We have simulated silicon nanowire field effect transistors in the ballistic transport regime using the effective mass theory and the mode space nonequilibrium Green's function method. In order to solve the two-dimensional Schrodinger equations on the nanowire cross-sectional planes as a part of the numerical procedure, we have developed an efficient numerical scheme, the product-space method, where the size of the eigenvalue problem is reduced to the number of subband modes that participate in the transport. We have investigated the scaling behavior of the nanowire transistors and found that their device characteristics sensitively depend on the aspect ratio of the channel length and width. (c) 2007 American Institute of Physics.