Electrometer with sub-attoampere current load.

By cooling a conventional junction field-effect transistor below 150 K, a simple and versatile electrometer with extremely high impedance can be realized. At operating condition, the leakage current to the gate amounts to a few hundredths of an attoampere. The electrometer can be used from DC up to a frequency of 10 kHz. Without reduction of the bandwidth, a sensitivity of a few µV is obtained. Working at low frequencies, currents as low as a few attoamperes can be detected. If the input voltage is out of the operational range, the forward current or the Zener current of the gate junction protects the transistor against destructive charging.