  A high performance triple-heterojunction (3HJ) design has been previously
proposed for tunneling FETs (TFETs). Compared with single heterojunction (HJ)
TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission
resonances that significantly improve the ON-state current ($I_{\rm{ON}}$).
Coherent quantum transport simulation predicts, that $I_{\rm{ON}}=460\rm{\mu
A/\mu m}$ can be achieved at gate length $Lg=15\rm{nm}$, supply voltage
$V_{\rm{DD}}=0.3\rm{V}$, and OFF-state current $I_{\rm{OFF}}=1\rm{nA/\mu m}$.
However, strong electron-phonon and electron-electron scattering in the heavily
doped leads implies, that the 3HJ devices operate far from the ideal coherent
limit. In this study, such scattering effects are assessed by a newly developed
multiscale transport model, which combines the ballistic non-equilibrium
Green's function method for the channel and the drift-diffusion scattering
method for the leads. Simulation results show that the thermalizing scattering
in the leads both degrades the 3HJ TFET's subthreshold swing through scattering
induced leakage and reduces the turn-on current through the access resistance.
Assuming bulk scattering rates and carrier mobilities, the $I_{\rm{ON}}$ is
dropped from $460\rm{\mu A/\mu m}$ down to $254\rm{\mu A/\mu m}$, which is
still much larger than the single HJ TFET case.
