Organic field-effect transistors (OFETs) have evolved beyond conventional architectures to meet the demands of emerging electronic applications. Unconventional designs, such as electrolyte-gated, ferroelectric-gated, and dual-gate OFETs, offer unique advantages in performance, functionality, and energy efficiency. These variants address limitations in traditional OFETs, enabling low-voltage operation, multi-functionality, and compatibility with flexible and bio-integrated systems. Their development has opened pathways for neuromorphic computing, biosensing, and adaptive electronics.

Electrolyte-gated OFETs (EGOFETs) utilize an ion-conducting electrolyte as the gate dielectric instead of a conventional solid insulator. The electrolyte enables electric double-layer formation at the semiconductor-electrolyte interface, leading to high capacitance and low operating voltages, typically below 1 V. This is due to the nanometer-scale effective dielectric thickness of the electric double layer, which results in carrier densities exceeding 10^14 cm^-2. EGOFETs are particularly advantageous in biosensing applications, where the electrolyte can directly interact with biological analytes. For instance, they have been used to detect glucose, DNA hybridization, and neural activity with high sensitivity. The ionic-electronic coupling in these devices also mimics synaptic behavior, making them suitable for neuromorphic computing. A key challenge is the stability of the electrolyte-semiconductor interface, as prolonged operation can lead to electrochemical doping or degradation.

Ferroelectric-gated OFETs incorporate a ferroelectric material as the gate dielectric, leveraging its non-volatile polarization to achieve memory functionality. The remanent polarization of ferroelectric polymers like poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) allows threshold voltage modulation without continuous power, enabling low-energy operation. These devices exhibit hysteresis in their transfer characteristics, which can be harnessed for non-volatile memory and synaptic transistors. The multi-level polarization states of ferroelectric materials permit analog switching, a critical feature for neuromorphic applications. Research has demonstrated ferroelectric OFETs with retention times exceeding 10^4 seconds and endurance over 10^5 cycles. However, issues such as polarization fatigue and interfacial charge trapping require further optimization for long-term reliability.

Dual-gate OFETs feature two independent gate electrodes, typically arranged in top- and bottom-gate configurations. This design provides enhanced control over charge transport by enabling threshold voltage tuning, ambipolar operation, and improved electrostatic coupling. The additional gate can compensate for trap states at the semiconductor-dielectric interface, leading to higher carrier mobility and reduced hysteresis. Dual-gate architectures have been employed in sensors to amplify signals and in reconfigurable logic circuits where the gate bias dynamically adjusts device behavior. For example, dual-gate OFETs have achieved mobility values over 5 cm^2/Vs in organic semiconductors like pentacene and polymer blends. The complexity of fabrication and the need for precise alignment between gates remain challenges for widespread adoption.

The low-voltage operation of these unconventional OFETs is a significant advantage for portable and wearable electronics, where power efficiency is critical. Electrolyte- and ferroelectric-gated devices often operate below 3 V, reducing energy consumption compared to conventional OFETs that may require tens of volts. This attribute is particularly beneficial for biomedical implants and environmental sensors powered by energy-harvesting systems. Additionally, the compatibility of these designs with solution processing enables large-area, low-cost fabrication on flexible substrates.

In neuromorphic computing, electrolyte- and ferroelectric-gated OFETs emulate synaptic plasticity through ionic dynamics and polarization switching. Short-term plasticity, spike-timing-dependent plasticity, and long-term potentiation have been replicated in these devices, providing building blocks for brain-inspired hardware. Ferroelectric OFETs, in particular, offer non-volatile memory with linear conductance modulation, essential for analog neural networks. Research has shown that arrays of these devices can perform pattern recognition and unsupervised learning tasks with accuracies comparable to software-based neural networks.

Biosensing is another niche application where unconventional OFETs excel. EGOFETs detect biomolecules through electrostatic or electrochemical interactions at the electrolyte-semiconductor interface. Their high sensitivity stems from the direct coupling between ionic charges in the analyte and the transistor channel. For instance, EGOFET-based pH sensors achieve sensitivities of 50–100 mV per pH unit, while glucose sensors exhibit detection limits below 1 μM. Dual-gate OFETs enhance sensing performance by providing a second gate to offset environmental noise or amplify signals. These devices have been integrated into wearable patches for real-time monitoring of biomarkers in sweat or interstitial fluid.

Despite their promise, unconventional OFETs face material and engineering challenges. Electrolyte-gated devices require stable electrolytes with minimal leakage and evaporation, prompting research into solid-state and gel-based alternatives. Ferroelectric OFETs need materials with high polarization and low coercive fields to minimize switching energy. Dual-gate designs demand precise alignment techniques to avoid parasitic capacitance or misalignment-induced performance variations. Advances in materials science and fabrication techniques are critical to overcoming these hurdles.

Future directions include hybrid architectures combining multiple gating mechanisms, such as electrolyte-ferroelectric dual-gate OFETs, to exploit synergistic effects. Scalable manufacturing methods, like roll-to-roll printing, will be essential for commercial viability. Furthermore, integrating these devices with flexible substrates and stretchable interconnects will expand their use in conformal and implantable electronics.

In summary, unconventional OFET designs push the boundaries of organic electronics by enabling low-power, multi-functional devices for neuromorphic computing, biosensing, and adaptive systems. Their development reflects a broader trend toward specialized transistors tailored for emerging technologies beyond traditional silicon-based electronics. Continued innovation in materials and device engineering will further unlock their potential in next-generation applications.