Modern electrometer techniques

Electrometers are used for measuring small currents, charges and voltages normally less than 10V, and sometimes resistance. Originally passive electrometers such as Kelvin´s quadrant electrometer were used. These earlier instruments have now been largely superseded, first of all by valve electrometers and more recently by electrometers incorporating semiconductors. The commonest and cheapest form of electrometer is probably the d.c. feedback current-voltage convertor. As well as the limitations of this instrument in detail, the vibrating-capacitor, varactor-bridge and other electrometer variations are also discussed. Techniques for measuring electrometer response times are covered together with methods for improving response times. In maximum-sensitivity applications, d.c. electrometers employing insulated-gate field-effect transistors are normally limited by low-frequency noise to a typical sensitivity of 0.2fA. It is anticipated that sensitivities of the order of 10aA should ultimately be possible with current improvements in device-processing technology.