Updates on GaN for RF & Microwave Applications
There are many predictions that the RF GaN industry has been, and will continue, to experience substantial growth over the next several years. Predictions estimate that the currently nearly US$ 400 million RF GaN market (2017 close) will likely grow to, or beyond, US$ 1 billion by 2023 [1, 2]. With the defense industry consuming roughly a third of all RF GaN devices in 2017, the remaining two-thirds of GaN devices is likely divided between wireless infrastructure and avionics radar/communication equipment, the majority of which is most likely designed for cellular applications. Though there are GaN-based low-noise amplifiers (LNA), mixers, filters, and other RF components and devices, it appears that the majority of the RF GaN market is focused around GaN power amplifiers (PA).
The defense industry desires extremely high power density and rugged devices for current, and next generation, radar and highly resilient communication systems. This is likely also the focus for high-performance L-band avionics radar, such as interference friend or foe secondary surveillance radar (IFF/SSR). In these cases, high peak power output is desired, with bandwidths ranging from S-band to Ka-band. Moreover, GaN-based devices have been demonstrated with extremely high mean-time-between-failure (MTBF) rates well beyond GaAs, Si, LDMOS, and other power amplifier semiconductor technologies.
What may possibly the largest GaN market segment, 4G and upcoming 5G cellular applications, are more concerned about GaN-based PA’s high efficiency over wide bandwidths, i.e. power-added efficiency (PAE). This indicates that even though GaN-based devices, mainly amplifiers, have been demonstrated in research reaching just over 100 GHz, the majority of GaN-based devices likely operate between several hundred megahertz and a several gigahertz (~600 MHz to 3 GHz). This bandwidth capability is actively being extended for future 5G cellular applications to 6 GHz to account for the latest 5G spectrum.
The two largest technology segments of GaN-based devices appear to be GaN on silicon carbide (GaN-on-SiC) and GaN on silicon (GaN-on-Si). GaN-on-SiC devices tend to operate at much higher power densities than GaN-on-Si, as SiC substrates demonstrate greater mechanical strength over temperature and coefficient of thermal expansion (CTE) than Si substrates. However, SiC substrates and GaN-on-SiC fabrication processes tend to also be more expensive than GaN-on-Si, leaving opportunity for GaN-on-Si for more cost-constrained applications.
Another attractive feature of GaN-based devices are their relatively high voltage operation. This enables much more efficient PAs, as higher operating voltages reduce resistance related losses. Moreover, this is also an attractive feature for high-survivability LNA devices used for defense, aerospace, and avionics applications. GaN-based devices high voltage operation can be leveraged by GaN-LNAs to prevent damaging overload from jamming signals or interference.
There are still challenges associated with modeling and accurately characterizing GaN transistors over these devices frequency of operation and power. Hence, continuing efforts are being made to develop more accurate models for GaN devices, as well as wider bandwidth and higher power load-pull testing systems specifically designed for GaN PAs.