Motivation
The FET is a device which is designed to
switch between insulating and conducting states of its channel
material under the control of the gate. Generically speaking,
the FET geometry is an efficient one for switch design
irrespective of channel material specifics.
It is believed
that silicon FET switching technology will no longer operate in
its present widely implemented form at nanoscopic scales,
leaving a serious gap in available switching technology in the
nanoscopic regime.
In a nanoscopic FET-geometry switch (say channel length 10 nm. or
below) the active channel thickness must be very small, say of
order 1 nm.. For a significant carrier number in the channel,
and for support of such a thin channel, high carrier
concentrations of order 10 to 21 or 22 are required. A solution
to this requirement compatible with switchability is a channel
material which is metallic when the device is in its conducting
state, and a Mott insulator in its insulating state.
Coulomb Blockaded ('Mott') insulators
The
Coulomb-Blockaded or Mott insulator is one where the electrons,
typically one per site, are localized
because the energy cost incurred if one electron were to
move on to a neighboring site (where it interacts repulsively
with the already resident electron) is too high relative to the
available electronic kinetic energy for this transfer to occur.
The Mott transition
The Mott transition between
the insulating, Coulomb Blockaded, state, and the metallic state
may be induced by altering the number of electrons from the
value of integral number per site ('doping'). If, for example,
some holes are present, then an electron transfer into a neighboring site may occur
if the site is empty (occupied by a hole). In the case of a
cuprate superconductor, the system becomes metallic at about 15%
doping. Under these conditions the conductivities of the doped Mott insulators such as cuprates or
organic 'synthetic metals' are competitive with those of the
existing Si FET channel.
The MTFET is a device with a
channel which, (in the case of the enhancement mode device
described here) consists of a Mott insulator (shown as a line of
spherical molecules in the figure).
When a potential is
applied to the gate, an additional number of holes (p-type case)
or electrons (n-type case) is induced in the active region of
the channel lying closest to the gate. This number is
sufficient to turn the device on via transition to the
conducting state in a working device.
Electrical
Characterstics
The characteristics
of the device in its conducting state are qualititatively
similar to those of the conventional FET.
In its insulating
state the device is dependent on the Coulomb Blockade gap for
its regime of insulating behavior.
Mott Insulator
Material
One possible choice of channel material is an
organic Charge Transfer salt, similar to TTF-TCNQ, but which is
Type I in Torrance's classification, i.e. charge transfer is
complete making it insulating. An example of the type of
material is KTCNQ . This material has
a Coulomb gap in the insulating state of approx. 1 eV.
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