Y-junction based carbon nanotube (CNT) transistors exhibit interesting switching behaviors, and have the structural advantage that the electrical gate for current modulation can be formed by any of the three constituent branches. In this letter, we report on the gating characteristics of metallic Y-CNT morphologies. By measuring the output conductance and transconductance we conclude that the efficiency and gain depend on the branch diameter and is electric field controlled. Based on these principles, we propose a design for a Y-junction based CNT switching device, with tunable electrical properties

Bandaru, P. R.

Daraio, C.

Gaillard, J.

Jin, S.

Park, J.

Rao, A. M.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS 88, 243113 2006Three-way electrical gating characteristics of metallic Y-junction carbon
nanotubes
J. Park, C. Daraio, S. Jin, and P. R. Bandarua
Materials Science and Engineering Program, Mechanical and Aerospace Engineering,
University of California, San Diego, La Jolla, California 92093-0411
J. Gaillard and A. M. Rao
Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634
Received 3 February 2006; accepted 18 May 2006; published online 15 June 2006
Y-junction based carbon nanotube CNT transistors exhibit interesting switching behaviors, and
have the structural advantage that the electrical gate for current modulation can be formed by any
of the three constituent branches. In this letter, we report on the gating characteristics of metallic
Y-CNT morphologies. By measuring the output conductance and transconductance we conclude that
the efficiency and gain depend on the branch diameter and is electric field controlled. Based on these
principles, we propose a design for a Y-junction based CNT switching device, with tunable electrical
properties. © 2006 American Institute of Physics. DOI: 10.1063/1.2213013While conventional silicon microfabrication technology
encounters barriers to further development due to difficulties
in miniaturization and increased cost, carbon nanotubes
CNTs show promise for a molecular electronics1,2 based
technology. CNTs both single walled nanotubes SWNTs
and multiwalled nanotubes MWNTs can be synthesized to
be either semiconducting or metallic.3 Consequently, they
can be used in a wide variety of electronics including diodes,
transistors, and high frequency devices.4 The ultimate aim is
an integrated nanoelectronics system, with the advantages of
reduced power consumption, radiation hardness, and faster
speeds of operation, which incorporates CNTs as both active
devices and interconnects. The electrical and thermal
conductivity5 properties of both SWNTs Ref. 6 and
MWNTs Ref. 7 have been well explored. While clean
SWNTs diameter 1 nm can be described as quantum
wires due to the ballistic nature of electron transport,8 the
transport in MWNTs diameter in the range of 2–100 nm is
found to be diffusive/quasiballistic.7 On this basis, Coulomb
blockade9 and field effect10 phenomena have been used for
demonstrating nanotube based transistors. While extremely
important in elucidating fundamental properties, the above
experiments have used external electrodes made through
conventional lithographic processes to contact the nanotubes
and do not truly represent nanoelectronic circuits. Addition-
ally, the well known metal oxide semiconductor field effect
transistor MOSFET architecture is used, where the nano-
tube serves as the channel between the electrodes source
and drain, and a SiO2/Si based gate modulates the channel
conductance. In other demonstrations of electrical switching,
cumbersome atomic force microscope AFM
manipulations11 of nanotube properties have been utilized.
It would, therefore, be very attractive to propose differ-
ent types of electronic elements and paradigms of switching,
to harness functionalities peculiar to CNT forms12 such as Y
and T junctions.13 In addition to the nanometer size of these
devices, there is also the additional feature of having a self-
contained gate, i.e., in a switching device, the gate does not
aAuthor to whom correspondence should be addressed; electronic mail:
pbandaru@ucsd.edu
0003-6951/2006/8824/243113/3/$23.00 88, 24311
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to have to be separately fabricated but forms a part of the struc-
ture. In earlier experiments, rectification,14 switching, and
logic gates15 were demonstrated in Y-shaped CNTs. An
abrupt switching action through two branches of the Y junc-
tion, when a control/gating dc voltage was applied to the
third terminal was seen. It was also proposed that all three
branches of a Y-CNT could be possibly used for a gate,
which we expand upon in this letter. We report on the current
I—voltage V characteristics of a metallic Y-CNT and re-
late the gating efficiency of each branch, together with the
voltage and current gain, to the nanostructure geometry.
