Saturday, February 20, 2010

Proton conduction in organic FETs?

This title is deliberately provocative. It is keeping with the notion of multiple alternative hypotheses. I note the following concerning organic Field Effect Transistors
  • The mobility of protons in water is 3 x 10-3 cm^2/Vs.
  • This is larger than the hole mobility in many OFETs.
  • Fabrication of many OFETs involves treatment with acids at some stage.
  • Gate dielectric surface treatments significantly affect device performance, as described in this review.
So can someone rule out the following hypothesis?
In some organic FETs there is actually a contribution to the current from protons (rather than holes) moving in the interface between the organic "semiconductor" and the gate dielectric.

Perhaps a systematic study of OFET performance as a function of humidity?


  1. Perhaps impedance spectroscopy would be a better experiment to settle this question.

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  3. Impedance spectroscopy may help Ben. However for the low mobility you are talking about, the phenomenon you are looking for is the low-frequency dispersion (mili Hz to kHz) which highly correlated electrons can also follow (see

  4. I would not be at all surprised if there is some amount of ionic conduction in some organic semiconductors. There are various "gate bias stress effects" (basically, measured mobility and mobile gated charge density decay when a gate voltage is applied to an organic FET for a long time) that are slow and very temperature dependent. I've often wondered if some of these are due to ion migration.

  5. Thanks for the link Majed - it looks really interesting

  6. One way to check is using muons in MuSR as a proxy for proton mobility. I think the best thing to do is map out the FET conductive behaviour as a function of temperature, and then compare and contrast it to a MuSR experiment as a function of temperature. The draw back to this is that you will need fairly massive samples in order to stop the muons in your sample.

  7. Protons are small enough and the distances short enough that many reactions considered electronic in (e.g.) organic polymer batteries and ultracapacitors seem to have a strong protonic component. Such protonic materials are also "self doping". It would not surprise if similar processes happen in FETs.

  8. Proton conduction will contribute to the off currents in the transport similar to the ionic currents seen in case of electrolyte gated FETs. However for a high mobility semiconductor impedance spectroscopy and temperature dependent especially at low T may be useful to distinguish them. Matthiessen's rule can be used to separate the contributions.