To meet the growing demands of flexible and wearable electronic systems, such as smart watches and biomedical sensors, electronics engineers are seeking high-performance transistors that can efficiently modulate electrical current while maintaining mechanical flexibility.

Thin-film transistors (TFTs), which are comprised of thin layers of conducting, semiconducting and insulating materials, have proved to be particularly promising for large-area flexible and wearable electronics, while also enabling the creation of thinner displays and advanced sensors.

Despite their potential, the energy-efficiency with which these transistors can switch electrical current has proved difficult to improve. This is due to the so-called thermionic limit, a theoretical threshold that delineates the lowest possible voltage required for a transistor to boost electrical current by a factor of 10 at room temperature when switching between “off” and “on” states.

Researchers at Soochow University and other institutes have developed a new TFT based on organic materials that could bypass this limitation, as it operates below the thermionic limit. The transistor, introduced in a paper published in Nature Electronics, was found to amplify signals with remarkable efficiency.

“Our work was driven by a fundamental challenge in wearable electronics and Internet of Things (IoT): the pursuit of high-performance devices with ultra-low-power consumption,” Jiansheng Jie, senior author of the paper, told Tech Xplore.

“Conventional organic thin-film transistors (OTFTs) are inherently limited by the thermionic emission mechanism, which sets a theoretical minimum for the subthreshold swing (SS)—a key metric that determines how efficiently a transistor can switch—of 60 mV dec^-1 at room temperature. This inherent limitation results in excessive power dissipation during switching operations, posing a major barrier to energy-efficient operation.”

This recent study builds on recent works that highlighted the promise of so-called tunnel field-effect transistors (TFETs) based on inorganic semiconductors. These transistors were found to overcome the limitations of conventional transistors, leveraging a quantum mechanical process known as band-to-band tunneling.

“We sought to translate these advantages into the field of organic electronics,” said Jie. “Our central objective was to develop organic thin-film tunnel transistors (OTFTTs) capable of sub-60 mV dec^-1 performance, thereby breaking the fundamental thermionic limit that has long governed conventional OTFTs.

“By demonstrating such behavior in a solution-processable, flexible organic platform, our research addresses a critical gap in the technological evolution of organic electronics and paves the way toward low-voltage, highly efficient flexible circuits for next-generation wearable and IoT applications.”

The new OTFTT developed by the researchers replaces the thermionic injection mechanism that drives the operation of conventional TFTs with band-to-band tunneling. This process allows charge carriers to pass through the energy barrier directly and at extremely low voltages, significantly boosting the devices’ switching efficiency.

“The key innovation lies in the design of a hybrid inorganic-organic source-channel heterojunction,” explained Jie.

“We combined molybdenum trioxide (MoO₃), an inorganic metal oxide with a deep-conduction-band, with the 2,7-dioctyl[1]-benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single-crystalline thin film, which has a relatively low highest occupied molecular orbital (HOMO) energy level. This creates a ‘broken-gap’ alignment, where the HOMO of C8-BTBT lies above the conduction band (CB) of MoO₃.”

The configuration of the team’s transistor prompts the thermally excited tail of carriers originating from the MoO₃ source to be sharply truncated. This in turn effectively suppresses classical thermionic emission processes, making band-to-band tunneling the dominant carrier injection mechanism.

“Meanwhile, by introducing a molecular decoupling layer (BPE-PDCTI) at the heterojunction interface, the Fermi-level pinning effect was effectively alleviated and the tunneling barrier height was further reduced,” said Jie.

“This strategic design enables the device to trigger charge band-to-band tunneling at an extremely low supply voltage. As a result, our OTFTTs overcame the 60 mV dec^-1 thermionic limit on SS, achieving the lowest SS of 24.2 ± 5.6 mV dec^-1 among the existing thin-film transistor technologies, alongside the record-high signal amplification efficiency of 101.2 ± 28.3 S A^-1.”

The ultra-low SS yielded by the newly developed transistor is highly favorable for the development of low-power signal amplification circuits. In initial tests, circuits based on the transistor were found to achieve a gain in amplification of over 537 V V⁻¹ at an ultra-low power consumption below 0.8 nW.

“Our OTFTTs break the fundamental thermionic limit—a long-standing theoretical ceiling on SS (60 mV dec⁻¹ at room temperature) that has constrained the energy efficiency of conventional thin-film transistors for decades,” said Jie.

“This breakthrough not only redefines the performance boundaries of organic electronics but also enables a new class of ultra-low-power devices. The practical implications are substantial. Our OTFTTs are ideally suited for energy-constrained applications such as wearable health monitors, implantable biosensors, and self-powered IoT nodes.”

Notably, the OTFTT developed by Jie and his colleagues is compatible with existing processing and electronics fabrication strategies. In the future, it could be improved further and used to develop a wide range of high precision sensing devices, including trackers for the diagnosis or monitoring of specific medical conditions, environmental sensing systems and neuromorphic (brain-inspired) computing hardware.

“In bridging the gap between the intrinsic physical limitations of organic semiconductors and the stringent efficiency demands of next-generation technologies, this work represents a critical step toward intelligent, pervasive, and environmentally benign electronic systems,” said Jie.

Other researchers could soon build on the team’s design and set out to develop similar OTFTTs. Meanwhile, Jie and his colleagues plan to continue improving their device, for instance, by optimizing its performance via the careful engineering of energy levels at the interface between the organic materials it is based on.

To do this, they will select organic semiconductors with reduced bandgaps and lower carrier effective mass, while also creating high-conductivity interfacial decoupling layers that could enhance the transistor’s tunneling efficiency and performance.

“We will also expand the technology to n-type OTFTTs to enable all-organic tunneling logic circuits, addressing the current gap in low-power organic logic applications,” added Jie.

“Moreover, we plan to deploy OTFTTs in high-precision biomedical signal amplification (e.g., EEG, EMG), ultra-sensitive environmental sensing (e.g., trace gas detection, low-light imaging), and low-power IoT signal processing.

“Finally, we will continue developing scalable integration techniques for the large-scale fabrication of the OTFTTs on flexible substrates, aiming to accelerate the industrial adoption of high-performance, energy-efficient organic electronic systems.”

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