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We have studied source-drain tunneling in double gate (DG) metal oxide semiconductor field effect transistors (MOSFETs) scaled down to 10 nm channel lengths. The full complex band structure is used, along with the nonequilibrium Green's function (NEGF) formalism, to calculate the quantum transport characteristics in the ballistic region. To get a good description of the Si band structure, the empirical tight-binding Hamiltonian including nearest and second nearest neighbor coupling is chosen. Far from the band edge, the energy versus wave vector relation is quite different from that based on effective mass theory, and could result in a higher tunneling current in the off state when compared to that obtained from effective mass theory. Overall, the analysis confirms the adequacy of an effective mass treatment for Si MOSFETs scaled to approximately 10 nm channel lengths. Below this scale or for low temperature operation, however, consideration of the complex band structure becomes important.