measurement application
electronic load. During source or sink operation, the SMU can simultaneously measure voltage, current, and resistance. This operating flexibility can be especially valuable when characterizing batteries, solar cells, or other energy generating devices.
Built-in sweep capabilities: The various sweep capabilities SMUs offer can simplify programming a test’s source, delay, and measure characteristics, significantly boosting testing productivity. All sweeps can be configured for single-event or continuous operation to simplify the process of capturing the data needed to characterize and test a wide range of devices. Sweeps can also be used in conjunction with other throughput-enhancing features like Hi-Lo limit inspection and digital I/O control to create high speed production
test systems. A fixed level sweep outputs a single level of voltage or current with multiple measurements his is typically done to bias or stress devices. Various types of fixed level sweeps can be generated, depending on the needs of the application.
Pulsed sweeps are often used to limit the amount of power that goes into a material sample or device over time and to minimize self-heating effects that could otherwise damage semiconductors and light emitting diodes (LEDs), experimental materials such as graphene, or other fragile nanotechnology-based devices.
Custom sweeps simplify creating application- specific waveforms.
SMU
vs.DMM Because of its built-in sourcing capabilities, an SMU can minimize overall measurement uncertainty in many applications. The first diagram in Figure 3 shows the basic voltmeter configuration for the SMU. Here, the built-in current source can be used to offset or suppress any system-level leakage currents (such as cable noise) that could cause unwanted errors in voltage measurement applications.
For current measurements, the SMU’s built-in source and “feedback ammeter” design works together to keep voltage burden low, and enable low current measurements to subpicoamp levels. DMMs do not have the built-in source, and typically have “shunt ammeter” designs that typically limit low current capabilities to microamp or nanoamp levels.
Finally, for resistance measurements, the SMU architecture offers full flexibility over the amount of
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current or voltage sourced to the DUT. DMMs have fixed current source values that are dependent on the range being used to measure resistance. SMUs offer fully programmable source values for measuring resistance. This can be valuable for protecting DUTs or for measuring extra high or extra low resistances. For high resistance measurements, the source voltage method is preferred; for low resistance measurements, the source current method is best. Some SMUs have a six-wire ohms feature that “guards out” the effects of unwanted parallel resistance paths in the circuit.
SMU Measurement Terminology One of the first considerations in choosing an SMU instrument must be the quality of the measurements it produces. Poor measurement integrity can cause those using the data produced to draw incorrect conclusions about the performance of a given DUT. In R&D, this can mean an imperfect understanding of a device’s operating parameters, leading to unnecessary rework and costly time-to-market delays. In production test, inaccurate measurements can result in rejection of good parts (false failures) or acceptance of bad ones, either of which can cause poor yields, customer dissatisfaction, and other problems.
When considering an SMU instrument’s measurement integrity, keep several key terms in mind: accuracy, repeatability or stability, resolution,
Figure
3.SMU voltmeter,ammeter, and ohmmeter configurations
Figure 2.A power supply (right) offers only two-quadrant operation; an SMU instrument (left) can source and sink power in all four quadrants
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