25-01-2013, 03:07 PM
Simulations of Carbon Nanotube Field Effect
Transistors
Simulations of Carbon Nanotube.pdf (Size: 76.95 KB / Downloads: 209)
Abstract
As the scaling of Si MOSFET approaches towards its limiting value, new
alternatives are coming up to overcome these limitations. In this paper first we
have reviewed carbon nanotube field effect transistor (CNTFET) and types of
CNTFET. We have then studied the effect of channel length and chirality on
the drain current for planer CNTFET. The Id~Vd curves for planer CNTFETs
having different channel lengths and diameters are plotted. For the same,
Id~Vd curves for different applied gate voltages are also plotted. We have then
discussed the effect of diameter on the characteristic curves for a cylindrical
CNTFET. Finally a brief comparison between the performance of Si-
MOSFET and CNTFET is given.
Introduction
Silicon-based technology has experienced phenomenal growth in the last few decades.
A large part of the success of the MOS transistor is due to the fact that it can be scaled
to increasingly smaller dimensions, which results in higher performance. Though this
trend still continues, bulk MOSFET will soon reach its limiting size. For this reason,
the semiconductor industry is looking for different materials and devices to integrate
with the current silicon-based technology and in the long run, possibly replace it. The
carbon nanotube field effect transistor is one among the most promising alternatives
due to its superior electrical properties.
This paper reviews different types of CNTFET which are one of the most
promising devices to replace Si MOSFET in near future and also gives an insight for
some basic characteristics of CNTFET. It is organized as follows. CNTFET and types
of CNTFET are discussed in section 2. Section 3 comprises of some simulation results
for planar CNTFET. Cylindrical CNTFET and the effect of diameter for cylindrical
CNTFET are discussed in section 4. A brief comparison between Si-MOSFET and
CNTFET is given in section 5 and we conclude the paper in section 6.
118 Rasmita Sahoo and R. R. Mishra
Carbon nanotube field effect transistor (CNTFET)
Single walled carbon nanotubes (SWCNTs) have huge potential for applications in
electronics because of both their metallic and semiconducting properties and their
ability to carry high current. CNTs can carry current density of the order 10 μA/nm2,
while standard metal wires have a current carrying capability of the order 10 nA/nm2.
Semiconducting CNTs have been used to fabricate CNTFETs, which show
promise due to their superior electrical characteristics over silicon based MOSFETs.
Since the electron mean free path in SWCNTs can exceed 1 micrometer, long channel
CNTFETs exhibit near-ballistic transport characteristics, resulting in high-speed
devices. The first CNTFET was fabricated in 1998[1]. In the same year R. Martel
et.al.[2] fabricated field-effect transistors based on individual single- and multi-wall
carbon nanotubes and analyzed their performance. The broad classifications of
CNTFET are discussed below.
Geometry dependent CNTFET
a. Back-gate CNTFET
The first back gate CNTFET was proposed by Tans et.al. [1]. In this structure a single
SWCNT was used to bridge two noble metal electrodes prefabricated by lithography
on an oxidized silicon wafer. Here the SWCNT plays the role of channel and the
metal electrodes act as source and drain. The heavily doped silicon wafer itself
behaves as the back gate. These CNTFETs behaved as p-type FETs with an I (on)/I
(off) ratio~105. The schematic diagram of back-gate CNTFET is shown in figure-1.
This suffers from some of the limitations like high parasitic contact resistance
(≥1Mohm), low drive currents (a few nanoamperes), and low transconductance gm ≈
1nS [3]. To reduce these limitations the next generation CNTFET developed which is
known as top gate CNTFET.
b. Top gate CNTFET
To get better performance Wind et al. proposed the first top gate CNTFET in 2003[4].
shows the schematic diagram of a top-gated CNTFET with Ti source, drain,
and gate electrodes. A 15-nm SiO2 film was used as the gate oxide. Here gate is
placed over the CNT. The advantage of top gated CNTFET over back gated CNTFET
is summarized in table-I. These data are taken from [3].
A back-gate CNTFET. Figure 2: A top-gate CNTFET [3].
Simulations of Carbon Nanotube Field Effect Transistors 119
Comparison between Back gate CNTFET and Top gate CNTFET.
Parameters
Back gate
CNTFET
Top gate CNTFET
Threshold voltage -12V -0.5V
Drain current
Of the order of
nanoampere
Of the order of
microampere
Transconductance 1nS 3.3μS
I(on)/I(off) 105
106
Electrodes dependent CNTFET
Based on the type of electrodes used CNTFET is classified into three categories.
(a) Schottky-barrier (SB) CNTFET (b) Partially gated (PG) CNTFET and ©
doped-S/D CNTFET. [5, 6]
a. Schottky-barrier (SB) CNTFET
As shown in figure-3(a), in this type of CNTFET an intrinsic CNT is used in the
channel region. This is connected to metal Source/Drain and forms Schottky barriers
at the junctions. Carbon nanotube transistors operate as unconventional Schottky
barrier transistors in which transistor action occurs primarily by varying the contact
resistance rather than the channel conductance. These types of FET require careful
alignment of the Schottky barrier and gate electrode which leads to manufacturing
challenge. Also the presence of Schottky barrier lowers the on-current. These are
explored further in references [7,8,9,10].
Different types of CNTFET: (a) Schottky-barrier (SB) CNTFET, (
partially gated (PG) CNTFET © doped-S/D CNTFET [5].
120 Rasmita Sahoo and R. R. Mishra
b. Partially gated (PG) CNTFET
PG-CNTFET, shown in fig.3 (b), is a depletion mode CNTFET in which the nanotube
is uniformly doped or uniformly intrinsic with ohmic contacts at their ends. PGCNTFETs
can be of n-type or p-type when respectively n-doped or p-doped. In these
devices the gate locally depletes the carriers in the nanotube and turns off the p-type
(n-type) device with an efficiently positive (negative) threshold voltage that
approaches the theoretical limit for room-temperature operation. The on-current of
such devices is given as ID (on) =qρvt where ρ is the carrier density per unit length
and vt is the uni-directional thermal velocity [5-6].
c. Doped- source or drain (S/D) CNTFET
Doped-S/D CNTFETs presented in fig.3© are composed of three regions. The region
below the gate is intrinsic in nature and the two ungated regions are doped with either
p-type or n-type. The ON-current is limited by the amount of charges that can be
induced in the channel by the gate and not by the doping in the source. They operate
in a pure p- or n-type enhancement-mode or in a depletion-mode, based on the
principle of barrier height modulation when applying a gate potential.
Out of these three, doped S/D CNTFETs are promising because (1) they show
unipolar characteristics unlike SB-CNTFETs; (2) the absence of SB reduces the OFF
leakage current; (3) they are more scalable compared to their SB counterparts; (4) in
ON-state, the source-to-channel junction has a significantly higher ON current.
Depending on the doping profile doped S/D CNTFETs can again be classified into
two groups.