Electrostatically tunable negative
differential resistance (NDR) is demonstrated in monolithic metal–semiconductor–metal
(Al–Ge–Al) nanowire (NW) heterostructures integrated
in back-gated field-effect transistors (FETs). Unambiguous signatures
of NDR even at room temperature are attributed to intervalley electron
transfer. At yet higher electric fields, impact ionization leads to
an exponential increase of the current in the ⟨111⟩
oriented Ge NW segments. Modulation of the transfer rates, manifested
as a large tunability of the peak-to-valley ratio (PVR) and the onset
of impact ionization is achieved by the combined influences of electrostatic
gating, geometric confinement, and heterojunction shape on hot electron
transfer and by electron–electron scattering rates that can
be altered by varying the charge carrier concentration in the NW FETs.