The Y-CNTs were synthesized by a modified chemical
vapor deposition CVD technique.16 Subsequent to synthe-
sis, the Y-CNTs were collected from the CVD chamber and
individually separated by ultrasonication in an ethanol sol-
vent. They were then dispersed on a Au electrode patterned
SiO2/Si substrate. As observed through transmission elec-
tron microscopy,7 the Y-CNTs comprise multiwalled nano-
tubes of the bamboo17 variety, and a detailed examination of
the nanostructure reveals the presence of randomly dispersed
catalyst particles. A scanning electron microscope SEM
was used to locate the nanotubes and focused ion beam FIB
induced deposition was used to lay down Pt lines to the three
Y terminals. Any possible ion-induced damage to the CNTs
was avoided through low operating voltages. It was verified
that this procedure results in high-quality Ohmic contacts,
with contact resistances of the order of 1 k.18
The experimental setup for the electrical characterization
is implemented on a Wentworth probe station and is shown
in Fig. 1. A dc control voltage V is applied on one of the
three terminals gate of the Y-CNT while the current I
through the other two terminals source and drain is moni-
tored. Phase sensitive electrical measurements using a
lock-in amplifier were also employed and qualitatively simi-
lar results were obtained on many samples. The I-V curves
Fig. 1 of the Y-CNTs, as a function of gate Vgs/bias Vds
voltage, are shown in Fig. 2. The notable features are a
Ohmic conduction, b a proportional displacement of the
current with Vgs and Vds, and c a geometry dependent con-
ductance. The devices are also seen to have both current and
19
voltage gains. In line with an earlier study, we define the
© 2006 American Institute of Physics3-1
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
243113-2 Park et al. Appl. Phys. Lett. 88, 243113 2006differential current gain gdiff defined as the ratio of
Igs /Vgs to Ids /Vgs, at a constant source-drain voltage
Vds. In our Y-CNT devices a gdiff of up to 10 is obtained at
room temperature. The voltage gain is calculated10 by con-
sidering the voltage change Vds for a given increment in
gate voltage Vg, at a constant value of the current. The
voltage gain when the stem 1 in Fig. 1 is used for electrical
gating is 6 Fig. 2b, while the value drops to 0.3 Fig.
2d when the branch 3 in Fig. 1 is used for a gate.
Other quantities of importance in quantifying the electri-
cal characteristics are the transconductance, gm
= Ids /VgsVds, the ratio of the output current Ids to the
modulating/gating voltage Vgs at constant bias voltage
Vds, and the output conductance, gd= Ids /VdsVgs, the ra-
tio of the output current to the bias voltage at a constant gate
voltage. The results for all the three possible configurations
of gate g, with the other two terminals as source s and
drain d are shown in Fig. 3a, and summarized in the inset.
FIG. 1. Color online The circuit arrangement used to probe the three-way
gating capability of a Y-CNT switching device SEM micrograph. The
Y-CNT is contacted by FIB deposited Pt lines to Ti/Au bonding pads not
shown. In the diagram, 1 is the control gate g while 2 and 3 are the source
s and drain d, respectively. A complete evaluation of electrical character-
istics was implemented by implementing each terminal 1, 2, and 3 succes-
sively as the gate while probing the electrical conduction through the other
two branches.
FIG. 2. Color online Current I-Voltage V characteristics for the Y-CNT.
a I23-V23=Vds with terminal 1 as the gate Vgs=V13. b I23-V13=Vgs as
a function of the bias voltage V23, =Vds applied across branches 2 and 3,
and c I12-V12 with terminal 3 as the gate V13=Vgs. d I23-V32=Vds as a
function of the bias voltage V12=Vds applied across stem 1 and branch 2.
