Analog/RF Playbook Blog

A gallium nitride (GaN) recap for next-gen RF applications

For nearly two decades, gallium nitride (GaN) semiconductor technology has been exposed to herald a paradigm shift in RF power capability. Though all of these promises haven’t materialized yet, GaN devices have made steady inroads into many RF, microwave, millimeter-wave (mmWave), and now even terahertz-wave (THz) applications.

Ongoing development efforts have enabled GaN active devices to cover a wide range of use cases, encompassing extreme temperature, power, and frequency ranges. These applications include both sub-6 GHz and mmWave 5G infrastructure devices. Further developments aim to decrease the cost of GaN devices, as well as enhance the integration capability of GaN with other common processes, including CMOS.

Where GaN stands today

The most common GaN devices seen today are GaN high electron mobility transistors (HEMTs) built as amplifier circuits. The majority of these GaN HEMTs are for power amplification applications. However, GaN HEMTs and other transistor variants are also used for low-noise and broadband amplifiers. GaN switches and diodes are also becoming increasingly common, generally replacing gallium arsenide (GaAs), silicon (Si), silicon germanium (SiGe) or indium phosphide (InP) devices.

Figure 1 A GaN-based HEMT can deliver major gains in terms of efficiency and power density in circuit topologies involving high-frequency operation and low on-state resistance. Source: STMicroelectronics

The reason GaN is becoming preferred over GaAs or InP for high-power or high-survivability applications is that GaN devices have a much higher breakdown voltage (critical field) and bandgap than other semiconductors. Along with a high saturation velocity and good electron mobility, GaN devices can provide highly efficient amplification and good power-added-efficiency (PAE), which is critical for communication applications such as 5G and Wi-Fi.

Common GaN variants

GaN-on-silicon (GaN-on-Si)

GaN-on-silicon carbide (GaN-on-SiC)

GaN-on-GaN

GaN-on-diamond

Common GaN devices

Power amplifiers (PA)

Rugged low-noise amplifiers (LNAs)

Broadband amplifiers

Switches

Diodes

Moreover, GaN devices, especially GaN-on-insulator devices, such as GaN-on-SiC, are generally more physically and thermally rugged than other semiconductors, and in some cases, have much better thermal conductivity. These factors show why GaN amplifiers typically have enhanced power density and survivability compared to other semiconductors, which makes GaN devices more attractive for military, aerospace, and industrial applications. In some cases, such as 5G and future 6G mmWave communications, GaN transmitters can exhibit higher power and efficiency than GaAs transmitters, which may allow less array elements and a more compact and lower cost active antenna system (AAS).

Figure 2 The structure of a GaN HEMT device is shown alongside comparison of output power against conventional technology. Source: Fujitsu

The future of GaN in RF

Most current GaN transistors are based on lateral heterojunction technologies, mainly aluminum-GaN and GaN HEMTs on Si or SiC substrates. With lateral transistor technologies, there is eventually a limit to the voltage/power limits devices can achieve within a given footprint of semiconductor material. It’s possible, however, to build transistors vertically. Using vertical transistor technologies can potentially increase the overall power density of GaN devices and require much less chip area for a given power/voltage capability. This would not only result in GaN transistors that are more compact, but also potentially lower cost than comparable performing lateral devices due to the reduced wafer area required per device.

For this to be realized, it is likely that a vertical GaN process would be a GaN-on-GaN technology, which is subject to GaN wafer costs and size constraints. Other possibilities with the development of high-voltage technologies could also include enhancement-mode GaN devices such as FinFETs and possibly trench MOSFETs.

There are still many process hurdles to overcome before this is possible, but in the next several years, it may be viable to have a CMOS-compatible GaN process. This would enable the integration of high-frequency and high-power GaN devices alongside high-density storage and digital logic circuits. Hence, whole high-power and high-frequency communication system-on-chips (SoCs) could be built; these devices will integrate RF transceivers, field programmable gate arrays (FPGAs), processors, and data storage.

JJ DeLisle, an electrical engineering graduate from Rochester Institute of Technology, moved to technical editing and writing work for design publications after spending six years in the industry as an IC layout and automated test engineer. He writes about analog and RF for Planet Analog.

 

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