Abstract
We describe a low-noise cryogenic probing system capable of measuring currents at the femtoAmpere (fA) level with integration time ⩽1 s and current noise spectra at the 1 fA/√Hz level with the sample cooled to cryogenic temperatures. The system consists of a low-noise electrometer and a low-noise cryogenic probe with completely guarded sample chamber. The low-noise electrometer has amplification of 0.1 V/1 pA with f3 dB=70 Hz, 1.1 V/pA with f3 dB=20 Hz, and/or 10.1 V/pA with f3 dB=1.4 Hz, and input current noise power density of 0.4 fA/√Hz rms and input voltage noise power density of ≈0.3 μV/√Hz rms at 1 Hz frequency. With the cryogenic probe, the entire measurement system has input current noise of 0.8 fA/√Hz rms at 1 Hz. The completely guarded sample chamber reduces parasitic conductance to <10−16 S and parasitic capacitance to ⩽15 fF between pairs of pins for current–voltage measurements. With this instrumentation, we are able to characterize the current and current noise of quantum well infrared photodetectors for sensitive zero-background applications.
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