In this letter, the occupancy and tunneling probabilities of interband tunneling devices are studied, pointing out the fundamental function of the source Fermi-Dirac distribution. Particularly, the reason for the degraded subthreshold swing, which is typical of devices with highly doped source, is explained, and its relation with the high-energy source Fermi tail is carefully analyzed. Simultaneously, the poor driving capability of Tunnel-FET devices is investigated, highlighting the primary role played by the occupancy functions.