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An ammeter is a current measuring instrument. In the circuit, it is represented by the symbol A. The ammeter is placed in series with the device or component to measure the current. A series connection is used because objects in series have the same current passing through them. If a circuit has multiple resistors and the current needs to be measured in each resistor, the number of ammeters required depends on whether the circuit is in series or parallel.
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Electrometer with sub-attoampere current load.

Paul Graf1, Meike Flebbe1, Detlef Utzat1

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Summary

Cooling a junction field-effect transistor below 150 K creates a high-impedance electrometer. This device achieves attoampere leakage current and microvolt sensitivity for detecting faint electrical signals.

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Area of Science:

  • Electronics
  • Instrumentation
  • Low-temperature physics

Background:

  • Electrometers are crucial for measuring small electrical charges and currents.
  • Conventional electrometers often face limitations in impedance and sensitivity.
  • Field-effect transistors (FETs) offer potential for high-impedance applications.

Purpose of the Study:

  • To develop a simple and versatile electrometer with extremely high impedance.
  • To investigate the performance of a cooled junction field-effect transistor (JFET) as an electrometer.
  • To characterize the electrometer's sensitivity, bandwidth, and leakage current.

Main Methods:

  • Utilizing a conventional junction field-effect transistor (JFET).
  • Operating the JFET at temperatures below 150 K.
  • Measuring leakage current, sensitivity, and frequency response.

Main Results:

  • Achieved an extremely high impedance electrometer by cooling a JFET below 150 K.
  • Demonstrated a gate leakage current of a few hundredths of an attoampere.
  • Obtained a sensitivity of a few microvolts (μV) without bandwidth reduction, usable from DC to 10 kHz.
  • Capable of detecting currents as low as a few attoamperes at low frequencies.
  • Incorporated protective mechanisms (forward or Zener current) against destructive charging.

Conclusions:

  • A cooled JFET provides a simple, versatile, and high-performance electrometer.
  • The developed electrometer is suitable for detecting extremely small currents and voltages.
  • The device offers robust protection against input overloads.