Model 4200A-SCS Source-Measure Unit (SMU) User's Manual
Section 6: Optimizing SMU measurements
4200A-SMU-900-01 Rev. A December 2020
6-3
Eliminating oscillations
In general, oscillations can be classified as high-frequency (100 kHz through 200 MHz) or low
frequency (below 100 kHz). The measures needed to eliminate oscillations depend on which type of
the oscillation is occurring.
Eliminating high-frequency oscillations
One or more of the following remedies may help to eliminate high-frequency oscillations; the
remedies are listed in order of preference:
•
Mount the preamplifiers as close to the DUT as possible.
•
Connect the COMMONs (outer shields) of all cables together at the DUT.
•
Use lossy ferrite beads or 100 Ω resistors in series with the DUT leads.
•
Disconnect the ground link between GNDU COMMON and chassis ground on the rear panel of
the mainframe. Connect the cable shields to the prober chassis.
•
Add a high-quality capacitor between the base and emitter of a bipolar junction transistor (BJT) or
between the gate and source of a field-effect transistor (FET). Use a 100 pF to 1000 pF capacitor.
Eliminating low-frequency oscillations
Oscillations at low frequencies (dc to 100 kHz) occur when the gain of a transistor under test interacts
with the output impedances of the connected SMUs. The following ratios of impedance (Z) determine
the gains of the transistors:
•
For a FET:
ZDrain SMU / ZSource SMU
•
For a BJT:
ZCollector SMU / ZEmitter SMU
A SMU measures current through the voltage drop across a resistance, which is in series with the
DUT. This series resistance is high for low-current ranges and low for high-current ranges. A large
current-range difference, or oscillation, occurs when two SMUs are connected to the BJT collector
and emitter terminals or FET source and drain terminals. This oscillation results in:
•
A large series-resistance difference
•
A large impedance ratio between the two series resistances connected to the transistor
•
A large circuit gain (potentially, the maximum, intrinsic transistor gain)
•
A potentially unstable circuit
