Since their introduction more than two decades ago, source measurement units (SMUs) have evolved into a category of multi-purpose instruments that are regularly called upon to address a rapidly expanding array of electronics industry applications:
• Semiconductor device fabrication, process development, and product research/design
• Production verification of electronic products such as portable wireless devices
• Production and development of new advanced materials for devices such as solar cells and HBLEDs
• Almost any electronic device test application.
Before exploring the factors that define SMU technology, it may be helpful to define exactly what an SMU is (and what it isn’t). Essentially, SMUs are fast-response, read-back voltage and current sources with high accuracy measurement capabilities, all tightly integrated in a single enclosure. They are designed for circuit and device evaluation where a DC signal must be applied to a device under test (DUT) and the response to that signal measured. They are capable of four-quadrant operation (Figure 1), acting as a positive or negative DC source or as a sink (load). They also provide highly repeatable measurements, typically with 5½- or 6½-digit resolution.
SMUs can typically be used to perform sweeps of both current and voltage that can be used to determine the I-V characteristics of a device under test. As a result of these advantages, SMUs have been widely adopted in industry and are a common component of many automated test systems.
Fig 1: Four-quadrant SMU design.
Despite any claims to the contrary, traditional instrumentation remains a vital, growing part of the test and measurement industry. Although specific communication interfaces (GPIB, RS-232, etc.) may become obsolete over time, instrument-based SMUs, used either alone or integrated with other SMUs in a system, typically provide the fastest, most accurate, most flexible solutions for the widest range of demanding applications. 'Component' SMUs often must compromise their performance to offer a specific form factor.
Widest available power and signal ranges
For testing many types of devices, it is desirable to have test equipment capable of operating with a wide range of signal levels. For example, devices such as power MOSFETs are designed to have very low resistance and handle very large currents when turned on but are also designed to have very high resistance and allow nearly zero current to flow when turned off. In the on state, this current is commonly as high as tens of amps; in the off state, this current may be less than nano-amps.
Power diodes and high brightness LEDs have similar dynamic range requirements as well for full characterization. On these kinds of devices, when a forward bias voltage below the threshold value is applied, device currents are very low. As voltage is swept from 0V to the threshold, device current goes from the sub-nano-amp range up to milli-amps. As the bias voltage reaches and then exceeds the threshold, test currents increase very rapidly, reaching as high as tens to hundreds of amps depending on the device.
Test equipment that is capable of taking accurate measurements over this wide range is desirable because it reduces the number of pieces of equipment required to test, thereby reducing both the complexity and the cost of the system.
Keithley SourceMeter instruments combine the most power with the widest range of signals available in a single instrument. The Model 2651A High Power SourceMeter instrument can deliver up to 200W of DC power and 2000W of pulsed power to a device. It can measure as much as 50A of current and is also capable of measuring with a maximum resolution of 1pA. The Model 2636A leads the industry in dynamic range with the ability to measure signals from 10A all the way down to 1fA, offering 16 decades of current resolution.
Some competitors’ instrument-based SMUs claim nearly the same coverage of dynamic range as the Model 2636A Dual-channel System SourceMeter instrument, measuring from 10A all the way down to 10fA. However, when one compares the available measurement ranges from each SMU (figure 2), it’s obvious that the Model 2636A has current ranges for the next two orders of magnitude lower than the competition. What this means is that the Model 2636A doesn’t have to rely on the least significant, least accurate digits of its measurement ranges to achieve a truly wide dynamic range. For instrument users, that provides far significantly greater confidence in the accuracy of their low current measurements.
Fig 2: Dynamic range of Keithley SMUs vs. competitive offerings.
Vendors of component SMUs also try to claim wide coverage. However, the form factor of these instruments limits them to a dynamic range several decades smaller than Keithley’s instrument-based SMUs. On the high end of the range, they are limited by how much power the chassis can provide and most component SMUs will top out at 100mA. On the low end, the electromagnetic interference of all the circuitry designed into a small space with inadequate room for shielding creates too much electrical noise for any kind of low-level measurement to be practical; as a result, it is uncommon to see a component SMU with any current ranges lower than 10 micro-amps.
The white paper continues with sections on analog-to-digital converters, multi-channel scalability, and I/O connectors.
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Article Courtesy Test & Measurement DesignLine