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WHITE
PA P E R
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.
Keithley Instruments, Inc.
28775 Aurora Road
Cleveland, Ohio 44139
(440) 248-0400
Fax: (440) 248-6168
www.keithley.com
A G r e a t e r M e a s u r e o f C o n f i d e n c e
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 109A/cm2,
whereas copper wire is limited to about 106A/cm2. 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.
Figure 1a. Schematic cross-section of IBM's CNFET (carbon Figure 1b. ISD versus VG for an IBM nanotube FET [2]. The
nanotube field effect transistor) [2] IBM Copyright. different color plots represent different source-drain
voltages. IBM Copyright
Figure 1b shows that the amount of current (ISD) flowing through a nanotube channel
can be changed by varying the voltage applied to the gate (VG) [2]. Other tests typically
performed on such devices include a transconductance curve (upper right corner of Figure
A G r e a t e r M e a s u r e o f C o n f i d e n c e
1b), gate leakage, leakage current vs. temperature, substrate to drain leakage, and sub-
threshold current. Since these types of devices are still in the research stage, measurements
that provide insight into fundamental properties of conduction, such as transport mechanisms
and I-V vs. temperature, are critical.
Figure 2. Nanopore structure [3]. Graphic courtesy of Mark A. Reed Research Group, Yale University.
Polyphenylene molecules are another approach to developing active electronic
components. The nanopore test structure shown in Figure 2 is based on polyphenylene
molecules deposited between two gold electrodes on a silicon wafer. This structure serves as
a probe pad, allowing a researcher to make probe connections for I-V characterization of
nanoscale devices, such as molecular diodes (see Figure 3).
With such I-V curves, researchers have determined that molecules can conduct small
amounts of electrical current. Although I-V measurement methods are typical for device
characterization, the levels of current measured are lower than those of many semiconductor
devices fabricated today.
I-V characterization of moletronic devices requires low level current measurements in
the nanoamp to femtoamp range. To complicate matters, these measurements are quite often
made at cryogenic temperatures. Therefore, highly sensitive instruments are required, and
appropriate measurement and connection techniques must be employed to avoid errors.
Typically, nanoelectronic and moletronic devices are characterized with semiconductor test
instruments and probe station systems, such as the one shown in Figure 4.
A G r e a t e r M e a s u r e o f C o n f i d e n c e
Figure 3. I-V curve for a molecular diode at room temperature [3]. Graphic courtesy of Mark A. Reed Research Group, Yale University.
Figure 4. Example of a Windows