Diameter-dependent photoresponse with high internal gain in a back gated single Si nanowire phototransistor

Size-dependent photodetection properties of back-gated single Si nanowire (NW) phototransistor with different nanowire diameters have been investigated. The nanowire transistor has been fabricated on silicon-on-insulator wafers with a buried oxide with a thickness of 300 nm and a top silicon layer with a thickness of 50 nm. The fabricated devices show 10³ times change in photocurrent under the exposure of small optical power ∼1 nW, which shows the high sensitivity of the devices. A high internal gain of the order of 10³–10⁴ has been observed from these devices. Diameter-dependent photoresponse was also observed from the devices, as the smaller NWs have shown a higher gain compared to larger NWs. NWs with a 50 nm diameter has shown maximum gain of 1.2  ×  10⁴ with a rise time of 120 µs. Finite-difference time-domain simulation shows that the confinement of incident optical energy is much more effective in the smaller NWs due to wave guiding effect in the NWs, which leads to confinement of photon energy and associated electric field. (paper)