Datasheet
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Current
Stored Current Measurement
Sample’s Background Current
Voltage Source
Current (pA)
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The alternating voltage source polarity method eliminates the effects of background currents in materials for making repeatable, accurate high resistance and
resistivity measurements.
consistent, repeatable, and accurate measurements for
a wide variety of materials and components, especially
when used in combination with the 8009 Resistivity
Improved High Resistivity
Measurements
Test Fixture.
Many test applications require measuring high levels
of resistivity (surface or volume) of materials. The
conventional method of making these measurements is to
apply a sufficiently large voltage to a sample, measure the
current that flows through the sample, then calculate the
resistance using Ohm’s Law (R=V/I). While high resistance
materials and devices produce very small currents that
are difficult to measure accurately, Keithley electrometers
and picoammeters are used successfully for such
measurements.
Alternating Polarity Method for
High Resistivity Measurements
The 6517B uses the Alternating Polarity Method,
which virtually eliminates the effect of any background
currents in the sample. First and second order drifts
of the background currents are also canceled out. The
Alternating Polarity Method applies a voltage of positive
polarity, then the current is measured after a specified
delay (Measure Time). Next, the polarity is reversed and
the current measured again, using the same delay. This
process is repeated continuously, and the resistance is
calculated based on a weighted average of the four most
recent current measurements. This method typically
produces a highly repeatable, accurate measurement of
resistance (or resistivity) by the seventh reversal on most
materials (i.e., by discarding the first three readings). For
example, a 1mm-thick sample of 1014 Ω-cm material
can be measured with 0.3% repeatability in the 8009
Resistivity Test Fixture, provided the background current
changes less than 200 fA over a 15-second period.
Even with high quality instrumentation, inherent
background currents in the material can make these
measurements difficult to perform accurately. Insulating
materials, polymers, and plastics typically exhibit
background currents due to piezoelectric effects,
capacitive elements charged by static electricity, and
polarization effects. These background currents are
often equal to or greater than the current stimulated by
the applied voltage. In these cases, the result is often
unstable, providing inaccurate resistance or resistivity
readings or even erroneous negative values. Keithley’s
6517B is designed to solve these problems and provides
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