Low-Current Measurements; Leakage Currents - Keithley 4200A-SCS Reference Manual

Model 4200A-SCS Parameter Analyzer Reference Manual
To avoid oscillations for a BJT, try the following:
1. Set (Collector-SMU current measure range) = (Emitter-SMU current measure range)
2. If necessary, set both SMUs to autorange.
3. For the emittter SMU, do not select the Common operation mode. This mode prevents prevents
you from configuring a current measurement range for the emitter SMU, which results in a lower
impedance than at the collector SMU; a potentially high gain; and an increased likelihood of low-
frequency oscillation. Instead, configure the emitter SMU for the Voltage Bias operation mode
and set it to 0 V. This allows you to configure the current measurement range.
Both Drain/Collector and Source/Emitter (SMU) must be set to measure current if they are set to
autorange.
For instructions on configuring tests, refer to

Low-current measurements

Low-current measurements made with a SMU or preamplifier are subject to error sources that can
have a serious impact on measurement accuracy. The following topics discuss low-current
measurement considerations, including leakage currents, generated currents, noise and source
impedance, and voltage burden. Refer to the Keithley Instruments Low Level Measurements
Handbook for more information.

Leakage currents

Leakage currents are generated by high-resistance paths between the measurement circuit and
nearby voltage sources. These currents can considerably degrade the accuracy of low-current
measurements.
Cable leakage currents are a common source of leakage. Typically, insulation resistance between
conductors in the type of triaxial cables supplied with the SMUs and preamplifiers is approximately
1 PΩ (10
through the cable insulation, affecting the measurement. Properly connecting the triaxial cables to the
SMU or preamplifier automatically drives the inner cable shield at guard potential, minimizing the
effects of cable leakage currents. See
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
4200A-901-01 Rev. C / February 2017
Ω). If the cables were used in an unguarded configuration, leakage current would flow
15
Test fixture guarding
Configure a complex test
Guarding (on page 3-23) for details.
(on page 3-25) for details.
Section 12: Maintenance
(on page 6-143).
12-11