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The effects of low-κ and high-κ spacer were investigated on the novel tunnel dielectric based tunnel field-effect transistor (TD-FET) mainly based upon ultra-thin dielectric direct tunneling mechanism. Drive currents consist of direct tunneling current and band-to-band tunneling (BTBT) current. Meanwhile, tunneling position of the TD-FET differs from conventional tunnel-FET in which the electron and hole tunneling occur at intermediate rather than surface in channel (or source-channel junction under gate dielectric). The 2-D nature of TD-FET current flow is also discussed that the on-current is degraded with an increase in the spacer width. BTBT current will not begin to play part in tunneling current until gate voltage is 0.2 V. We clearly identify the influence of the tunneling dielectric layer and spacer electrostatic field on the device characteristics by numerical simulations. The inserted Si3N4 tunnel layer between P+ region and N+ region can significantly shorten the direct and band-to-band tunneling path, so a reduced subthreshold slope (SS) and a high on-current can be achieved. Above all the ambipolar current is effectively suppressed, thus reducing off-current. TD-FET demonstrates excellent performance for low-power applications.