Graphene has emerged as a promising material for radio-frequency (RF) transistors due to its exceptional electronic properties. Its high carrier mobility and saturation velocity make it an attractive candidate for high-frequency applications, where traditional semiconductors like silicon face limitations. However, despite its potential, graphene RF transistors also present significant challenges, including the lack of a natural bandgap and high contact resistance. This article evaluates the performance and limitations of graphene-based RF transistors, focusing on carrier mobility, cutoff frequency, and key engineering obstacles.

One of the most critical advantages of graphene for RF applications is its ultra-high carrier mobility. At room temperature, graphene exhibits carrier mobilities exceeding 15,000 cm²/V·s in suspended form and around 10,000 cm²/V·s on substrates like silicon dioxide. This is significantly higher than silicon, which typically has mobilities in the range of 1,400 cm²/V·s for electrons and 450 cm²/V·s for holes. The high mobility translates to faster carrier transport, enabling graphene transistors to achieve higher cutoff frequencies. The cutoff frequency (fₜ) is a key metric for RF performance, representing the frequency at which the current gain drops to unity. Experimental graphene field-effect transistors (GFETs) have demonstrated cutoff frequencies exceeding 400 GHz in research settings, surpassing conventional III-V semiconductor-based high-electron-mobility transistors (HEMTs) in some cases.

The intrinsic velocity saturation of carriers in graphene also contributes to its RF performance. Unlike silicon, where velocity saturation limits high-field operation, graphene’s carriers can reach saturation velocities as high as 5 × 10⁷ cm/s under high electric fields. This property is crucial for maintaining high-frequency operation even at short channel lengths, a necessity for scaling down transistor dimensions. The combination of high mobility and saturation velocity allows graphene transistors to maintain performance advantages as device geometries shrink, a critical requirement for next-generation RF electronics.

However, graphene’s lack of a natural bandgap poses a fundamental challenge for transistor applications. A bandgap is essential for achieving sufficient on/off current ratios, a requirement for digital and analog switching. In RF applications, while a large on/off ratio is less critical than in digital logic, some degree of current modulation is still necessary for efficient signal amplification. Researchers have explored several methods to induce a bandgap in graphene, including bilayer graphene with vertical electric fields, nanoribbon structures with quantum confinement, and chemical functionalization. Bilayer graphene, for instance, can exhibit a tunable bandgap of up to 250 meV under high displacement fields, but this often comes at the cost of reduced carrier mobility. Nanoribbons, while capable of opening larger bandgaps, suffer from edge roughness and reduced conductivity. These trade-offs complicate the integration of bandgap-engineered graphene into practical RF devices.

Another major challenge is contact resistance between graphene and metal electrodes. Unlike conventional semiconductors, graphene lacks surface states that facilitate ohmic contact formation. As a result, the contact resistance in graphene transistors can be a significant bottleneck, particularly for high-frequency operation where low parasitic resistance is essential. Reported contact resistances for graphene devices range from 200 Ω·µm to over 1 kΩ·µm, depending on the metal used and the interface quality. Palladium and nickel are commonly employed due to their relatively low work function mismatch with graphene, but even these metals struggle to achieve resistances below 100 Ω·µm consistently. High contact resistance degrades the extrinsic cutoff frequency and increases power dissipation, limiting the overall RF performance.

Scalability and reproducibility are additional hurdles for graphene RF transistors. While lab-scale demonstrations have shown impressive performance metrics, transferring these results to large-scale manufacturing remains difficult. Variability in graphene quality, substrate interactions, and processing conditions can lead to inconsistent device characteristics. For instance, defects introduced during transfer or lithography can scatter carriers and reduce effective mobility. Substrate phonons also play a significant role in limiting mobility, as graphene’s electronic properties are highly sensitive to the underlying dielectric environment. Hexagonal boron nitride (hBN) has been used as an ideal substrate to preserve high mobility, but its integration into wafer-scale processes is still under development.

Despite these challenges, progress in material synthesis and device engineering continues to advance graphene RF transistor technology. Heterostructure designs combining graphene with other two-dimensional materials, such as transition metal dichalcogenides, offer new avenues for improving performance. These structures can provide electrostatic control while maintaining high carrier mobility. Additionally, advances in contact engineering, including the use of edge contacts and optimized annealing processes, have shown promise in reducing contact resistance.

The potential applications of graphene RF transistors span a wide range of high-frequency systems. They could enable ultra-high-speed analog amplifiers, low-noise mixers, and reconfigurable RF front-ends for wireless communication. Their compatibility with flexible substrates also opens possibilities for conformal and wearable RF electronics. However, for widespread adoption, further improvements in bandgap engineering, contact resistance reduction, and manufacturing uniformity are necessary.

In summary, graphene RF transistors offer compelling advantages in carrier mobility and cutoff frequency, making them strong candidates for next-generation high-frequency electronics. However, the absence of a bandgap and high contact resistance remain critical obstacles. Ongoing research into material modifications, heterostructures, and contact engineering is essential to overcome these limitations and unlock the full potential of graphene in RF applications. The progress in this field will determine whether graphene can transition from a promising material to a mainstream technology in RF electronics.