  The Hamiltonian size reduction method or the mode space method applicable to
general heterogeneous structures is developed in this work. The effectiveness
and accuracy of the method are demonstrated for four example devices of
GaSb/InAs tunnel field effect transistor (FET), MoTe2/SnS2 bilayer vertical
FET, InAs nanowire FET with a defect, and Si nanowire FET with rough surfaces.
The Hamiltonian size is reduced to around 5 % of the original full Hamiltonian
size without losing the accuracy of the calculated transmission and local
density of states in a practical sense. The method developed in this work can
be used with any type of Hamiltonian and can be applied to virtually any
hetero-structure, so it has the potential to become an enabling technology for
efficient simulations of hetero-structures.
