Because of its different current injection mechanism, a tunnel field-effect transistor (TFET) can achieve a sub-60-mV/decade subthreshold swing at room temperature, which makes it very attractive in replacing a metal-oxide- semiconductor field-effect transistor, particularly for low-power applications. It is well known that some specific TFET structures show a good drain current ID saturation in the output characteristics, whereas other structures do not. A detailed investigation, through extensive device simulations, of the role of the channel on the drain-potential dependence of double-gate TFET characteristics is presented in this paper for the first time. It is found that a good saturation of ID is observed only for devices in which a thin silicon body is used. A relatively thick silicon body or gate-drain underlaps result in the penetration of the drain electric field through the channel, which does not allow the drain current to saturate, even at higher drain voltages. © 2011 IEEE.