Researchers in Taiwan have developed the first thin-film piezo-gated transistor that works in dual accumulation and depletion mode for switching.
Along with an analytical model to explain its workings, using force for switching the transistor could guide the development of next-generation ‘piezotronic’ sensors that are lower cost and lower power.
Piezoelectric force sensors are typically governed by either a strain-induced Schottky barrier height (SBH) modulation or by a piezo-gating effect that redistributes the charge carriers in an induced piezoelectric field. However, while SBH piezo devices have been well-explored, piezo-gating based devices remain relatively less understood. This, in turn, has limited the fabrication of piezo-gated transistors. This piezo-gating effect is also often confused with the piezoresistive effect as the two co-exist and are similar in response.
So the researchers from National Cheng Kung University (NCKU) in Taiwan built a a “dual-mode” piezo-gated thin-film transistor (PGTFT) along with an analytical model explaining its mechanism in depth. The PGTFT operates in both depletion and accumulation modes, and has a record ratio of relative change in current to mechanical strain, the gauge factor, of 2780, indicating its sensitivity.
“PGTFTs that rely solely on piezo-gating effect are essential for developing advanced piezotronic devices. But, most PGTFTs reported so far have shown indistinct piezo-gating effect through SBH modulation induced by piezoelectric fields, and can detect only one-dimensional strain,” said Prof Chuan-Pu Liu, corresponding author of a study on Nano Letters.
The researchers used zinc oxide (ZnO) to build the thin-film transistors, varying the charge carrier concentrations in the ZnO thin films by changing the gas used during their preparation. The thin films were then fully characterized and used to prepare two distinct PGTFT configurations.
The team tested the current-voltage characteristics of the PGTFTs by subjecting them to strain and analyzed the results both analytically and using numerical simulations. Additionally, they explored the effect of changing carrier concentrations on the depletion and accumulation modes of the PGTFTs to gauge the influence of the piezo-gating effect.
The team found that increasing the strain on the PGTFT reduced the current in the top electrode while increasing it in the bottom electrode. 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 in their design.
Additionally, the carrier concentration dependence revealed, both experimentally and analytically, that the gauge factor is highly sensitive to the carrier concentration, with the experimental results showing a 44% enhancement in the gauge factor.
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