Photocurrent: what changes when the light turns on?
Hold the voltage across a device fixed and switch the light on and off. The change in current is the photocurrent. To find out what produced it, compare measurements at different wavelengths, biases or optical powers.
From absorbed light to an electrical signal
An absorbed photon can excite an electron in a semiconductor and leave behind a hole. A built-in electric field at a junction can separate the two, allowing current to flow through a closed external circuit even at zero applied voltage. Light can also change a material’s conductance, which changes the current when an external bias is applied.
From two current readings to responsivity
Subtract the dark current from the illuminated current at the same bias, gate voltage and temperature. The sign gives the direction of the change. To compare magnitudes, divide the absolute difference by the optical power incident on the active area. If the beam is wider than the device, account for the fraction that actually reaches that area.
For example, a rise from 2 nA to 12 nA under 50 nW of incident power gives a photocurrent of 10 nA and a responsivity of 0.20 A/W. To estimate the weakest signal the detector can resolve, you also need the current noise in the measurement bandwidth.
Measurement schematic and relation
I(light) and I(dark) are the illuminated and dark currents in amperes, measured under identical electrical conditions. P(in) is the incident power on the active area in watts. R is the responsivity magnitude in A/W at that wavelength, bias and power.
Reading Figure 2: why look near zero bias?
For the MoTe₂/graphene/SnS₂ device, Figure 2b shows how short-circuit current and open-circuit voltage vary with optical power at 655 nm. Both were small at 1550 nm, yet Figure 2d shows a photocurrent under applied bias at that wavelength. The authors used this contrast to distinguish a photovoltaic junction response from a photoconductive response at the longer wavelength.
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