Commercial off-the-shelf graphene transistors have been used to achieve high on-off ratios, with potential for integration in digital logic circuits. Researchers have achieved this feat by coating the transistors with a liquid dielectric that induces an electrochemical effect that gates the graphene field effect transistors (GFETs). On-off ratios larger than 104 were achieved, as demonstrated in the recent publication in IEEE Access.
Since the first experimental isolation of graphene, the material has been considered as a potential replacement for silicon, due to its ultrathin form, flexibility, transparency, and high carrier mobility. However, graphene lacks an essential feature that enables semiconductors to be at the heart of digital logic components– an energy bandgap. The lack of a bandgap makes graphene switches impossible to fully turn off, resulting in poor on-off ratios.
Image: Graphenea GFET-S20 graphene field effect transistors.
Researchers have been exploring different avenues to exploiting the favourable properties of graphene in transistors. It was found that GFETs can be gated (switched) by electrochemical modification with a thin layer of silicon dioxide, which enabled the use of graphene for digital logic. Nevertheless, the switching times of such devices were on the order of 10 seconds, which is far too slow for use in circuitry. The most recent work demonstrates devices that have a high switching ratio while maintaining high speed.
Image: Liquid dielectric gated graphene transistor. CC4.0 license. Hayashi et al, IEEE Access 8, 92314 (2020).
The devices were made with a simple procedure that utilizes off-the-shelf commercial components. A wafer housing an array of graphene transistors was purchased, while the liquid dielectrics, also purchased, were deposited at precise locations using a micromanipulated probe. The scientists demonstrated good operation with liquid gates made of glycerine, honey, and caramelized honey. Caramelized honey has lower water content than pure honey, which results in a higher resistance, favourable for gate dielectrics. Achieved resistance was as high as 10 MOhm at a low operating voltage of 4V. The speed of operation was ranged from 2 to 120 milliseconds, depending on the gate dielectric material.
Image: GFET switching. CC4.0 license. Hayashi et al, IEEE Access 8, 92314 (2020).
While the current devices demonstrate limited repeatability, they are still a promising candidate for non-volatile memory or reconfigurable devices. Pending further device optimization, liquid dielectric gated graphene field effect transistors could be incorporated in digital logic devices. Due to the ultrahigh carrier mobility of graphene, these devices may even operate at lower subthreshold levels than what is currently possible with CMOS.