Flexible Submicrometer-Channel Organic Transistors for High-Frequency Applications via Low-Resolution Fabrication

Organic field-effect transistors (OFETs) have garnered significant attention due to their versatility and numerous advantages, including low fabrication costs, mechanical flexibility, and compatibility with large-area deposition techniques. However, OFETs have yet to realize their full potential in mass-market applications. The Internet of Things (IoT), represents an area of applications in which flexible low-cost organic transistors could find a real-world scenario of use. High-performance devices capable of real-time, high-frequency communication, particularly in the MHz to GHz range, are essential for such applications. However, achieving these frequencies with OFETs remains challenging due to existing material limitations and the constraints given by the scalable, large-area manufacturing processes commonly employed with organic devices. As a result, OFETs development has been primarily focused on fields where rapid response times are not critical, such as sensing and biosensing. A possible approach to enhance OFETs performances is to enlarge the transistor form factor by reducing the channel length. Among the various device architectures proposed, vertical-channel transistors (v-OFETs) and particularly the so-called step-edge structure represent an intriguing and very effective approach to obtain short-channel devices. Such peculiar architectures are characterized by the fact that the channel, differently from standard coplanar structures, develops perpendicularly with respect to the substrate. In this work, we propose a Bottom-Contact Top-Gate (BCTG) step-edge short-channel OFET based on a Parylene C spacer developed employing an up-scalable fabrication method that allows obtaining submicron channels using a low-resolution fabrication process and a convenient large-area patterning technique compatible with flexible substrates. In particular, the devices were fabricated on a 250-µm thick polyethylene naphthalate (PEN) substrate. A gold (bottom) contact was deposited and patterned via thermal evaporation and low-resolution photolithography. A 350-nm thick Parylene C spacer was applied through chemical vapor deposition (CVD), followed by a second gold (top) contact that was deposited and patterned, partially overlapping the bottom contact. These two metallizations act as Source and Drain contacts. The channel, whose length is defined by the easily controllable and tunable spacer thickness, was formed by exposing the substrate to oxygen plasma, with the top contact acting as a self-aligned mask, thus the Parylene C was dry etched from everywhere except under the top contact. After cleaning and surface treatment, a polymer semiconductor (P(NDI2OD-T2)) was spin-coated and annealed. A dielectric layer (PMMA and Parylene C) was deposited, followed by an inkjet-printed gold top gate. The final device was annealed inside a nitrogen glovebox before characterization. In particular, the proposed devices showed an average frequency response of 2.5 MHz, with a maximum value of 5.5 MHz, a transition frequency that is higher than those obtained by other step-edge v-OFETs. The obtained results highlight a good reproducibility of the process and a good electrical stability of the devices, which can be operated in continuous mode and repeatedly characterized for extensive periods of time. In fact, another important aspect of our device is the possibility to operate it both in DC and AC modes without the need of a complicated bias setup and without any specific heat dissipation layer. This aspect, although yet to be specifically characterized, represents a crucial feature of this structure, since to fully exploit the possibilities offered by high-frequency organic transistors, a continuous-mode operation is highly desirable. These features, together with the low-cost and low-resolution fabrication method, can open up interesting new scenarios in the field of organic electronics for sensing and flexible circuits in the IoT context.