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to The values have been normalized to equivalent lengths of the
constituent nanotubes. It is found that the magnitude of the
gd is inversely proportional to the ratio of the diameters of
the branches through which the current is flowing Fig. 3b,
i.e., a larger discrepancy between the branch diameters low-
ers the conductance. On the other hand, the gm which denotes
the gating efficiency for current modulation, seems to be
directly proportional to the diameter of the gating nanotube.
While each of the constituent nanotubes in the Y-CNT is
metallic, the necessity of maintaining a uniform electro-
chemical potential =−eV in the overall nanostructure
gives rise to nonuniform/nonlinear interactions.20 For ex-
ample, when V2 the voltage on branch 2 decreases,
2=−eV2 increases, and an excess electron current flows
towards the central junction. The balance between the incom-
ing current and the outgoing currents, at the junction itself, is
achieved by increasing 1 decreasing V1. On the other
hand, when 2 decreases, 1 decreases also, but cannot de-
crease past a certain critical point, viz., the fixed electro-
chemical potential, dictated by geometry/defects, of the junc-
tion. This limitation results in a saturation of the current, and
is observed in Fig. 2b.
Due to the interconnected nature of the Y junction, the
gating is not perfect and there is a current leakage which is
manifested in the I-V curves, i.e., the current is diverted from
one path say, 2-3 to another say, 1-2 when a gating volt-
age is applied on 1, and results in current displacement.
However, at a certain fixed voltage Vds, the control voltage
Vg can be modulated and it is seen that positive, zero, or
negative currents can flow through the device, which could
be interpreted as switching. While operation up to 100 kHz
is possible, the upper limit seems to be set by the nanotube
capacitance related to the details of the Y-CNT structure.
FIG. 3. Color online a The variation of the output conductance gd and
transconductance gm with the diameters of the constituent branches of the
Y-CNT. The inset table summarizes the data for all the possible permuta-
tions of the gate terminal. b The conductance gd through two nanotubes
of different diameters in the Y-CNT is inversely proportional to their ratio
=d /d.This switching is of a different form than the one observed in
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
243113-3 Park et al. Appl. Phys. Lett. 88, 243113 2006a previous experiment15 where the simultaneous presence of
an ac voltage on the source-drain channel and a dc voltage
on the control/gate terminal resulted in an abrupt turn off,
possibly due to defect mediated negative capacitance
effects.21
From the Y-CNT electrical characteristics it is seen that
the mode of operation of the device is quite different from
that of a conventional FET and more akin to a Schottky-
barrier-type FET.22 The current modulation may be con-
trolled by the electric field =r ·V at the gating terminal,
and not the carrier density in the channel. In this case, the
device performance can be improved by increasing the diam-
eter of the gate so as to have a greater modulating effect on
the source-drain current. The assumption of electric field
control can also be tested23 by varying the angle  between
the branches of the Y-CNT. From elementary electrostatics,24
the surface-charge density at a distance  from the intersec-
tion of the gate with the junction varies as /−1 which
implies a greater modulating effect at a sharper angle. As an
example, for the Y-CNT in Fig. 4a, the stem would have a
higher gating efficiency than in case b. The manipulation of
the junction region to be dielectric/insulating say, by local-
ized ion implantation can also be used to change the elec-
trochemical potential and vary device characteristics. It is
interesting to note the similarity of Fig. 4a to a MOSFET,
where the stem is analogous to a gate, the dielectric particle
plays the role of the oxide, and the channel is formed by
conduction through the branches. However, the latter option
leads to a loss of flexibility in the Y-CNT. On the other hand,
uniform electrical gating/switching characteristics could be
obtained by making all the nanotubes the same diameter and
length, at an angle of 120° to each other Fig. 4b.
We also suggest that even more interesting applications
are facilitated by our experimental results and observations.
