Vacuum nanoelectronics: Back to the future?—Gate insulated nanoscale
vacuum channel transistor
Jae Sub Oh,
and M. Meyyappan
Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035, USA
National Nanofab Center, 335 Gwahangno, Yuseong-gu, Daejeon 305-806, Korea
(Received 24 February 2012; accepted 22 April 2012; published online 23 May 2012)
A gate-insulated vacuum channel transistor was fabricated using standard silicon semiconductor
processing. Advantages of the vacuum tube and transistor are combined here by nanofabrication. A
photoresist ashing technique enabled the nanogap separation of the emitter and the collector, thus
allowing operation at less than 10 V. A cut-off frequency f
of 0.46 THz has been obtained. The
nanoscale vacuum tubes can provide high frequency/power output while satisfying the metrics of
lightness, cost, lifetime, and stability at harsh conditions, and the operation voltage can be
decreased comparable to the modern semiconductor devices.
2012 American Institute of Physics.
Early electronics centered around the vacuum tube used
to amplify, switch, or modulate electrical signals. It has been
many decades since the vacuum tubes have been replaced by
solid-state devices such as the metal-oxide-semiconductor
ﬁeld-effect transistor (MOSFET) and diode.
the vacuum tubes are still used in niche applications such as
premier sound systems and high-power radio base stations.
The transition from the vacuum tube to the solid-state device
was not driven by the superiority of the semiconductor as a
carrier transport medium but by the ease of fabrication, low-
cost, low-power consumption, lightness, long lifetime, and
ideal form factor for integrated circuits (ICs). The vacuum
tubes were fabricated by mechanical machining and used as
discrete components, whereas modern solid-state devices are
batch processed in assembling the integrated circuits. The
vacuum device is more robust than solid-state devices in
extreme environments involving high temperature and expo-
sure to various radiations. The critical tradeoff is that the
vacuum tubes yield higher frequency/power output but con-
sume more energy than the MOSFET. The vacuum is
intrinsically superior to the solid as carrier transport medium
since it allows ballistic transport while the carriers suffer
from optical and acoustic phonon scattering in semiconduc-
tors. The velocity of electrons in vacuum is theoretically
3 Â 10
cm/s, but is limited to about 5 Â 10
cm/s in semi-
conductors. As the cathodes of vacuum tubes need to be
heated for thermionic emission of electrons, the energy for
heating adversely overwhelms the energy required for ﬁeld
emission. The vacuum device is, therefore, not suitable for
low power devices. For high power ampliﬁcation (e.g.,
>50 W), however, the solid state device needs a complex cir-
cuit architecture including many transistors, microstrips, and
thermal management systems.
The advantages of both devices can be achieved together
if the macroscale vacuum tube is miniaturized to the nano-
meter scale. The nano vacuum tubes can provide high fre-
quency/power output while satisfying the metrics of
lightness, cost, lifetime, and stability at harsh conditions.
More importantly, further downscaling can allow a cold
cathode because the electric ﬁeld itself is strong enough to
emit electrons. Also, an ultimate downscaling in conjunction
with low workfunction materials may decrease the turn-on
gate and drain voltages to less then 1 V, thus enabling these
devices to be competitive with modern semiconductor tech-
nology. These beneﬁts can be attained by the use of matured
IC technology to fabricate nanoscale vacuum tubes and facil-
itate circuit integration.
The most common design of vacuum microelectronic is
a vertical ﬁeld emitter consisting of the emitter, gate, and
collector as shown in Fig. 1(a). The emitter is a sharp conical
tip, the gate is a circular aperture, and the collector is ﬂapped
at the top. The movement of electrons between the emitter
(cathode) and the collector (anode) is controlled by the gate.
An array of vertical ﬁeld emitters forms a large-area ﬂat
Unfortunately, the vertical structure may
be undesirable for circuit implementation due to the difﬁcul-
ties in achieving geometrical dimensions such as gap spacing
to be identical over all devices on the substrate. In contrast,
FIG. 1. Structures of vacuum devices and analogues to conventional MOS-
FET. (a) Vertical ﬁeld-emitter, (b) planar lateral ﬁeld-emitter, (c) MOSFET,
and (d) gate-insulated air channel transistor.
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2012 American Institute of Physics100, 213505-1
APPLIED PHYSICS LETTERS 100, 213505 (2012)