  We present a scaling theory of two-dimensional (2D) field effect transistors
(FETs). For devices with channel thickness less than 4 nm, the device
electrostatics is dominated by the physical gate oxide thickness and not the
effective oxide thickness. Specifically, for symmetric double gate (DG) FETs
the scale length ({\Lambda}) varies linearly with the gate oxide
thickness(t_{ox}) as {\Lambda} ~ 3/4t_{ox}. The gate oxide dielectric
permittivity and the semiconductor channel thickness do not affect the device
electrostatics for such device geometries. For an asymmetric device such as
single gate (SG) FETs, the fringing fields have a second order effect on the
scale length. However, like symmetric DG FETs, the scale length in asymmetric
FETs is also ultimately limited by the physical gate oxide thickness. We
compare our theoretical predictions for scaled monolayer MoS2 DG FETs.
