Scientists at the Laboratory of Nanophotonics Materials and Technology at Beijing Institute of Technology, China, are studying room temperature FET-based photodetectors. The concept of a phototransistor, such as a photo-field-effect-transistor, offers an attractive possibility: gain united with a lowered dark current compared with the photo-conductor, achieved if the thickness of the current-carrying channel can be chosen independently from the thickness of the light-absorbing layer.

In the case of conventional field effect transistor photodetectors, an electrical gate bias is used to modulate the lateral field across the active layer between source and drain electrodes. This electric field causes the separation of photogenerated carriers and greatly extends the carrier recombination lifetime leading to a much higher sensitivity.

As shown in the figure, the output characteristics of the FET-based photodetector ITO/PMMA(180 nm)/P3HT:PCBM:PbS(110 nm)/A1, in which the nanocomposite blend is in the weight ratio of MP3HT:MPCBM:MPbS=1:1:1, show an n-type enhanced operational mode. That is, the drain-source current (IDS) increases more greatly at positive gate biases than that at negative gate biases under 600 nm illumination with an intensity of 30 µW/cm2.

The team’s experimental data shows that the absorption mechanism and charge carrier transport mechanism of the active layer under different applied gate voltages can be revealed by selecting different wavelengths of incident light. The researchers report that the FET-based photodetector displays ambipolar behaviour in the dark under forward and backward biases, but for near-infrared illumination (980 nm laser irradiation) only PbS can absorb light and photoinduced holes from PbS CQDs are dominant in the active layer. As a consequence, IDS increases more greatly under negative gate voltages than that under positive gate biases and the device shows p-type enhanced characteristics.

In this way, developers can control the dominant charge carriers in the active layer using different incident illumination. Also, because this electric field causes the separation of photogenerated carriers and greatly extends the carrier recombination lifetime, a much higher sensitivity can be obtained for this kind of FET-based phototransistor.

More information can be found in the journal Nanotechnology.