For example, a single Y-CNT switching device can be made
to have diverse operating characteristics through the use of
different gating terminals. Y-junction based technology can
be used for devices with 1 multiple programmable charac-
FIG. 4. Color online a A proposed Y-junction CNT based switching
device, where a smaller angle 1 between the branches B2 and B3 can
result in a higher gating efficiency for the stem S. A defect at the junction
region can also help see text. b A Y-CNT with uniform gating/electrical
switching characteristics can be fabricated by synthesizing all the constitu-
ent nanotubes to be of the same diameter and with 2=120°.teristics and as 2 components in field programmable gate
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to arrays FPGAs for reconfigurable computing,25 where it
would be possible to dynamically alter circuit paths. The
large scale integration density and high speed of signal
propagation intrinsic to nanotubes would be advantageous in
this regard.
For such applications to be feasible, it would be impor-
tant and necessary to gain better control of the geometry,
through synthesis or processing, of Y-CNTs. It is plausible
that remarkable features of Y-junction based transistors such
as the abrupt switching15 and differential gain19 observed in
earlier studies could be related to the presence and location
of defects. In situ engineering of CNT morphology, e.g., ex-
posure to intense electron-beam radiation13 could also be uti-
lized to tailor individual Y-CNT characteristics.
The authors gratefully acknowledge support from the
National Science Foundation Grant No. ECS-05-08514 and
the Office of Naval Research Award No. N00014-06-1-
0234. The authors also appreciate the use of facilities at the
National Center for Electron Microscopy NCEM at the
Lawrence Berkeley National Laboratory, Berkeley.
1P. Avouris, Acc. Chem. Res. 35, 1026 2002.
2P. Avouris, J. Appenzeller, R. Martel, and S. Wind, Proc. IEEE 91, 1772
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Walters, M. J. Casavant, J. Schmidt, and R. A. Smalley, Appl. Phys. Lett.
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and C. Dekker, Nature London 386, 474 1997.
7L. Forro and C. Schonenberger, in Carbon Nanotubes—Topics in Applied
Physics, edited by P. Avouris Springer, Heidelberg, 2001, Vol. 80, pp.
329–391.
8C. T. White and T. N. Todorov, Nature London 393, 240 1998.
9J. Nygard, D. H. Cobden, M. Bockrath, P. L. McEuen, and P. E. Lindelof,
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293, 76 2001.
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Phys. Lett. 77, 2530 2000.
13M. Terrones, F. Banhart, N. Grobert, J.-C. Charlier, H. Terrones, and P. M.
Ajayan, Phys. Rev. Lett. 89, 075505 2002.
14C. Papadapoulos, A. Rakitin, J. Li, A. S. Vedeneev, and J. M. Xu, Phys.
Rev. Lett. 85, 3476 2000.
15P. R. Bandaru, C. Daraio, S. Jin, and A. M. Rao, Nat. Mater. 4, 663
2005.
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Lett. 4, 213 2004.
17A. V. Melechko, V. I. Merkulov, T. E. McKnight, M. A. Guillorn, K. L.
Klein, D. H. Lowndes, and M. L. Simpson, J. Appl. Phys. 97, 041301
2005.
18V. Gopal, V. R. Radmilovic, C. Daraio, S. Jin, P. Yang, and E. A. Stach,
Nano Lett. 4, 2059 2004.
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Zaslavsky, Appl. Phys. Lett. 87, 123504 2005.
