IEEE Spectrum July, 2016 - 28

so future progress must still involve improving the underlying switches.
modifying materials and device geometries can create transistors with better
electronic properties than silicon transistors have today. But such evolutionary approaches will yield only small benefits. to get bigger gains, we're left
with one option: use novel nanotechnologies to supplant silicon altogether.
there are many candidates for this role. But the most exciting and mature
contender by far is the single-walled carbon nanotube, a rolled-up sheet
of linked carbon atoms. if you're experiencing some déjà vu now, it's no
wonder. there was great enthusiasm for carbon nanotubes in the 1990s,
and a number of big-name semiconductor companies began investigating
them. But the carbon nanotube fell out of fashion when researchers ran into
obstacles in circuit fabrication that seemed too hard to overcome. When
the first measurements of the electronic properties of graphene-flat lattices of carbon atoms-were reported in 2004, many were all too eager to
abandon carbon nanotubes and take up that shiny new technology.
now the carbon nanotube is back and better than ever. unlike its muchhyped two-dimensional cousin, the carbon nanotube comes in a natural
semiconducting form, which means it can be turned on and off to make
a binary switch. And recent research into the material has moved well
beyond the demonstration of simple logic gates. in 2013, for example,
our group at stanford university built the first complete digital system-a
basic computer-entirely out of carbon-nanotube circuits. What's exciting is that the fabrication and design techniques we employed to build
that computer can be seamlessly integrated into a semiconductor fabrication facility today and used to manufacture chips with the billions of
transistors needed to compete with state-of-the-art silicon.
not only can carbon nanotubes make traditional chips more powerful, they can also be used to create entirely new computing systems-
ultrafast and efficient three-dimensional chips that can be employed in
data centers as well as the wide world of mobile computers used in cars,
smartphones, and the sensor-filled internet of things. such 3d systems
could mix carbon nanotubes and emerging memory technologies, and
they could also be built directly on top of conventional silicon circuitry.
so while carbon nanotubes may eventually leap ahead of silicon, they
could also share the road with it. in both cases, the resulting massive
advances in computational capability would affect our lives profoundly.
Source

Gate

Carbon
nanotubes
Drain

Silicon oxide
Silicon

Gate oxide

switching with carbon: a carbon-nanotube field-effect transistor (FEt)

resembles a conventional silicon FEt. the main difference is the current-carrying
channel, which is made from carbon nanotubes [yellow] instead of silicon.

jul 2016

North AmericAN

SPectrum.ieee.orG

he initiAl discovery of carbon nanotubes is hard to pinpoint
in time. But many cite a landmark
1991 paper by physicist sumio
iijima, who grew "graphitic carbon needles" on an electrode in a gas-filled vessel. iijima's work inspired a flurry of interest,
and soon electrical engineers began suspecting that carbon nanotubes might be an ideal
transistor technology.
to build a transistor, you need several things:
a source and drain-the origin point and destination for current; a channel between the two
where the current can flow; and a gate near the
channel to control that flow. semiconductordevice physics is complex, particularly when
it comes to today's nanoscale switches, but as
a general rule of thumb, there are two basic
properties that determine a transistor's performance. First, the more current that flows
through the channel, the faster a circuit can be.
that's because greater current allows the gate
of the next device in the circuit to be charged
faster. second, the thinner the channel, the more
energy efficient the device will be, because it will
be easier for the gate to exert the control needed
to turn the transistor on and off. think of the
channel as a water hose and the gate as your
foot. the thinner the hose, the less effort you'll
need to make in order to stem the flow of water.
unfortunately, in today's ultrasmall silicon
devices, these two factors-the thinness of the
channel and the speed at which charge can
flow through it-are at odds with each other.
like other three-dimensional bulk materials,
as silicon gets thinner, the sides and edges of
the crystal have a stronger and stronger effect
on its electrical properties. these so-called surface effects impede the movement of charge
through the material.
But the carbon nanotube naturally and
simultaneously fulfills both of those transistor
channel requirements. the nanotube's smoothwalled structure is naturally devoid of dangling
bonds, permitting charges to flow through it
quite rapidly, many times faster than in silicon. And because carbon nanotubes are also
very small-in the realm of about a nanometer
in diameter-a gate can easily control the flow
of current along its length.
A carbon-nanotube transistor looks much
the same as a silicon transistor. the main difference is that the channel is made of carbon
nanotubes instead of silicon. Just as in a silicon transistor, the electric field created by a
illustration by

Jean-Luc Fortier

http://SPectrum.ieee.orG

Table of Contents for the Digital Edition of IEEE Spectrum July, 2016