Graphene field-effect transistors (FETs) represent a revolutionary advancement in semiconductor technology due to their exceptional electronic properties. The ultra-high carrier mobility and ambipolar conduction behavior of graphene make it a promising candidate for high-performance electronics, particularly in high-frequency applications. This article explores the fundamental characteristics of graphene FETs, their fabrication challenges, contact resistance issues, and potential applications.

The defining feature of graphene is its linear energy dispersion near the Dirac points, which results in charge carriers behaving as massless Dirac fermions. This unique band structure enables electron and hole mobilities exceeding 200,000 cm²/Vs at room temperature in suspended graphene, though practical devices on substrates typically exhibit values between 1,000 and 50,000 cm²/Vs depending on substrate interactions and defect density. Such high mobility allows for fast carrier transport, making graphene FETs ideal for high-speed electronics.

Ambipolar behavior is another critical characteristic of graphene FETs. Unlike conventional semiconductors, graphene lacks a bandgap, leading to a continuous transition between electron and hole conduction as the gate voltage is swept. The charge neutrality point, where the Fermi level crosses the Dirac point, separates the n-type and p-type regimes. This ambipolarity enables reconfigurable logic circuits but also poses challenges for digital applications due to the absence of a natural off-state.

Fabrication of graphene FETs involves several critical steps. The most common method begins with mechanical exfoliation of graphene from highly ordered pyrolytic graphite (HOPG), though chemical vapor deposition (CVD) on metal substrates like copper has become the preferred approach for large-scale production. The transfer process of CVD-grown graphene onto insulating substrates such as SiO₂/Si or hexagonal boron nitride (hBN) is crucial, as impurities and wrinkles can degrade performance. Electron-beam lithography or photolithography is then used to pattern electrodes, typically made of metals like palladium, gold, or chromium, which form contacts to the graphene channel.

Contact resistance remains a significant bottleneck in graphene FET performance. The formation of a Schottky barrier at the metal-graphene interface leads to non-ideal charge injection, increasing access resistance and reducing overall device conductance. Studies have shown that contact resistance can range from 200 to 1,000 Ω·µm depending on metal choice and interface quality. Approaches to mitigate this issue include edge contacts, where metals interact with graphene's reactive edges rather than its basal plane, and the use of phase-engineered contacts that reduce the Schottky barrier height.

High-frequency performance is one of the most compelling applications of graphene FETs. The combination of high mobility and low sheet resistance enables cutoff frequencies (fT) and maximum oscillation frequencies (fmax) surpassing 100 GHz in sub-micron gate-length devices. Experimental demonstrations have achieved fT values up to 427 GHz for 67 nm gate-length graphene FETs. These metrics suggest potential for graphene in millimeter-wave and terahertz electronics, including high-speed analog amplifiers and mixers.

The absence of a bandgap in pristine graphene limits its use in digital logic, where a high on/off ratio is essential. However, for radio-frequency (RF) applications, the on/off ratio is less critical than transconductance and cutoff frequency. Graphene FETs have demonstrated outstanding RF performance, with power gains exceeding 10 dB at 10 GHz and linearity suitable for communication systems. Their symmetric ambipolar conduction also enables novel circuit designs, such as frequency multipliers and phase detectors.

Thermal management is another consideration in graphene FETs. While graphene's high thermal conductivity (up to 5,000 W/mK) is beneficial for heat dissipation, the interfaces between graphene and substrates or electrodes often introduce thermal resistance. Self-heating effects can become significant at high power densities, affecting device reliability and performance.

Scalability remains a challenge for commercial adoption of graphene FETs. While CVD growth allows wafer-scale production, uniformity and defect control must be improved to achieve consistent device characteristics across large areas. Integration with existing semiconductor manufacturing processes also requires further development to enable hybrid graphene-silicon technologies.

Despite these challenges, graphene FETs continue to show promise for specialized applications where their unique properties provide unmatched advantages. Their ultra-high mobility and ambipolar behavior position them as leading candidates for next-generation high-frequency electronics, pushing the boundaries of speed and performance in semiconductor devices. Continued research into contact engineering, scalable fabrication, and device integration will determine their ultimate role in the electronics industry.