Tungsten Diselenide Transistors: Layer Number Dependence, Channel Passivation and In‐Plane Gates

High‐performance and few‐layer WSe 2 field‐effect transistors with p ‐type channels are fabricated through sequential mono‐layer WSe 2 film transferring. With 30 nm Al 2 O 3 passivation layer and polycrystalline gold electrodes, increasing field‐effect mobility values from 16.3 (1‐layer WSe 2 ), 45.4 (2‐layer WSe 2 ) to 81.3 (3‐layer WSe 2 ) cm 2 · V −1 · s −1 are observed with increasing WSe 2 layers. To further improve the device performances, tri‐layer WSe 2 transistors with in‐plane gates (IPGTs) and reduced channel width/length 400/500 nm are fabricated. High field‐effect mobility value 79.7 cm 2 · V −1 · s −1 and ON/OFF ratio > 10 8 are observed for the device, which demonstrates that the in‐plane gates can more effectively modulate the carrier concentration in the WSe 2 channel. When the IPGTs are operated as photodetectors, since the Au electrode is deposited on the topmost WSe 2 layer of the tri‐layer WSe 2 channel, the topmost WSe 2 layer may act as the main current flow path. The bottom WSe 2 layers in the tri‐layer WSe 2 channel would act as a light absorption layer to the photodetector. The hole accumulation resulted from the photo‐excited electron extraction from the bottom to top WSe 2 layers would lead to a Fermi level shift of the top WSe 2 layer. High responsivity 585 A/W and detectivities 10 12 Jones are, therefore, observed for the device.