Using bundles of vertical zinc oxide nanowires, Georgia Tech researchers have fabricated arrays of piezotronic transistors capable of converting mechanical motion directly into electronic controlling signals. The arrays could help give robots a more adaptive sense of touch, provide better security in handwritten signatures, and offer new ways for humans to interact with electronic devices.
The arrays include more than 8,000 functioning piezotronic transistors, each of which can independently produce an electronic controlling signal when placed under mechanical strain. These touch-sensitive transistors – dubbed “taxels” – could provide significant improvements in resolution, sensitivity and active/adaptive operations compared to existing techniques for tactile sensing. Their sensitivity is comparable to that of a human fingertip.
The vertically aligned taxels operate with two-terminal transistors. Instead of a third gate terminal used by conventional transistors to control the flow of current passing through them, taxels control the current with a technique called “strain-gating.” Strain-gating based on the piezotronic effect uses the electrical charges generated at the Schottky contact interface by the piezoelectric effect when the nanowires are placed under strain by the application of mechanical force.
Mimicking the sense of touch electronically has been challenging, and is now done by measuring changes in resistance prompted by mechanical touch. The devices developed by the researchers rely on a different physical phenomenon – tiny polarization charges formed when piezoelectric materials such as zinc oxide are moved or placed under strain. In the piezotronic transistors, the piezoelectric charges control the flow of current through the wires just as gate voltages do in conventional three-terminal transistors.
The arrays are transparent, which could allow them to be used on touch-pads or other devices for fingerprinting. They are also flexible and foldable, expanding the range of potential uses.