A Greater Measure of Confidence

Keithley Instruments, Inc.

28775 A ur ora Road

Cleveland, Ohio 44139

(440) 248-0400

Fax: (440) 248-6168

www.keithley.com

PAPER

Improving Low Current Measurements on

Nanoelectronic and Molecular Electronic Devices

Jonathan L. Tucker

Keithley Instruments, Inc.

Nanotech Development

Moore’s Law states that circuit density will double every 18 months.

However, in order to maintain this rate of increase, there must be fundamental

changes in the way circuits are formed. Over the past few years, there have

been significant and exciting developments in nanotechnology, particularly in

the areas of nanoelectronics and molecular electronic (also called moletronic)

devices. The 2001 International Technology Roadmap for Semiconductors

projects that by 2004, devices should shrink to 0.09 micron (90nm) structures,

the upper end of the nanostructure size range. However, a few semiconductor

companies that claim to be fabricating devices smaller than 100nm are already

challenging that level.

Below a semiconductor scale of 100nm, the principles, fabrication

methods, and ways to integrate silicon devices into systems are not fully

developed, but apparently not impossible. Still, the increasing precision and

quality control required for silicon devices smaller than 100nm will

presumably require new fabrication equipment and facilities that may not be

justified due to high cost. This cost barrier is likely to be reached within the

next ten years. Even if cost were not a factor, silicon devices have physical

size limitations that affect their performance. That means the race is on to

develop nanodimensional and moletronic devices and associated production

methods.

Carbon Nanotube and Organic Chain Devices

Two types of molecules that are being used as current carrying, nano-scale electronic

devices are carbon nanotubes and polyphenylene-based chains. Researchers have already

demonstrated carbon nanotube based FETs, nanotube based logic inverters, and organic-chain

diodes, switches, and memory cells. All of these can lead to early stage logic devices for

future computer architectures.

Carbon nanotubes (CNTs) have unique properties that make them good candidates for

a variety of electronic devices. They can have either the electrical conductivity of metals, or

act as a semiconductor. (Controlling CNT production processes to achieve the desired

property is a major area of research.) CNT current carrying densities are as high as 10

A/cm

whereas copper wire is limited to about 10

A/cm

. Besides acting as current conductors to

interconnect other small-scale devices, CNTs can be used to construct a number of circuit

devices. Researchers have experimented with CNTs in the fabrication of FETs, FET voltage

inverters, low temperature single-electron transistors, intramolecular metal-semiconductor

diodes, and intermolecular-crossed NT-NT diodes [1].

The CNT FET uses a nanotube that is laid across two gold contacts that serve as the

source and drain, as shown in Figure 1a. The nanotube essentially becomes the current

carrying channel for the FET. DC characterization of this type of device is carried out just as

with any other FET. An example is shown in Figure 1b.

A Greater Measure of Confidence

Figure 1b shows that the amount of current (I

) flowing through a nanotube channel

can be changed by varying the voltage applied to the gate (V

) [2]. Other tests typically

performed on such devices include a transconductance curve (upper right corner of Figure

Figure 1a. Schematic cross-section of IBM’s CNFET (carbon

nanotube field effect transistor) [2] IBM Copyright.

Figure 1b. I

versus V

for an IBM nanotube FET [2]. The

different color plots represent different source-drain