Fig. 1: Contact gating in field-effect transistors (FETs).
a Schematic of a back-gated FET, where inset b shows the parallel field from the back gate directly gating the entire contact. c Schematic of a top-gated FET, where inset d shows fringing fields gating the contact edge. (Top-gated devices without underlaps would have their contact edges gated directly by the parallel field instead.) As a result of contact gating, the channel resistance Rch and contact resistance RC can decrease at different rates as Vgs increases (e), creating a distinct kink in the resultant Id vs. Vgs characteristics (f). After exiting the subthreshold region, the channel may turn on even as the contacts remain off, suppressing Id (e, f, Region I). The contacts then turn on as Vgs further increases, leading to a sharp increase of Id (e, f, Region II). The magnitude and slope of Id here are dictated by the contacts rather than by the channel; attempting to extract the channel mobility from this region with conventional techniques can result in severe overestimation. Finally, the FET returns to a channel-limited regime when 2RC < Rch (e, f, Region III); μ can usually be safely extracted from this region. The diagrams shown here are for an n-channel FET.
