Proposal of an Intrinsic-Source Broken-Gap Tunnel FET to Reduce Band-Tail Effects on Subthreshold Swing: A Simulation Study

High source doping is essential in tunnel FETs (TFETs) to support high electric fields necessary to provide sufficient drive currents. High doping is associated with band-tails in the density of states that decay exponentially into the bandgap, comparable with the decay of the carrier density above the band edge due to Fermi-tails. This is expected to limit the subthreshold swing (SS) and OFF-current in the TFETs. In this paper, we analyze the effects of band-tails on the subthreshold characteristics of III-V TFETs based on GaSb/InAs heterojunction. It is shown that the decay constant comparable with room-temperature thermal energy, k
_B
T = 26 meV, can result in significant degradation of SS in TFETs from their ideal value. Electrically doping the intrinsic source could be an alternative, but not only does it increase the coupling between the gate and source contact due to proximity but also complicates the fabrication process. As a solution, we propose an undoped-source GaSb/InAs broken-gap TFET in an i-i-n configuration, which achieves similar performance to that of a corresponding p-i-n TFET while having exactly the same geometry. In the proposed device, the high drive current is supported by the broken-gap tunnel junction, while the Fermi-level pinning near the valence band of GaSb helps in realizing an ohmic source contact for the carriers in the valence band, which eventually participate in interband tunneling at the broken-gap tunnel junction in the ON-state.