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A G R E AT E R M E A S U R E O F C O N F I D E N C E toamp range requires a device that can meas-
ure currents with a few tens of attoamps
RMS noise in the range of 0.1 to 0.01Hz and,
in the case of a demonstration setup, a cur-
rent source with a resolution better than one
attoamp. A category of instruments known
in the industry as Source-Measure Units
(SMUs) contain a precision voltage source,
a precision current source, a voltmeter, and
an ammeter. The Keithley Model 6430 Sub-
femtoamp Remote SourceMeter instrument
Counting Electrons:
has an ultra-low noise current amplifier and
provides these functions in a single instru-
ment. Figure 1 shows a typical SourceMeter
How to measure
instrument with the source block represent-
ing both the voltage source and current
source capability. The Vmeasure circle repre-
currents in the sents the built-in voltmeter which gives feed-
back to the source block and can be used to
attoampere range
control it. The Imeasure circle represents the
built-in ammeter, and it, too, can control the
source block. Note also that the instrument
can provide both Vmeasure and Imeasure func-
tions simultaneously.
Adam Daire, Keithley Instruments, Inc. A good SMU can source very small cur-
rents, and the 6430 SourceMeter instrument
T
can source current with 50 attoamp resolu-
HE tiny world of nanotechnology Typically, measuring such low currents tion. But this experiment called for a source
is making big waves in industry has meant using expensive test equipment, capable of repeatedly producing accurate ten
these days. Nanoscale materials upwards of $100,000, and costly cryogenic attoamp currents. The voltage source within
hold promise for areas such as current comparators. However, it is possible the Model 6430 was used to create a current
medicine, homeland security, de- to measure changes in current as small as one source with this high resolution. To reduce
fense, and many other industries. Research- attoamp at room temperature using commer- the difficulty of working with extremely high
ers in R&D and development labs all over cially available test and measurement equip- value resistors, the voltage source was first
the globe are investigating the physical and ment. This article looks at a simple setup and divided down by a factor of ten, then applied
electrical properties of nanoscale compo- technique for making repeatable and reliable across a two-teraohm resistor to the current
nents as in single-electron transistor (SET) measurements at the attoamp level. measurement input. The result is ten attoamps
and quantum-dot research. With this reduc- of current flowing with 200 microvolts ap-
tion in physical size of the material under in- Equipment required plied. The five microvolt digital resolution
vestigation come new problems; specifically, Making current measurements in the at- of the source yields a 0.25 attoamp current
accurately measuring electrical parameters
such as resistance, voltage, and current.
To keep pace, test and measurement in- IMeasure Force
struments and techniques have had to adapt R
to the changing needs of researchers. Al- Sense HI
Ifeedback
ready, improvements in instrumentation
make it fairly simple to measure currents in
Source VMeasure
the low picoamperes (1