate of such thermal processes depends not only on the active area, but also critically on the temperature and on the band gap energy of the material (and possibly of energy levels of common defects), and also on the operation voltage (particularly near the breakdown voltage, where impact ionization can occur). At high voltages, tunneling through the depletion region may also contribute.

For visible light detectors such as silicon-based photodiodes, the dark current can be very small (e.g. in the picoampere region) (even for significant bias voltages) and is then negligible for most applications. Germanium photodiodes exhibit much higher dark currents which is however mostly not due to their somewhat lower band energy. Indium gallium arsenide diodes, which also have a reduced bandgap energy compared with silicon, also exhibit a relatively low dark current.

For materials with substantially smaller band gap, dark current can be a serious problem and may thus enforce the operation at substantially reduced temperatures. Therefore, some mid-infrared cameras, for example, need to be equipped with a Stirling cooler for operation around 100 K or even lower.

For operation near the break-down voltage, the dark current can become far stronger than for lower voltages.

Dark currents may also be generated by some leakage currents which are not related to thermal excitation.

In any case, a dark current can normally not occur for operation with zero bias voltage, since there is no energy supply available for it – at least as long as the temperature of the device is uniform, excluding any Peltier effects. Therefore, one may operate a photodiode, for example, with zero bias voltage in cases where influences of a dark current must be avoided.

Of course, drifts of output signals may also occur in related electronics, for example due to bias drifts of operational amplifiers. Therefore, a non-zero output signal does not necessarily indicate a dark current of the detector.

Dark Current in Photodetectors with External Photoelectric Effect

The primary cause for a dark current is usually thermionic emission on the photocathode. This means the thermal excitation of electrons. Thermionic emission can be substantial for cathode materials with very low work function, as required for infrared detection. It is also strongly temperature-dependent; low-temperature operation is thus a very effective measure for reducing the dark current. The dependence on the operation voltage is weak.

For quite high operation voltages, there can be a steeper rise of dark current due to field emission at various locations in the bulb. That can lead to accelerated aging.

Some current is contributed by the ionization of residual gas, i.e., due to the non-perfect vacuum. This is particularly the case for devices operated with higher voltages, for example photomultipliers.

A typically quite weak contribution comes from the leakage current due to non-perfect electrical isolation.

It is also possible that some unwanted light is generated by scintillation, e.g. when electrons hit the glass tube. At a usually very low level, there are weak flashes of light caused by cosmic rays and radioactive substances e.g. in the glass tube or the near surroundings.

See also: photodetectors, photodiodes

Questions and Comments from Users


What is surface dark current?

The author's answer:

That is dark current caused by effects occurring at surfaces, rather than in the bulk medium.


Which of the following detectors suffer less from dark currents: PMTs, MCT, or photodiode arrays?

The author's answer:

That really depends very much on the circumstances. Some photodiodes have relatively strong dark currents, but when you operate them with an amplifier which keeps the bias voltage close to zero, that problem is completely eliminated. A photomultiplier tube can have very low dark current, but it also gets high if you have a very large detection area or if you operate it with very high voltage, or if it gets hot.


Why is the dark current small in phototransistors?

The author's answer:

I don't think that phototransistors fundamentally exhibit lower dark currents than photodiodes, for example. However, they are often based on silicon, which has a relatively high band gap energy and thus leads to low dark currents.


How can I measure dark current? Can I simply use a multimeter?

The author's answer:

Yes, if that multimeter is sufficiently sensitive, just measure the current with no light on the detector.


I am reviewing a visible light detector that specifies dark current and operating dark current. Could you please explain the difference?

The author's answer:

I can't – I guess it is the same.


Is there a difference between the dark current and back current?

The author's answer:

“Back current” is unknown to me, but maybe you mean backward current; more common would be reverse current.

Reverse current is generally an electric current which you get when applying a reverse voltage to a diode – meaning a voltage in a direction where the diode is generally not conducting. Only, you get some dark current, and possibly a photocurrent. The dark current is the reverse current which you get without any incident light.


Hello, how can you explain the position of minimum dark current voltage of a photodetector ?

I don't know why it should be placed at 0 V and it often changes, sometimes it's in reverse or direct voltage area even if it often is at 0 V. I'm referring to the voltage position of the “tip shaped” part of dark current curve on semi-log graphs or when dark current crosses x-axis of a linear real y-axis.

The author's answer:

In the dark, the photocurrent must always be zero for zero voltage. So there cannot be a non-zero voltage leading to even less current.

However, you might have some additional energy source, such as thermoelectric effects. That may happen, for example, if you do measurements shortly after exposure of the detector with intense light, which causes temperature gradients. Of course, we are then not just talk about photocurrents – we have currents driven by another effect.


How to test whether the measured current is a dark current or not?

The author's answer:

The best way is to check whether it stays if you really prevent any light from getting to the detector. In a lab, you may e.g. switch off the room illumination.

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