is whether the device will scale down
easily and fulfill the promise of impact
ionization devices to provide significant energy advantages over conventional transistors.
Koopmans says when the Eind-
hoven team found silicon could display
large magnetoresistance, “It was a new
and very interesting effect. But, at the
time, I was not sure whether it would
be easy to handle in miniaturized
nanodevices. We had been working on
rather large devices.”
Chiara Ciccarelli, a researcher at the
University of Cambridge who has inves-
tigated magnetoresistance in silicon,
says as devices get smaller, “although
there is no fundamental limit to room-
temperature magnetoresistance, its
Ciccarelli points out that high-
mobility materials, of which InSb
is an example, show stronger mag-
netoresistance than silicon, which
should help in smaller devices. Scaling
could compensate in other ways. “As
the length of the channel decreases,
the electric field increases,” she says.
“That suggests miniaturization could
play a positive role in the magnetore-
sistance of these devices. However, this
is not what has been observed in the
devices studied so far.”
Even if magnetoresistance can be
maintained at a high-enough level,
there is a limit to how small the device
can be made before impact ionization
“The minimum length of the chan-
nel is determined by a ‘dead space,’ the
distance a carrier travels before acquir-
ing enough energy from the electric
field to participate in impact ioniza-
tion,” says Hong. This dead space may
not shrink past 20nm—the length of
the channel in today’s most advanced
Even if it cannot be made as small as
a logic transistor, the magnetically controlled device could still have a future.
The nonvolatile nature of spintronic
devices also means that for systems
that are active only intermittently—
such as smart sensors—the energy savings achieved by only having to power
a circuit while it is processing could be
immense, and outweigh silicon’s likely
advantage in size.
The magnetoresistive device may
lend itself to plastic electronics, in
which organic polymers are printed
onto a surface to form circuits. These
processes can be performed at very low
temperatures and with much cheaper
equipment than that used in conventional semiconductor fabs.
Delmo says the electron dynamics
are different in organic semiconductors from those in silicon or InSb: “If
these mechanisms do not depend on
the device size, organic semiconductors could be strong candidates for
magnetoresistive-based semiconductor logic technology.”
Joo S., Kim, T., Shin, S.H., Lim J. Y., Hong, J.,
Song J.D., Chang J., Lee, H. W., Rhie, K.,
Han, S.H., Shin, K.H. and Johnson, M.
semiconductor logic, Nature 494, 72,
Feb. 7, 2013
Schoonus, J.J.H., Bloom, F.L., Wagemans, W.,
Swagten, H.J.M., Koopmans, B.
Extremely large magnetoresistance in
boron-doped silicon. Physics Review Letters
100, 127202, Mar. 27, 2008
Ciccarelli, C., Park, B.G., Ogawa, S.,
Ferguson, A.J., and Wunderlich, J.
Gate-controlled magnetoresistance in a
silicon metal-oxide-semiconductor field-effect transistor. Applied Physics Letters 97,
082106, Aug. 25, 2010.
Datta, S., Behin-Aein B., and Salahuddin, S.
non-volatile spin switch for Boolean and
non-Boolean Logic, Applied Physics Letters
101, 252411, Dec. 20, 2012. Applied Physics
Letters 95, 132106, Sep. 30, 2009.
Chris Edwards is a Surrey, u.k.-based writer who reports
on electronics, It, and synthetic biology.
© 2013 aCm 0001-0782/13/09 $15.00
colleagues at the Eindhoven University
of Technology stumbled across a large
magnetoresistive effect in the non-
magnetic material silicon. “We saw
very large effects that resulted from im-
The Seoul team used the non-magnet-
ic material indium antimonide (InSb)—
which has been proposed as a possible
high-speed successor to silicon for fu-
ture transistors—because its proper-
ties “lead to easy control of impact ion-
ization by magnetic fields,” Hong says.
The experiment, reported in the
science journal Nature earlier this
year, used externally applied magnetic fields to control the motion of
electrons along a channel of InSb. Underneath this layer was a thin strip of
InSb doped to have a low electron concentration—if electrons moved into
this region, they would have a high
probability of forming bonds with nuclei there and drop out of the free-car-rier pool, preventing them from ionizing any more electrons. A magnetic
field applied in one direction provided
the necessary force, with the result of
turning the device off. When the field
reversed, the electrons were able to
move from one end of the device to
the other more freely and take part in
Building magnetic-field control into
the device itself is a major stumbling
block, but Hong envisages tiny ferromagnets being deposited alongside
the InSb diode channels, pointing to
work performed in the lab five years
ago. “We demonstrated the fabrication of 1µm magnets,” he says, noting
that the techniques used in magnetic
memories that are just beginning to
move into production could be used to
switch field directions as needed.
Michael Delmo, a postdoctoral fellow at Osaka University who has studied transport mechanisms that lead to
magnetically controlled impact ionization, says incorporating magnetic elements into what is primarily an electronic device is a realistic proposition.
“But this technology is in its infancy,
and many things about it are still unknown,” he adds.
In addition, the magnetically controlled impact-ionization device is
large compared with today’s logic transistors, and its current ratio is less than
one order of magnitude. The question
device may lend
itself to plastic
polymers are printed
onto a surface
to form circuits.