This paper presents an analytical model for the on-current (I/sub ON/) of ballistic MOSFETs that points out how the reduction of the in-plane masses implies a trade-off between the increase of the electron velocity and the reduction of the 2D density of states (D/sub 2D/). Numerical simulations confirm the analytical results and demonstrate that the I/sub ON/ is deteriorated for materials with a very small D/sub 2D/.