Science - USA (2021-12-24)

voltageof0.5V,theONcurrentwasashighas
~0.74mA, corresponding to a current density
of ~1233mA/mm with the channel width nor-
malized to the tube diameter. The OFF current
was ~0.2 nA, and theION/IOFFratio was ~3000.
The subthreshold swing (SS) was ~1.33 V/dec,
which is lower than that of previously reported
CNT transistors in a suspended configuration
( 20 ), where for a channel length of ~30 nm,
the SS value was ~4.9 V/dec.
The transistor performance is closely depen-
dent on the geometry, interface bonding, and
electrical resistance of the source and drain
contacts ( 21 ). Atomically thin graphene con-
tacts were reported to improve electrostatic

control of the gate ( 3 ), and end-contacts were

critical to reducing the contact resistance for

CNT nanotransistors ( 4 ). In our work, the

semiconducting CNT channel was covalently

bonded to the metallic CNT leads, which nat-

urally made good contacts to external electrodes.

Such seamless contact with a needlelike geom-

etry has been theoretically predicted to be ideal

for nanotube transistors ( 22 ), where the strong

sbonds enhance the cohesion and the de-

localizedpelectrons result in low charge-carrier

scattering at the interface.

The metallic-semiconducting-metallic nano-

tube junctions form a symmetric structure of

double Schottky barriers. A schematic of the

band structure of the CNT intramolecular junc-

tion is shown in the inset of Fig. 2I. Given that

the Fermi level of a semiconducting nanotube

is equal to that of a metallic nanotube ( 19 ),

there is no band bending when the bias is zero.

When a positive potential is applied to the left

terminal, the Fermi levels of the semiconduct-

ing channel and the right metallic tube are

raised. There will be a Schottky barrier at the

right junction for the electron transport. Be-

cause the junctions are symmetric, there will

be a Schottky barrier under forward bias and

another one under reverse bias. TheI-Vchar-

acteristic withVGat zero (Fig. 2I) is symmetric

for the negative and positive biases, with the

1618 24 DECEMBER 2021¥VOL 374 ISSUE 6575 science.orgSCIENCE

Fig. 2. Metal-to-semiconductor transition for fabricating SWCNT
intramolecular transistors.(AtoC) TEM images of a SWCNT through
consecutive thermomechanical processing cycles. (D) Schematic of a
SWCNT intramolecular transistor. M, metal; S, semiconductor. (Eand
F)ISD-VGandISD-VSDtransport curves of the initial tube and after
transformations, showing the transition from metallic to semiconducting
behavior and the widening of the bandgap. (G) TEM image of a SWCNT

intramolecular transistor with a channel length of ~2.8 nm. (H)ISD-VG

transport measurements of the ultrashort channel transistor. Resonant

conductance peaks in the gap are indicated by arrows. (I)ISD-VSD

characteristic withVGat 0 V. Insets are schematics of the band structures of

the metallic-semiconducting-metallic nanotube junctions.EFM, Fermi level

of the metallic nanotube;EC, conduction band;EFS, Fermi level of the

semiconducting nanotube;EV, valence band; SB, Schottky barrier.

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