Piezoelectric transistors for force sensing?

Author: EIS Release Date: Aug 22, 2022

National Cheng Kung University in Taiwan is experimenting with piezoelectric gated thin-film transistors.

 

NCKU piezo gated thin film transistor force sensorNational Cheng Kung University in Taiwan is experimenting with piezoelectric gated thin-film transistors.

 

“Piezotronic force sensors are typically governed by either a strain-induced Schottky barrier height modulation or by a piezo-gating effect that redistributes the charge carriers in an induced piezoelectric field,” according to NCKU. “While Schottky barrier height based piezotronic devices have been well-explored, piezo-gating based devices remain relatively less understood.”

 

The experimental piezo-gated transistor (diagram right) has a ohmic contacts on a zinc oxide thin film to remove Schottky barriers.

 

NCKU piezo gated thin film transistor force sensor graph 904

 

Putting up or down force into the structure shifts carriers to different depths in the thin film, allowing the device to act like a transistor whose channel resistance increased or decreased from its initial value depending on the direction of applied force.

 

At the same time as this piezo-gating occurs, the piezo-resistive effect also alters the device’s conductivity.

 

To separate piezo-gating from piezo-resistive behaviour, the researcher built similar devices with the contacts under the thin film, to see if this flipped the slope of the piezo-gating curve without affecting any piezo-resistive curve.

 

And that is what happened, allowing, depending on the force direction to be sensed, top or bottom contacts to be chosen so that the piezo-gating and piezo-resistive behaviours add rather than partially cancel.

 

With downward force, “increasing the strain reduced the current in the top electrode while increasing it in the bottom electrode”, said NCKU. “This happened due to the movement of electrons from the top to the bottom in response to the force, creating a depletion at the top and an accumulation of electrons at the bottom. This, in turn, affected the output current and showed the co-existence of piezo-gating effect and piezoresistive effect, with the piezo-gating effect being dominant.”

 

A force transistor was created with a “record gauge factor – the ratio of relative change in current to mechanical strain – of 2780, indicating its extreme sensitivity”, 44% better than before, according to NCKU.

 

Prior the building the transistors, the team created a theoretical model to predict their thin film’s  behaviour, confirming it by rf sputtering ZnO under different host gasses to vary charge carrier characteristics.

 

The work is described in ‘Analytical and experimental investigation of dual-mode piezo-gated thin film transistor for force sensors‘, published in ScienceDirect.

 

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