Scaling of the performance of carbon nanotube transistors

1
Scaling of the performance ofcarbon nanotube
transistors
S. Heinze1, M. Radosavljevic2, J. Tersoff3, and
Ph. Avouris3
1 Institute of Applied Physics, University of
Hamburg, Germany 2 Novel Device Group, Intel
Corporation, Hillsboro, OR 3 IBM Research
Division, TJ Watson Research Center, Yorktown
Heights, NY

• Why carbon nanotube transistors?
• Evidence for Schottky barriers
• Carbon nanotube Schottky barrier transistors
• Gas adsorption versus doping
• Scaling of transistor performance
• New device designs capabilities
• Conclusions

2
Carbon nanotube field-effect transistors
comparable with Si MOS-FETs

• Nanotube FETs with top gates
• turn-on gate voltage is about 1V

S. J. Wind et al., Appl. Phys. Lett. 80, 3817
(2002).
3
Evidence for Schottky barriersscanned gate
microscopy at contacts
map transport current as a function of moving,
charged AFM tip
current increase when gating the source
junction ? barrier thinning.
M. Freitag et al., Appl. Phys. Lett. 79, 3326
(2001).
4
Evidence for Schottky barriersambipolar
conduction in SWNTs
R. Martel et al., PRL 87, 256805 (2001).
5
Evidence for Schottky barriers Influence of the
contacts for CNFETs
-6
10
-7
L300nm tox5nm
10
-8
10
-9
10
Current A
-10
10
-11
10
Vd-0.9V to -0.5V 0.2V steps
-12
10
-13
10
-2.0
-1.0
0.0
1.0
Gate Voltage V
6
Conventional vs. Schottky barrier FET
dNT1.4nm ? Eg0.6eV Typical SBs for NTs 0.3eV
p-type Characteristic
7
Transmission through Schottky barrier
8
Self-consistent SB-transistor modelfor
needle-like contact
Gate

• Cylindrical gate at RGate
• Metal electrode of NT diameter
• Analytic electrostatic kernel G
• Test of approximations for ?

NT
Metal
9
Needle-like contactconductance vs. gate voltage
10
Carbon nanotube transistors with planar gates

• Solve a 2D boundary value problem ? Vext(x)
• Local approximation for potential from NT charge

11
Influence of the contact geometry
PRL 89, 106801 (2002)
12
Gas adsorption vs. dopingExperimental
observations
Doping with Potassium
Gas Adsorption (O2)
Increase of Potassium
V. Derycke et al., APL 80, 2773 (2002).
13
Uniform dopingExperiment vs. SB model
Doping with Potassium
Needle-Contact Model
14
Uniform doping of nanotube
n-doped at 5?10-4 e/atom
Calculated Doping Characteristics
15
Uniform doping of nanotube
n-doped at 1?10-3 e/atom
Calculated Doping Characteristics
16
Gas adsorption Experiment vs. SB model
Gas Adsorption (O2)
Needle-Contact Model
Increase of O2
17
Gas adsorptionChange in metal workfunction
18
How does the performance of Schottky barrier
CNFETs scale?
Scaling law with oxide thickness?
? Why is the thermal limit of 60 mV/decade not
reached?
J. Appenzeller et al., PRL 89, 126801 (2002).
19
Turn-on vs oxide thickness for bottom gate
SB-CNFETs
? Vscale sqrt(tox)
20
Analytic model for thin sheet contact
21
Analytic model applied to bottom gate SB-CNFETs
Single, empirical factor for bottom gate devices
22
Scaling of turn-on performance of CNFETs with
oxide thickness
Largest improvements by optimization of
the contact geometry
PRB 68, 235418 (2003)
23
Scaling of drain voltage for ultra-thin oxide
CNFETs?
24
Effect of drain voltage for ultra-thin oxide
CNFET
Bottom-gate tox2nm
? exponential increase of OFF current with Vd
25
Scaling of drain voltage model vs. experiment
tox2nm
APL 83, 2435 (2003)
26
OFF state problem for transistor? light emission
device
Infrared light emission from a SWNT J. Misewich
et al., Science 300, 783 (2003).
27
Asymmetric device design to solve OFF state
problem

• Symmetric CNFET (tox2nm)
• unfavorable OFF state

Asymmetric CNFET ? low OFF current
p- and n-type device for Vdlt0 and Vdgt0
APL 83, 5038 (2003)
28
Conclusions
CN Transistors competetive with Si MOSFETs,
however

• Transistor action in CNFETs due to Schottky
  barriers
• ? ambipolar transfer characteristic (I vs Vg)
• Nanoscale features of contacts are essential
• Gas adsorption modifies band line-up at the
  contact
• Scaling in turn-on regime with sqrt(tox)
• Scaling of drain voltage at ultra-thin oxides
  necessary
• New device physics light emission device
• New device designs may be favorable