femtoampere

A femtoampere (symbol fA) is the SI unit of electric current equal to 10^-15 amperes. It is derived from the prefix femto-, meaning one quadrillionth, applied to the base unit ampere. The femtoampere is used to express extremely small currents that arise in high-impedance electronics, nanoscale devices, and precision detectors.

Typical contexts include leakage currents in insulating materials, dark current in photodetectors, currents in single-electron devices, and other noise-limited measurements in nanoscale sensors. In specialized experimental settings, femtoampere-level signals can occur in ultra-low-current measurements, though practical use is most common in instrumentation specifications and theory.

Measurement and challenges: Measuring femtoampere-level currents requires low-noise instrumentation, careful guarding, and shielding. Instrumentation may include electrometers or transimpedance amplifiers with very low input bias currents and high feedback resistances. Noise sources such as Johnson-Nyquist thermal noise, flicker (1/f) noise, dielectric leakage, and surface contamination can dominate. Techniques include averaging, temporal integration, guarding, shielding, and meticulous calibration against known standards.

Relation to other units: 1 fA equals 10^-15 A, which is 0.001 pA. Conversely, 1 A equals 1×10^15 fA. The femtoampere is primarily a conceptual and practical unit for discussing extremely small currents in advanced electronic and scientific contexts.