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This letter reports the results on measurements and modeling of the ultralow current measurement capability of a silicon-on-sapphire current integrator circuit. We have tested the lowest possible current measurable with the device and the noise performance with picoampere input currents. The device is capable of resolving subpicoampere currents with an rms noise of 350 fA in a 110-Hz bandwidth. The device is also capable of digitally measuring currents up to 100 muA by employing a pulse-based A/D converters.