GaN still has great potential

As demand for bandwidth continues to grow and existing radio spectrum becomes congested, the telecommunications industry is looking for new technologies to meet the needs of future mobile communications. The pursuit of more bandwidth is inextricably linked to the use of higher radio frequencies, and higher operating frequencies mean more available bandwidth.

While researchers study new III-V materials, such as indium phosphide, for frequencies above 100GHz, they expect GaN-based technologies to become important in the lower millimeter-wave portion of the RF spectrum (i.e., below 50GHz) effect. Because of this, GaN is expected to serve next-generation 5G networks and possibly early versions of 6G.

GaN technology attributes its potential for RF/lower millimeter-wave communications to its excellent physical properties: it has high current density, high electron mobility, and high breakdown voltage. Due to its high mobility, this technology can handle higher switching frequencies than today's silicon-based technologies.

In addition to speed, GaN-based technology is also highly regarded for its power handling capabilities, which allows it to deliver high output power with good energy efficiency. These properties could make GaN an attractive technology for use in power amplifiers (PAs) located in front-end modules for next-generation mobile handsets and small base stations. These front-end modules send RF signals to and receive from the antenna. Compared with traditional Si or SiGe-based technologies, GaN's higher power handling capabilities translate into higher transmission range and/or fewer components required to drive the antenna.

To be suitable for use as a PA in user equipment and small base stations, the cost and form factor of the device are as important as its electrical characteristics. As mentioned earlier, GaN helps reduce the form factor of front-end modules due to the inherent characteristics of the technology. But achieving highly scalable form factors requires integrating various components of RF front-end technology. To help achieve this goal, imec is adapting its GaN-on-Si technology platform for RF applications as part of its Advanced RF program.

Imec chose GaN-on-Si over GaN-on-SiC for cost-saving reasons: not only is the Si substrate cheaper, but the CMOS-compatible process also enables large-scale manufacturability. GaN-on-Si technology was originally developed for power electronics applications and is envisioned for power conversion in battery chargers, computers, servers, automobiles, lighting systems and photovoltaic devices. However, making GaN-on-Si suitable for mobile RF applications requires several technological innovations. Parasitics within the device structure must be suppressed as much as possible to reach high frequencies. Examples of these efforts include reducing source access resistance through methods such as developing technology modules with raised source/drain and reducing gate-related parasitic capacitance. Optimizing devices for higher operating frequencies will also require further reductions in gate length. This favors higher fT and fmax, which is a measure of the inherent speed of the device. Additionally, the buffer layer must be RF compatible to minimize RF substrate losses.

Imec's GaN-on-Si process flow for RF starts with the metal-organic chemical vapor deposition growth of epitaxial structures on 200 mm silicon wafers. The epitaxial structure consists of a proprietary GaN/AlGaN buffer structure, GaN channel, AlN spacer and AlGaN barrier. The GaN HEMT device with TiN Schottky metal gate is then integrated with a low-temperature three-level Cu back-end process, as shown in Figure 1.

As demonstrated at the 2020 International Electronic Devices Meeting (IEDM 2020), Imec researchers used this CMOS-compatible platform to fabricate GaN HEMTs. Optimization of the gate metal stack, contact resistance and gate length scaling to 110nm resulted in a device with fmax of 135GHz, representing a step towards millimeter wave applications.

The key figures of merit for a PA are the output power and efficiency the transistor can deliver. Competitive results were obtained on imec's GaN-on-Si platform for 0.19μm gate length (LG) devices at 6GHz. These results were presented at European Microwave Week 2022 and are shown in Figure 2a.

Figure 2b, presented at IEDM 2022, compares the performance of the imec GaN-on-Si process with other GaN-on-Si and GaN-on-SiC processes. The imec data in red is one of the best reported for GaN-on-Si devices and is comparable to GaN-on-SiC devices. Using a shorter gate length improves measurement performance at 28GHz. With these improvements, imec believes that, for the first time, the PAE of amplifiers designed specifically to meet user equipment requirements and manufactured using the GaN-on-Si process is on par with equivalent GaN-on-SiC amplifiers.

GaN-on-Si technology has matured considerably in recent years, driven by the growth of the power electronics market, largely due to the development of a technology originally intended for power electronics applications. With maturity in mind, delving deeper into the physical mechanisms behind device operation provides an additional tool to improve device characteristics. Imec complements technology development with modeling activities that will ultimately help achieve better performance and reliability. The insights gained will not only benefit the development of GaN HEMT devices for millimeter-wave applications, but will also improve performance in other application areas, including GaN-based power electronics.

