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Optimizing Low-Current Measurements and Instruments Jonathan L. Tucker Senior Marketer, Research and Education Business, and Sensitive Measurements Product Line Manager Keithley Instruments, Inc. Characterizing devices at low current levels requires knowledge, skill, and the right test equipment. Even with all three, achieving accuracy at low current levels can be a challenge because the level of the current is often at or below the noise level of the test setup. To ensure success in low-level current measurements, it is important to know the type of test equipment to use, the different sources of measurement error, and the appropriate techniques to minimize these errors. Examining several test examples, such as characterization of a field-effect transistor (FET) and a carbon nanotube, can help in the learning process. How low is low? The term low current is relative, of course. A current level considered low for one application, such as 1mA, may be high for a device operating at 10nA. In general, an instrument's noise level will establish its low- level sensitivity, with low-current measurements referring to those made near an instrument's noise level. Trends in portable and remote electronic devices, along with advances in semiconductors and nanotechnology, are requiring greater use of low-current measurements. Small-geometry devices, photovoltaic devices, and carbon nanotubes are a few examples of devices designed to operate at extremely low current levels, and all of these devices must be characterized in terms of their current-voltage (I-V) characteristics. A number of instruments are available for low-current measurements, depending upon the type of device under test (DUT) and the level of current to be measured. Perhaps the most ubiquitous tool on production lines and in field service is the digital multimeter (DMM), which typically provides capabilities Keithley Instruments, Inc. 28775 Aurora Road Cleveland, Ohio 44139 (440) 248-0400 Fax: (440) 248-6168 www.keithley.com 1 for measuring current, voltage, resistance, and tem