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AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Three-way electrical gating characteristics of metallic Y-junction carbon nanotubes

APPLIED PHYSICS LETTERS 88, 243113 2006Three-way electrical gating characteristics of metallic Y-junction carbon
nanotubes
J. Park, C. Daraio, S. Jin, and P. R. Bandarua
Materials Science and Engineering Program, Mechanical and Aerospace Engineering,
University of California, San Diego, La Jolla, California 92093-0411
J. Gaillard and A. M. Rao
Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634
Received 3 February 2006; accepted 18 May 2006; published online 15 June 2006
Y-junction based carbon nanotube CNT transistors exhibit interesting switching behaviors, and
have the structural advantage that the electrical gate for current modulation can be formed by any
of the three constituent branches. In this letter, we report on the gating characteristics of metallic
Y-CNT morphologies. By measuring the output conductance and transconductance we conclude that
the efficiency and gain depend on the branch diameter and is electric field controlled. Based on these
principles, we propose a design for a Y-junction based CNT switching device, with tunable electrical
properties. © 2006 American Institute of Physics. DOI: 10.1063/1.2213013While conventional silicon microfabrication technology
encounters barriers to further development due to difficulties
in miniaturization and increased cost, carbon nanotubes
CNTs show promise for a molecular electronics1,2 based
technology. CNTs both single walled nanotubes SWNTs
and multiwalled nanotubes MWNTs can be synthesized to
be either semiconducting or metallic.3 Consequently, they
can be used in a wide variety of electronics including diodes,
transistors, and high frequency devices.4 The ultimate aim is
an integrated nanoelectronics system, with the advantages of
reduced power consumption, radiation hardness, and faster
speeds of operation, which incorporates CNTs as both active
devices and interconnects. The electrical and thermal
conductivity5 properties of both SWNTs Ref. 6 and
MWNTs Ref. 7 have been well explored. While clean
SWNTs diameter 1 nm can be described as quantum
wires due to the ballistic nature of electron transport,8 the
transport in MWNTs diameter in the range of 2–100 nm is
found to be diffusive/quasiballistic.7 On this basis, Coulomb
blockade9 and field effect10 phenomena have been used for
demonstrating nanotube based transistors. While extremely
important in elucidating fundamental properties, the above
experiments have used external electrodes made through
conventional lithographic processes to contact the nanotubes
and do not truly represent nanoelectronic circuits. Addition-
ally, the well known metal oxide semiconductor field effect
transistor MOSFET architecture is used, where the nano-
tube serves as the channel between the electrodes source
and drain, and a SiO2/Si based gate modulates the channel
conductance. In other demonstrations of electrical switching,
cumbersome atomic force microscope AFM
manipulations11 of nanotube properties have been utilized.
It would, therefore, be very attractive to propose differ-
ent types of electronic elements and paradigms of switching,
to harness functionalities peculiar to CNT forms12 such as Y
and T junctions.13 In addition to the nanometer size of these
devices, there is also the additional feature of having a self-
contained gate, i.e., in a switching device, the gate does not
aAuthor to whom correspondence should be addressed; electronic mail:
pbandaru@ucsd.edu
0003-6951/2006/8824/243113/3/$23.00 88, 24311
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to have to be separately fabricated but forms a part of the struc-
ture. In earlier experiments, rectification,14 switching, and
logic gates15 were demonstrated in Y-shaped CNTs. An
abrupt switching action through two branches of the Y junc-
tion, when a control/gating dc voltage was applied to the
third terminal was seen. It was also proposed that all three
branches of a Y-CNT could be possibly used for a gate,
which we expand upon in this letter. We report on the current
I—voltage V characteristics of a metallic Y-CNT and re-
late the gating efficiency of each branch, together with the
voltage and current gain, to the nanostructure geometry.
The Y-CNTs were synthesized by a modified chemical
vapor deposition CVD technique.16 Subsequent to synthe-
sis, the Y-CNTs were collected from the CVD chamber and
individually separated by ultrasonication in an ethanol sol-
vent. They were then dispersed on a Au electrode patterned
SiO2/Si substrate. As observed through transmission elec-
tron microscopy,7 the Y-CNTs comprise multiwalled nano-
tubes of the bamboo17 variety, and a detailed examination of
the nanostructure reveals the presence of randomly dispersed
catalyst particles. A scanning electron microscope SEM
was used to locate the nanotubes and focused ion beam FIB
induced deposition was used to lay down Pt lines to the three
Y terminals. Any possible ion-induced damage to the CNTs
was avoided through low operating voltages. It was verified
that this procedure results in high-quality Ohmic contacts,
with contact resistances of the order of 1 k.18
The experimental setup for the electrical characterization
is implemented on a Wentworth probe station and is shown
in Fig. 1. A dc control voltage V is applied on one of the
three terminals gate of the Y-CNT while the current I
through the other two terminals source and drain is moni-
tored. Phase sensitive electrical measurements using a
lock-in amplifier were also employed and qualitatively simi-
lar results were obtained on many samples. The I-V curves
Fig. 1 of the Y-CNTs, as a function of gate Vgs/bias Vds
voltage, are shown in Fig. 2. The notable features are a
Ohmic conduction, b a proportional displacement of the
current with Vgs and Vds, and c a geometry dependent con-
ductance. The devices are also seen to have both current and
19voltage gains. In line with an earlier study, we define the
© 2006 American Institute of Physics3-1
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
243113-2 Park et al. Appl. Phys. Lett. 88, 243113 2006differential current gain gdiff defined as the ratio of
Igs /Vgs to Ids /Vgs, at a constant source-drain voltage
Vds. In our Y-CNT devices a gdiff of up to 10 is obtained at
room temperature. The voltage gain is calculated10 by con-
sidering the voltage change Vds for a given increment in
gate voltage Vg, at a constant value of the current. The
voltage gain when the stem 1 in Fig. 1 is used for electrical
gating is 6 Fig. 2b, while the value drops to 0.3 Fig.
