Generated Currents; Reducing Leakage Currents; Offset Currents - Keithley 4200-SCS Reference Manual

Section 5: Source-Measure Concepts

Reducing leakage currents

Several methods to reduce leakage currents include:
• Use good quality insulators, such as Teflon or polyethylene, in the test fixture.
• Reduce the humidity of the test environment. Insulators and even the test circuit itself may
absorb water, causing spurious currents to be generated.
• Use guarding in the test fixture to isolate the high-impedance nodes from leakage current
due to voltage sources. See

Generated currents

Any extraneous generated currents in the test system will add to the desired current, causing
errors. Currents can be internally generated, as in the case of preamp input offset current, or they
can come from external sources such as insulators and cables. The following paragraphs discuss
the various types of generated currents.
generated currents discussed in this section.
Table 5-1
Typical generated currents

Offset currents

The preamp has a small current, known as the input offset current, that flows at all times. As
shown in
measures the sum of the two currents. Note that input offset current can be nulled by performing
system calibration, as discussed in
NOTE
Offset currents can also be generated externally from such sources as triboelectric and
piezoelectric effects (discussed below). As shown in
adds to the source current, and the SMU again measures the sum of the two. These external
offset currents can be suppressed manually by subtracting them via the KITE Formulator or the
KITE Calc worksheet (for more information about the Formulator and the Calc worksheet, refer to
Analyzing test data using the
Section 6).
5-20
Effect
Triboelectric
Mechanical stress (Teflon)
Mechanical stress (ceramics)
Clean epoxy circuit board
Dirty epoxy circuit board
Figure 5-16A, the input offset current adds to the measured current so that the SMU
The meaning of the word "nulled" in this context is to bring the offset current down to
within specifications.
Formulator, and
Return to
Test fixture guarding earlier in this section for details.
Table 5-1 summarizes the typical ranges of a number of
Generated current range
1 fA to 10 nA
1 fA to 1 pA
100 aA to 100 fA
100 fA
100 pA
Configuring the Project Plan ITMs
Figure
Displaying and analyzing data using the Sheet tab
Section Topics
Model 4200-SCS Reference Manual
in Section 6.
5-16B, the external offset current also
4200-901-01 Rev. S / May 2017
in