Blue sensitive sub-band gap negative photoconductance in SnO 2 /TiO 2 NP bilayer oxide transistor.

Large negative photoconductance (NPC) of SnO 2 /TiO 2 nanoparticles (NPs) heterostructure has been observed with thin film transistor (TFT) geometry and has been investigated using sub-bandgap light (blue) illumination. This negative photoconduction has been detected both in accumulation and depletion mode operation, which effectively reduces the carrier mobility ( μ ) of the TFT. Moreover, the threshold voltage ( V th ) widely shifted in the positive direction under illumination. The combined effects of the reduction of mobility and V th shifting led to a faster reduction of On (or Off) state current under illumination. The negative photosensitivity of this system is as high as 3.2 A W -1 , which has been rarely reported in the earlier literature. Moreover, the variation of On (or Off) current, μ and V th shift is linear with low-intensity blue light. This SnO 2 /TiO 2  NP bilayer channel has been deposited on top of an ionic dielectric (Li-Al 2 O 3 ) that reduces its operating voltage of this TFT within 2 V. Furthermore, the device has achieved a saturation mobility of 0.4 cm 2 V -1 s -1 with an on/off ratio of 7.4 × 10 3 in the dark. An energy band diagram model has been proposed based on the type-II heterostructure formation between SnO 2 /TiO 2 semiconductors to explain this NPC mechanism. According to the energy band diagram model, adsorbed H 2 O molecules of TiO 2 NPs created a depleted layer in the heterostructure that accelerated the recombination process of photo-generated carriers rather than its transport.