AC Impedance Spectroscopy

The electrochemical properties of a system are investigated by the impedance spectroscopy. In our case a sinusoidal signal over a frequency range is applied and the averaged effect by for instance dipoles, chemical reactions, ionic or electrical conduction is recorded. In case of a small voltage stimulus, smaller than the equivalent thermal energy V<kT/q, the influence of the measurement on the characteristics of the band structure and the charge distribution are negligible. Based on a physical model an electrically equivalent circuit representing the elements of the probe is designed and the expected resulting signal is compared to the experimental data.


The sensitive layer of the standard n-type metal oxide sensor consists of interconnected grains. In the presence of oxygen electrons are trapped at the surface and the the surface layer of every grain becomes depleted of charge carriers. In case that the grain size is greater than the depletion layer an energy barrier results and a high resistive layer forms. By solving the Poisson’s equation with the boundary conditions for the flat band situation, a relation between the depletion layer thickness z0, the energy band bending qVS and the relative permittivity ?S of the surface can be obtained :

Modeling the surface region by a plan parallel capacitor, the capacitance per area unit of the grain-grain contact can be simplified expressed by

This capacitive element is, speaking in electrical terms, parallel to the high resistive element of the depletion layer. In an equivalent circuit these two elements would appear in series to the the low resistive bulk.

Similar to the intergranular contact the semiconductor-metal contact of the metaloxide and the electrode form a resistive and capacitive element in parallel. However, in contrast to the grain–grain contacts, their Schottky barrier height is determined by the difference in the work functions of the metal and semiconductor and not by the surface processes. Changes in the electron concentration near the metal–semiconductor contact due to the gas interaction at the grains are immediately compensated by electron transfer from the electrode and, as a result, the resistance of the metal–semiconductor contact does not depend on the ambient atmosphere.

The analysis of thick film metal oxide gas sensor by ac impedance spectroscopy provides detailed information about the way in which the conduction takes place.



Schematic diagram of the grain and electrode contacts and their energy band representation and the equivalent circuit of the sensitive material
Typical impedance spectrum of an n-type sensor at different CO concentrations
Fitted values for the gas sensitive resistor R1 at different CO concentrations

Related References

  • Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity, N. Barsan and U. Weimar, J. Phys. Condens. Matter, 15, 2003, R813-R83.
  • Investigations of conduction mechanism in Cr2O3 gas sensing thick films by ac impedance spectroscopy and work function changes measurements, S. Pokhrel, C.E. Simion, V. Quemener, N. Barsan, U. Weimar, Sensors and Actuators B 133, 2008, 78-83.
  • Impedance Spectroscopy during The Sensing of CO With Tin Dioxide Gas Sensors, O. Sachlara, N. Barsan and U. Weimar, IV International Workshop on Semiconductor Gas Sensors, SGS’04,2004, 49, Ustron, Poland.
  • Empfindliche Prozesse bei dünnen Polyacrylsäure-Schichten unter Ammoniak-Atmosphäre durch Impedanzspektroskopie aufgedeckt, M. Sahm, A. Oprea, N. Barsan and U. Weimar, 7. Dresdner Sensor-Symposium, Dresdner Beiträge zur Sensorik, Band 24, TUDpress,, 2005, 267-270, Dresden.
  • Influence of the Nature of the Electrode on the Sensing Performance of SnO2 Sensors; Impedance Spectroscopy Studies, J. Bertrand, D. Koziej, N. Barsan, J.P Viricelle, C. Pijolat and U. Weimar, Eurosensors XX, 2006, 100-101, Goeteborg, Sweden, ISBN 978-91-631-9280.
  • Morphology Influence upon the CO Sensitivity for WO3 Based Gas Sensors – Impedance Spectroscopy Investigations, C.E. Simion, N. Barsan, S. Pokhrel, A. Tomescu, U. Weimar, Conf. Proc. Eurosensors XXII, Sept. 07-10, 2008, 372, Dresden.