  Gunn (or Gunn-Hilsum) Effect and its associated negative differential
resistivity (NDR) emanates from transfer of electrons between two different
energy bands in a semiconductor. If applying a voltage (electric field)
transfers electrons from an energy sub band of a low effective mass to a second
one with higher effective mass, then the current drops. This manifests itself
as a negative slope or NDR in the I-V characteristics of the device which is in
essence due to the reduction of electron mobility. Recalling that mobility is
inversely proportional to electron effective mass or curvature of the energy
sub band. This effect was observed in semiconductors like GaAs which has direct
bandgap of very low effective mass and its second indirect sub band is about
300 meV above the former. More importantly a self-repeating oscillation of
spatially accumulated charge carriers along the transport direction occurs
which is the artifact of NDR, a process which is called Gunn oscillation and
was observed by J. B. Gunn. In sharp contrast to GaAs, bulk silicon has a very
high energy spacing (~1 eV) which renders the initiation of transfer-induced
NDR unobservable. Using Density Functional Theory (DFT), semi-empirical 10
orbital ($sp^{3}d^{5}s^{*}$) Tight Binding (TB) method and Ensemble Monte Carlo
(EMC) simulations we show for the first time that (a) Gunn Effect can be
induced in narrow silicon nanowires with diameters of 3.1 nm under 3 % tensile
strain and an electric field of 5000 V/cm, (b) the onset of NDR in I-V
characteristics is reversibly adjustable by strain and (c) strain can modulate
the value of resistivity by a factor 2.3 for SiNWs of normal I-V
characteristics i.e. those without NDR. These observations are promising for
applications of SiNWs in electromechanical sensors and adjustable microwave
oscillators.
