The ambipolar behavior of tunneling field-effect transistors (TFETs) has been investigated quantitatively by introducing a novel parameter: ambipolarity factor (${ u}$). It has been found that the malfunction of TFET can result from the ambipolar state which is not on- or off- state. Therefore, the effect of ambipolar behavior on the device performance should be parameterized quantitatively, and this has been successfully evaluated as a function of device structure, gate oxide thickness, supply voltage, drain doping concentration and body doping concentration by using ${ u}$.