GaN RF HEMTs: Powering ahead with native substrates

GaN-on-GaN high-power amplifiers with through-substrate vias are delivering record-breaking power-added efficiencies and continuous-wave operation
BY NAOYA OKAMOTO AND YUSUKE KUMAZAKI FROM FUJITSU LIMITED

Electronic devices make our lives richer and more convenient. They are now taking us towards an advanced society, formed through the fusion of cyberspace and physical space. On entering this era, we will benefit from vast numbers of sensors in physical space collecting big data, quantitatively analysed by artificial intelligence in cyberspace. New value will be created from this, including the provision of high-quality services. Underpinning this new world order will be an increase in the use of radio waves above the microwave band to sense and collect data, alongside networks that exchange huge amounts of data, as well as sectors within industry that manufacture substances that bring new value. Supporting this introduction of a higher sensing resolution and a roll-out of a higher network capacity will be an increase in the frequency of solid-state power amplifiers deployed in radio equipment. A shift from microwaves to millimetre waves is already underway, and migration to the terahertz domain will follow.

Moving to higher frequencies is not trivial. Challenges are not limited to simply ensuring that devices can operate at higher speeds – there is also the issue of a reduction in the power conversion efficiency of solid-state power amplifiers at higher frequencies, leading to greater power consumption. This is at odds with a sustainable society, as to curb the carbon footprint of the communication sector, the power consumption of radio equipment must fall as the number of units increases.

The most efficient, powerful solid-state power amplifiers are based on GaN HEMTs. These RF transistors are typically fabricated on non-native substrates, such as SiC or silicon. However, the power and the efficiency of these devices are held back by electron traps that form in the GaN epilayers – mainly the buffer layer – and lead to current collapse.

Our team from Fujitsu, Japan, is tackling this issue head-on by switching the substrate to free-standing GaN. It is a solution we have been working on for many years. About ten years ago, when characterising the metal-insulator-semiconductor interface and the Schottky junction of GaN-based epitaxial layers grown on GaN substrates, we observed excellent crystal quality in this GaN-on-GaN heterostructure. But at this time only n-type, rectangular GaN substrates that were small in size were available. This restricted our development to the basic research phase.

Around 2017, when the availability of semi-insulating 2-inch GaN substrates began, we embarked on full-fledged work on RF GaN-on-GaN HEMTs. Initially, we directed our efforts at developing RF GaN-on-GaN HEMT power amplifiers for microwave heating. This project, supported by the Japan Ministry of the Environment, required transistors to run under severe conditions of high-power, continuous-wave operation. Realising success on these fronts would create a device that could serve in other applications, such as radar and wireless communication. However, progress would not be easy, due to the lower thermal conductivity of the GaN substrate compared with that made from SiC.