In recent years, the popularity of smart phones worldwide, the LTE network has been commercially available, affected by this, the use of global mobile data continues to soar. According to GSA mobile industry classification data, as of March 2015, the number of LTE users worldwide increased by 151% over the previous year to 635 million. This growth momentum will continue, and by 2020, the number of LTE users will reach 2 billion 500 million.
Mobile network operators face many challenges. On the one hand, it is necessary to expand the capacity to support incremental users. On the other hand, it is necessary to minimize network interruption and reduce costs. In the long run, the 5G network is expected to significantly increase capacity and data rate. However, the 5G specification is still in the definition stage, and it is impossible to deploy at least 5 years. In addition, 5G may involve a drastic change in network architecture.
In order to meet the urgent needs of substantially increasing capacity before the arrival of 5G, operators have begun to do their best to expand the capacity of the 4G network without redesigning the infrastructure architecture. Their focus is on technologies that enable them to get more capacity from existing LTE spectrum allocation, so as to reduce the need to purchase high additional spectrum.
Operators are targeting a variety of key capacities and performance upgrading technologies. Short term plan is centered on carrier aggregation (CA), which is a characteristic of LTEAdvanced standard. Medium-term enhancement technologies include a variety of enhanced technologies called 4.5G, 4G+, or pre-5G, including high order (up to 64X) multiuser multiple input and multiple output (MU-MIMO), higher order modulation, and the use of non licensed 5GHz spectrum.
These short-term and medium-term expansion technologies and the final 5G network will require a base station power amplifier (PA) that provides higher power output and efficiency and supports broadband operation and high frequency bands.
The prospect of GaN on SiC
Historically, base station power amplifiers mainly use silicon based lateral diffusion metal oxide semiconductor (LDMOS) technology. However, more and more stringent requirements have gradually exposed the limitations of LDMOS, which has led to many suppliers turning to gallium nitride (GaN) in high power base station power amplifier technology. For example, the power output requirements are increased every year; the requirements for base station power amplifiers increase from 30W-40W a year ago to this year's 60W, and the requirements for a new generation of base stations may reach 100W or more. Current and planned expansion requirements also require broadband power amplifiers that support higher frequencies. LDMOS has bandwidth limitations even at lower RF frequencies, and the bandwidth of LDMOS power amplifiers will decrease significantly with the increase of frequency. Although LDMOS is valid only in the frequency range of no more than 3.5GHz, GaN power amplifier has been able to handle millimeter wave frequencies of 50GHz or above. In addition, GaN power amplifiers support higher bandwidth even at higher frequencies.
The two main GaN technologies currently exist are silicon carbide GaN (SiC) and silicon GaN (Si). The advantage of GaN on Si is that the substrate cost is low, and it can be produced in Si plant and has the advantage of scale economy. But GaN on SiC supports much higher power density and supports higher power output. This is because SiC has better thermal conductivity: about three times higher than Si. The power density of GaN on SiC is about 5W/mm, which is about 7 times the power density of LDMOS. Therefore, the GaN on SiC power amplifier can provide about two times the power output in the same size. As a result, GaN on SiC has become the preferred technology for high power RF applications.
The advantages of GaN on SiC power amplifier are directly related to the three major concerns of operators, namely the so-called three C problem: capacity, coverage and cost. As I will describe in this article, higher output power can significantly increase capacity while maintaining cellular coverage. The GaN on SiC power amplifier has higher efficiency, which can reduce the huge electricity charges of operators and reduce the heat dissipation problem. To explore these advantages in more detail, I will discuss the possible role of GaN on SiC in the various stages of the evolution of the wireless network, first from the carrier aggregation, then 4.5G, and finally 5G.