Raytheon Technology Today 2011 Issue 1

The concept of a single atomic layer of
crystalline material is easy enough to
grasp, yet creating such a layer was
not achieved until 2004, when two scientists,
Andre Geim and Konstantin Novoslov,
demonstrated the existence of a single
atomic layer of carbon called graphene.
This discovery was followed by a flurry of
research activities, with the proposal of
several applications for defense and commercial
products. Through its support of
several multidisciplinary research initiatives
and DARPA-funded programs during the
past three years, the U.S. Dept. of Defense
has also indicated the importance of
graphene for its military applications.
The theoretically predicted and experimentally
verified values of graphene properties
have provided the impetus for a vast area
of opportunity, from nano-scale devices to
system-level advances. The implication of a
single crystalline layer of pure carbon has
spun many new startup businesses, which
is likely to continue. Each is based on a
unique finding aimed at anticipated and
emerging markets. Among the technologies
that can benefit from graphene in the near
future are:
Ultracapacitors. While batteries are high
energy density power sources, they cannot
deliver the energy to the load in short
time, due to the natural process of ionic
movement through an electrolyte between
the battery electrodes. On the other hand,
capacitors can release all their energy to
the load in a very short time; however, they
can store only a relatively small amount of
energy. Replacement of the carbon charcoal
with crumpled sheets of graphene
provides several orders of magnitude higher
charge storage capacity as in a battery,
while allowing for faster charge/discharge
time as in a capacitor — thus merging and
improving the two power storage technologies.
Ultracapacitor technology has
the potential to significantly improve many
Raytheon products such as radar front-end
electronics, which we are considering as the
first insertion point.
Thermal management. In power electronics,
heat removal from the active part of
the device is a substantial challenge. The
measured thermal conductivity (TC) of
single layer graphene is reported to be
nearly three times that of bulk diamond at
55 W/cm-K. This is mainly attributed to the
ability of phonons to propagate through the
crystalline layer without suffering from any
scattering processes. Engineering schemes
need to be developed to exploit such high
TC values successfully. Any method that
can harvest the superior TC of graphene for
thermal management in electronics circuitry
can have extensive implications in all areas
of digital, RF and optoelectronics.
Transparent conductors. Due to its high
sheet electronic charge density of 10 13 cm -2 ,
and high electron mobility, graphene is a
near perfect conductor. Furthermore, with
its single atomic layer nature, graphene
absorbs little visible light, making it an excellent
transparent conductor. Commercial
applications of this technology are already
underway for use on touch-screen monitors,
where large square-meter areas are being
processed at one time. Such a low sheet
resistance, low absorption layer is an ideal
material for many Raytheon electro-optics
applications, some of which currently
use indium oxide. The same low sheet
resistivity property of graphene can be
exploited in interconnect technologies
where material and fabrication cost
can be a significant factor.
THz Electronics. The superb material and
electrical properties of this unique material
system provide the potential for improved
performance in the terahertz (THz) frequency
range — performance that has been
difficult to attain in conventional gallium
arsenide (GaAs)- and gallium nitride (GaN)based
material. A single atomic layer of
crystalline carbon has been reported to have
Special Interest
Carbon-Based Electronic Devices Open a New Window
to Electronics
a room temperature electron mobility of
greater than 200,000 cm 2 /(V-s), two and
half times that of the best semiconductor.
Such high electron mobility allows for ballistic
electron transport in today’s transistors
with state of the art geometries, hence
making THz device fabrication highly feasible.
This attribute is shown graphically in
Figure 1. Recently, experimental field effect
transistor (FET) devices have validated this
figure by demonstrating the first such devices
with a cutoff frequency (f t ) of 300 GHz.
Fmax, Ft (GHz)
800
700
600
500
400
300
200
100
Carbon-Based Electronics
10 mW, 80% PAE, 20 dB Gain
130 nm CMOS
InP HEMT
InGaAs PHEMT
NextGen GaN
GaN HEMT
10
Power W/mm
Figure 1. Power-frequency space showing the
niche for carbon-based RF devices
-4 10-3 10-2 10-1 1 10
A truly two-dimensional (2D) crystal of graphene
has a number of unusual properties,
which can be exploited in new ways. One
such property is its ambipolar conductivity,
which produces a positive current whether
the device is forward or reverse biased. This
property arises from the unusual symmetry
in the band structure of 2D graphene with
zero bandgap energy and nearly symmetrical
behavior of electrons and holes in
the material.
The ambipolar property is illustrated in
Figure 2, which shows the operation of a
graphene-based FET (GFET) as a frequency
doubler, as demonstrated by Professor
Palacios at MIT [1] and more recently by
J.S. Moon at HRL [2]. In this configuration,
the gate bias of the FET is centered at zero,
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