  We report the capability to simulate in a quantum mechanical tight-binding
(TB) atomistic fashion NW devices featuring several hundred to millions of
atoms and diameter up to 18 nm. Such simulations go far beyond what is
typically affordable with today's supercomputers using a traditional real space
(RS) TB Hamiltonian technique. We have employed an innovative TB mode space
(MS) technique instead and demonstrate large speedup (up to 10,000x) while
keeping good accuracy (error smaller than 1 percent) compared to the RS NEGF
method. Such technique and capability open new avenues to explore and
understand the physics of nanoscale and mesoscopic devices dominated by quantum
effects. In particular, our method addresses in an unprecedented way the
technological relevant case of band-to-band tunneling (BTBT) in III-V nanowire
MOSFETs and broken gap heterojunction tunnel-FETs (TFETs). We demonstrate an
accurate match of simulated BTBT currents to experimental measurements in a
[111] InAs NW having a 12 nm diameter and a 300 nm long channel. We apply the
predictivity of our TB MS simulations and report an in-depth atomistic study of
the scaling potential of III-V GAA nanowire heterojunction n and pTFETs
quantifying the benefits of this technology for low-power, low-voltage CMOS
application. At VDD = 0.3 V and IOFF = 50 pA/um, the on-current (Ion) and
energy-delay product (ETP) gain over a Si NW GAA MOSFET are 58x and 56x
respectively.
