Two kinds of coer effects existing in double-gate (DG) and gate-all-around (GAA) MOSFETs have been investigated by three-dimensional (3D) and two-dimensional (2D) simulations.It is found that the coer effect caused by conterminous gates,which is usually deemed to deteriorate the transistor performance,does not always play a negative role in GAA transistors.It can suppress the leakage current of transistors with low channel doping,though it will enhance the leakage current at high channel doping.The study of another kind of coer effect,which exists in the coer at the bottom of the silicon pillar of DG/GAA vertical MOSFETs,indicates that the D-top structure with drain on the top of the device pillar of vertical transistor shows great advantage due to lower leakage current and better DIBL (drain induced barrier lowering) effect immunity than the S-top structure with source on the top of the device pillar.Therefore the D-top structure is more suitable when the requirement in leakage current and short channel character is critical.