2d when the branch 3 in Fig. 1 is used for a gate.
Other quantities of importance in quantifying the electri-
cal characteristics are the transconductance, gm
= Ids /VgsVds, the ratio of the output current Ids to the
modulating/gating voltage Vgs at constant bias voltage
Vds, and the output conductance, gd= Ids /VdsVgs, the ra-
tio of the output current to the bias voltage at a constant gate
voltage. The results for all the three possible configurations
of gate g, with the other two terminals as source s and
drain d are shown in Fig. 3a, and summarized in the inset.
FIG. 1. Color online The circuit arrangement used to probe the three-way
gating capability of a Y-CNT switching device SEM micrograph. The
Y-CNT is contacted by FIB deposited Pt lines to Ti/Au bonding pads not
shown. In the diagram, 1 is the control gate g while 2 and 3 are the source
s and drain d, respectively. A complete evaluation of electrical character-
istics was implemented by implementing each terminal 1, 2, and 3 succes-
sively as the gate while probing the electrical conduction through the other
two branches.
FIG. 2. Color online Current I-Voltage V characteristics for the Y-CNT.
a I23-V23=Vds with terminal 1 as the gate Vgs=V13. b I23-V13=Vgs as
a function of the bias voltage V23, =Vds applied across branches 2 and 3,
and c I12-V12 with terminal 3 as the gate V13=Vgs. d I23-V32=Vds as a
function of the bias voltage V12=Vds applied across stem 1 and branch 2.
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to The values have been normalized to equivalent lengths of the
constituent nanotubes. It is found that the magnitude of the
gd is inversely proportional to the ratio of the diameters of
the branches through which the current is flowing Fig. 3b,
i.e., a larger discrepancy between the branch diameters low-
ers the conductance. On the other hand, the gm which denotes
the gating efficiency for current modulation, seems to be
directly proportional to the diameter of the gating nanotube.
While each of the constituent nanotubes in the Y-CNT is
metallic, the necessity of maintaining a uniform electro-
chemical potential =−eV in the overall nanostructure
gives rise to nonuniform/nonlinear interactions.20 For ex-
ample, when V2 the voltage on branch 2 decreases,
2=−eV2 increases, and an excess electron current flows
towards the central junction. The balance between the incom-
ing current and the outgoing currents, at the junction itself, is
achieved by increasing 1 decreasing V1. On the other
hand, when 2 decreases, 1 decreases also, but cannot de-
crease past a certain critical point, viz., the fixed electro-
chemical potential, dictated by geometry/defects, of the junc-
tion. This limitation results in a saturation of the current, and
is observed in Fig. 2b.
Due to the interconnected nature of the Y junction, the
gating is not perfect and there is a current leakage which is
manifested in the I-V curves, i.e., the current is diverted from
one path say, 2-3 to another say, 1-2 when a gating volt-
age is applied on 1, and results in current displacement.
