Integration of Ni2Si/Si Nanograss Heterojunction on n-MOSFET to Realize High-Sensitivity Phototransistors

We report a top-down fabrication technique for direct integration of Ni₂Si/Si heterojunction arrays on n-type MOSFET gate terminal to realize sensitive field-effect phototransistors (PTs). It was observed that exposing gate area to light (λ = 655 nm) leads to significant modulation of threshold voltage (V_TH) of PT in comparison with a conventional MOSFET. By analyzing optical absorption spectra of Ni₂Si/Si nanostructures on SiO₂ of this novel PT and planar Ni₂Si/poly-Si on SiO₂ of conventional transistors, we concluded that, nanostructures strongly improve light absorption. This results in higher optically generated electron-hole pairs in which excess holes induce an electric field on the transistor channel and help the threshold happen at lower voltages. To recognize the appropriate operating point of such nanostructure-based PT, effect of V_GS on the sensitivity of drain current was investigated. Experimental results show that device sensitivity (S) is related to gate-source voltage via a homographic like function. Theoretical analysis shows S ∝ 1/(V_GS - V_TH) for high V_GS, which has been confirmed by experimental data. In addition, external quantum efficiency and photoresponsivity of PT as functions of gate voltage were studied. The achieved results suggest that this device would be a promising candidate for fabricating low-power PTs based on silicide nanostructures.