Design of a transmit-receive front-end in GaN for the FR3 band

Recently, new spectrum has been allocated for wireless communication between 7.125 GHz and 24.25 GHz. This frequency band, called FR3, has been allocated to accommodate the projected growth of wireless communication. Existing wireless communication hardware that is used in today’s cell phones operates below 6 GHz. Most of the functions are realized in CMOS, except for the RF front-end. This is the electronics between frequency conversion stages of a transceiver and the antenna: it roughly consists of a power amplifier (PA), a low-noise amplifier (LNA), switches and passive filters. The latter ones are made with discrete components such as acoustic wave filters and they are not considered here. In most high-end cell phones today, the PA is based on GaAs while switches use RF SOI (silicon on insulator) technology. The LNA is often put on the CMOS transceiver chip.

The adoption of the FR3 band for mobile user equipment might shake up this partitioning of front-end electronics into different chips and modules. The reason is that GaN has shown to be a semiconductor technology that can yield higher powers and superior PA efficiencies compared to GaAs. Also, low-loss switches that should stand high voltages can benefit from the use of GaN. The GaN devices with the best power and efficiency performance are processed on silicon carbide (SiC) wafers. These wafers are smaller and much more expensive than 300 mm silicon wafers. Imec is developing a GaN technology starting from silicon wafers. At the expense of some performance loss, these GaN-on-Si devices are much cheaper than their GaN-on-SiC counterparts.

The goal of this PhD is to design a GaN-based front-end module (FEM) chip for FR3 that will contain PA, LNA and switches. Such chip will reduce the footprint compared to today’s sub-6 GHz front-end electronics that use various semiconductor technologies. You will compare GaN-on-SiC devices with imec’s GaN-on-Si devices. Most of your circuit design will use GaN-on-SiC from foundries as this technology is more available today than GaN-on-Si. However, circuit design in imec’s GaN-on-Si technology will also be included in this PhD.

Although GaN can yield a high power with a high efficiency, the nonlinear behavior of the PA might degrade the quality of the modulated signals that are transmitted. To compensate for this, you will  use predistortion algorithms that you will fine tune to your GaN devices and circuits, which can exhibit nonlinear distortion with memory.

Your PhD will mainly consist of circuit design, while the experimental characterization and the application of predistortion will take you to the measurement lab. You will also interact with device engineers from imec when designing in imec’s GaN-on-Si technology.