As an example of these modeling activities, this section focuses on device isolation. This is one of the technology building blocks of the GaN-on-Si platform. When integrating GaN HEMTs into a common Si platform, the devices must be electrically isolated from each other, with as few leakage paths between adjacent devices as possible. This electrical isolation reduces power losses and improves the breakdown behavior of active devices. For GaN HEMTs, ion implantation has proven to be a more attractive isolation method than other isolation techniques such as mesa etching, providing lower leakage and higher isolation area breakdown voltage. The technology was originally developed for GaN-based power electronics applications and it remains one of the isolation technologies actively used today.

Ion implantation introduces several defects in the GaN heterostructure, which act as trapping centers for charge carriers. In terms of physics, these defects move the Fermi level away from the conduction or valence band of GaN. Implanting ions, such as nitride (N) ions, into the area surrounding the device will reduce the number of conducting free carriers, thereby creating an electrically insulating region. In experiments, the researchers also observed that damage caused by ion implantation disappears after annealing at high temperatures (usually above 600°C), thus affecting isolation quality.

imec's GaN-on-Si manufacturing process ensures high-quality isolated HEMT devices with a low post-epitaxy thermal budget. Imec has demonstrated a GaN HEMT ion implantation isolation technology that helps report the highest sheet resistance, with values ​​in the range of 10 ¹³ to 10 ¹⁵ Ω/sq. This is the basic metric for quantifying isolation. Figures 3a and 3b illustrate benchmarks for sheet resistance (R sh ) of AlGaN/(AlN)/GaN heterostructures isolated by ion implantation with different activation energy magnitudes and peak heating temperatures. The benchmark in Figure 3a demonstrates the common physical mechanisms behind isolation, while the benchmark in Figure 3b demonstrates the main effect of processing temperature on isolation quality.

Why this technique is so effective and precise where residual current leakage paths form remains a mystery. A basic understanding and modeling of leakage mechanisms in the ion implantation region is required. This helps improve process conditions such as thermal budget, implant dose, and energy for a variety of applications, including millimeter-wave communications.

There's a reason why understanding the exact mechanisms behind insulation is so difficult. The ion implantation area is filled with defects of various natures. Point defects exist, such as vacancies or interstitial atoms, defect complexes, foreign ionic impurities, and lattice disorders, among others. Furthermore, polarization charges exist at the interface between AlGaN and GaN. This complex mixture of defects and charges makes it extremely challenging to model charge behavior and locate leakage paths within isolated heterostructures.

Through a combination of experimental and modeling work, imec researchers have revealed for the first time leakage mechanisms in isolated GaN-based heterostructures. Details of the work have been published in the Journal of Applied Physics. By setting up dedicated experiments with different AlGaN and AlN thicknesses, the researchers extracted and analyzed the sheet resistance and corresponding activation energies of the isolated regions. The conclusion from these experiments is that the main leakage occurs through the ohmic path of the electrons at the GaN surface. Back to physics terms, this translates into a downward bending of the GaN conduction band near the GaN surface. These insights lay the foundation for more detailed modeling of isolated heterostructures and reconstruction of their energy band diagrams. This theory helps extract the net defect density in these isolated implant regions, which for these experiments is ∼2×10 ¹⁹ cm ^-3 and ∼2×10 ¹⁸ cm ^-3 for GaN and AlGaN, respectively . Most of these defects are found to be point defects. Point defects are created through ion implantation technology and avoid recombination through imec's low thermal budget HEMT manufacturing. A high density of point defects is critical to limit GaN surface band bending and therefore leakage. Figure 4a and Figure 4b illustrate the leakage mechanism in GaN heterostructures. Figure 4a shows the surface leakage path and the overall leakage path in the transmission line model structure. Figure 4b illustrates the energy band diagram of the AlGaN/AlN/GaN heterostructure, showing the band bending of the GaN surface.

For the first time, imec researchers have revealed the exact mechanism behind ion implantation as a technique for electrically isolating GaN HEMT devices. These insights can help improve process conditions to achieve good isolation quality when targeting RF/mmWave communications. These findings can also be extended to power electronics applications.

Furthermore, the study leads to a new method to estimate the net defect density in isolated GaN-based heterostructures. These activities fit into the broader GaN device optimization framework for RF applications through technology and modeling. These efforts and results illustrate how uncovering the physical secrets behind the technology's building blocks can help advance these GaN-based devices to higher levels of maturity.