However, at a certain fixed voltage Vds, the control voltage
Vg can be modulated and it is seen that positive, zero, or
negative currents can flow through the device, which could
be interpreted as switching. While operation up to 100 kHz
is possible, the upper limit seems to be set by the nanotube
capacitance related to the details of the Y-CNT structure.
FIG. 3. Color online a The variation of the output conductance gd and
transconductance gm with the diameters of the constituent branches of the
Y-CNT. The inset table summarizes the data for all the possible permuta-
tions of the gate terminal. b The conductance gd through two nanotubes
of different diameters in the Y-CNT is inversely proportional to their ratio
=d /d.This switching is of a different form than the one observed in
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
243113-3 Park et al. Appl. Phys. Lett. 88, 243113 2006a previous experiment15 where the simultaneous presence of
an ac voltage on the source-drain channel and a dc voltage
on the control/gate terminal resulted in an abrupt turn off,
possibly due to defect mediated negative capacitance
effects.21
From the Y-CNT electrical characteristics it is seen that
the mode of operation of the device is quite different from
that of a conventional FET and more akin to a Schottky-
barrier-type FET.22 The current modulation may be con-
trolled by the electric field =r ·V at the gating terminal,
and not the carrier density in the channel. In this case, the
device performance can be improved by increasing the diam-
eter of the gate so as to have a greater modulating effect on
the source-drain current. The assumption of electric field
control can also be tested23 by varying the angle  between
the branches of the Y-CNT. From elementary electrostatics,24
the surface-charge density at a distance  from the intersec-
tion of the gate with the junction varies as /−1 which
implies a greater modulating effect at a sharper angle. As an
example, for the Y-CNT in Fig. 4a, the stem would have a
higher gating efficiency than in case b. The manipulation of
the junction region to be dielectric/insulating say, by local-
ized ion implantation can also be used to change the elec-
trochemical potential and vary device characteristics. It is
interesting to note the similarity of Fig. 4a to a MOSFET,
where the stem is analogous to a gate, the dielectric particle
plays the role of the oxide, and the channel is formed by
conduction through the branches. However, the latter option
leads to a loss of flexibility in the Y-CNT. On the other hand,
uniform electrical gating/switching characteristics could be
obtained by making all the nanotubes the same diameter and
length, at an angle of 120° to each other Fig. 4b.
We also suggest that even more interesting applications
are facilitated by our experimental results and observations.
For example, a single Y-CNT switching device can be made
to have diverse operating characteristics through the use of
different gating terminals. Y-junction based technology can
be used for devices with 1 multiple programmable charac-
FIG. 4. Color online a A proposed Y-junction CNT based switching
device, where a smaller angle 1 between the branches B2 and B3 can
result in a higher gating efficiency for the stem S. A defect at the junction
region can also help see text. b A Y-CNT with uniform gating/electrical
switching characteristics can be fabricated by synthesizing all the constitu-
ent nanotubes to be of the same diameter and with 2=120°.teristics and as 2 components in field programmable gate
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to arrays FPGAs for reconfigurable computing,25 where it
would be possible to dynamically alter circuit paths. The
large scale integration density and high speed of signal
propagation intrinsic to nanotubes would be advantageous in
this regard.
For such applications to be feasible, it would be impor-
tant and necessary to gain better control of the geometry,
through synthesis or processing, of Y-CNTs. It is plausible
that remarkable features of Y-junction based transistors such
as the abrupt switching15 and differential gain19 observed in
earlier studies could be related to the presence and location
of defects. In situ engineering of CNT morphology, e.g., ex-
posure to intense electron-beam radiation13 could also be uti-
lized to tailor individual Y-CNT characteristics.
The authors gratefully acknowledge support from the
National Science Foundation Grant No. ECS-05-08514 and
the Office of Naval Research Award No. N00014-06-1-
0234. The authors also appreciate the use of facilities at the
National Center for Electron Microscopy NCEM at the
Lawrence Berkeley National Laboratory, Berkeley.
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Three-way electrical gating characteristics of metallic Y-junction carbon nanotubes

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