Source / Measure Units

The term source meter (or sourcemeter) and source measure unit (SMU) can be used interchangeably. A source meter is an instrument that measures direct current (DC) or voltage while simultaneously sourcing / supplying or sinking / receiving them.

A source meter combines the functionalities of a power supply and a digital multimeter. Unlike a power analyzer, an SMU typically does not have any oscilloscope or signal generator features to generate waveforms and view them in real-time. However, some specialized SMUs, like the PZ2100 series, have built-in digitizers to characterize response waveforms and pulsers to generate pulsed waveforms. In terms of performance, source meters are very high-speed instruments with very fast settling times compared to other instruments.

The regular power supply mode is considered a first-quadrant operation where both the voltage and current are positive. A source meter or an SMU behaves as follows in each of the quadrants:

• In the first quadrant, the source meter is a source with a positive voltage across it and is supplying a positive current from its output terminal.
• In the second quadrant, the source meter acts as a load with a positive voltage across it. It sinks or receives current from the DUT at its output terminal.
• In the third quadrant, the source meter is again a source supplying current, but its polarity is the opposite of conventional polarity and hence negative. That means the return terminal is set at a higher voltage than the output terminal but without changing the wiring to the DUT.
• In the fourth quadrant, the SMU again acts like a load. The voltage is negative, which means the return terminal is at a higher potential than the output terminal. However, the current flows conventionally.

An SMU or a source meter combines the functionality of a power supply (or power source), electronic load, voltmeter, and ammeter. It can both source and sink power. This is termed as operating in all four quadrants of the voltage-current plane. Source meters can rapidly reach set voltages or currents compared to other instruments. The typical settling times of good SMUs are of the order of tens to hundreds of microseconds. A critical capability of a source measure unit, which is essential for many characterization tests, is its ability to rapidly transition between different source and sink modes to simulate real-world conditions.

The compliance voltage is the configurable maximum limit on the measured voltage. If the voltage goes above this high-voltage limit, the source meter can be configured to quickly cut off power and shut down to protect both the instrument and the DUT. Similarly, there is also a compliance current limit to protect the SMU and DUT from high currents.