Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra-sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick TiO_x-based ultrathin TFTs exhibiting on-off ratio of ~10⁶, predominantly driven by ~6-decade gate-induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent regimes reflect the bi-exponential distribution of the density of band-tail states. This improved understanding would be significant in fabricating high-performance ultrathin oxide devices.

近来，超薄金属氧化物薄膜晶体管（TFT）已显示出非常高的通断比和超锐度的亚阈值摆幅，使其成为有望在常规大面积电子产品之外应用的候选材料。虽然典型TFT中的开-关操作主要是由栅极电压对载流子密度的调制引起的，但超薄氧化物TFT中的高开/关比可能与大的载流子迁移率调制有关，其起源仍然未知。我们研究了3.5 nm厚的TiO _x基超薄TFT，其通断比约为10 ⁶，主要由约6个十年的栅极感应迁移率调制驱动。迁移率的幂律特性具有两种状态，在低栅极电压下具有很高的指数，这对于氧化物TFT来说是空前的。我们发现，这种现象可以通过在导带边缘附近存在高密度尾态来很好地解释，这可以通过可变范围跳变来支持载流子传输。观察到的双指数态反映了带尾态密度的双指数分布。这种改进的理解对于制造高性能超薄氧化物器件将具有重要意义。