Temperature-Dependent Opacity of the Gate Field Inside MoS₂ Field-Effect Transistors

The transport behaviors of MoS₂ field-effect transistors (FETs) with various channel thicknesses are studied. In a 12 nm thick MoS₂ FET, a typical switching behavior is observed with an I_on/I_off ratio of 10⁶. However, in 70 nm thick MoS₂ FETs, the gating effect weakens with a large off-current, resulting from the screening of the gate field by the carriers formed through the ionization of S vacancies at 300 K. Hence, when the latter is dual-gated, two independent conductions develop with different threshold voltage (V_TH) and field-effect mobility (μ_FE) values. When the temperature is lowered for the latter, both the ionization of S vacancies and the gate-field screening reduce, which revives the strong I_on/I_off ratio and merges the two separate channels into one. Thus, only one each of V_TH and μ_FE are seen from the thick MoS₂ FET when the temperature is less than 80 K. The change of the number of conduction channels is attributed to the ionization of S vacancies, which leads to a temperature-dependent intra- and interlayer conductance and the attenuation of the electrostatic gate field. The defect-related transport behavior of thick MoS₂ enables us to propose a new device structure that can be further developed to a vertical inverter inside a single MoS₂ flake.