Generally, the RF front-end of smart phones mainly includes power amplifiers (PA), filters, RF switches, low noise amplifiers (LNA), etc. Among them, the value of filters accounts for 53%, followed by power amplifiers (33%) and RF switches (7%).

In terms of value and design difficulty, RF switches are inferior to filters and power amplifiers, but the former are used in a large number of RF front ends. More importantly, Chinese manufacturers have achieved a high market share in the field of RF switches, and have more say than filters and power amplifiers.

The Value of RF Switches

RF switch, also known as microwave switch, is mainly used to control the conversion of microwave signal channels and is widely used in mobile terminals such as smart phones. According to the purpose, RF switches can be divided into mobile communication conduction switches, Wi Fi switches, antenna switches, etc; According to the number of poles and throws, it can also be divided into single pole single throw, single pole double throw, single pole multiple throw and multiple pole multiple throw switches.

The RF switch is composed of two parts: a conductive switch and an antenna tuning switch. Tuner's technical difficulty is higher than that of the switch, because Tuner has extremely high withstand voltage requirements. The on resistance and off capacitance have a great impact on the performance, which puts forward higher design and process requirements for the product.

Affected by the trend of modularization, more than half of switches are integrated in modules. The split switches are mainly used in low-end mobile phones.

In smart phones, RF switches are at the key position of the RF front-end and are essential. Their performance, such as insertion loss, return loss, isolation, harmonic suppression and power capacity, has an important impact on the RF front-end link. The main function of RF switch is to switch signals in different directions (receiving or transmitting) and different frequencies through control logic, so as to share the antenna and save the cost of terminal products.

In general, every time a smart phone supports more than one frequency band, its RF chip will add a receiving channel. In order to reduce chip area and cost, a receiving channel will often support more than one frequency band. This requires increasing the number of switches in the RF front-end to meet the requirements for receiving and transmitting signals in different frequency bands.

The number of frequency bands supported by 5G mobile phones has increased by 50 on the basis of 66 4G LTEs. Since 5G is all downward compatible, the total number of frequency bands supported by 2G/3G/4G/5G networks in the world exceeds 110. Therefore, compared with 4G mobile phones, the number of radio frequency switches in 5G will increase by 50% from 10 to 15. Moreover, with the development of technology and application requirements, this number will further increase in the future.

In addition, with the decrease of package size, RF front-end presents a trend of modularization. In the future, RF switch performance and stand-alone value are expected to be further improved.

Process technology

At the manufacturing process level, RF switches are mainly divided into two types: electromechanical and solid-state RF switches. Solid state switches are manufactured using semiconductor technologies, such as silicon or PIN diodes, field effect transistors (FETs), and hybrid technologies (i.e., a combination of PIN diodes and FETs), and are constructed using silicon based substrates.

RF switch manufacturing technology mainly includes CMOS, GaAs, RF SOI and RF MEMS, among which RF SOI is the mainstream technology, CMOS technology occupies a small share, and GaAs is facing elimination. With the continuous innovation of RF switch product design, RF MEMS technology can better meet the needs of future RF system multi-mode and multi frequency communication, and is becoming the focus of the industry layout.

RF SOI is a silicon based semiconductor process material with a unique silicon/insulating layer/silicon three-layer structure. It realizes the full dielectric isolation of devices and substrates through insulating buried layers (usually SiO2). It has many advantages in device performance, mainly including: reducing parasitic capacitance and improving operating speed. Compared with bulk silicon materials, the operating speed of SOI is increased by 20% - 35%; With lower power consumption, the power consumption of SOI devices can be reduced by 35% - 70% due to the reduction of parasitic capacitance and leakage; The latch effect is eliminated; The pulse current interference of the substrate is suppressed, and the occurrence of errors is reduced; Compatible with the existing CMOS silicon process, it can reduce 13% - 20% of the process.

With the continuous improvement of RF SOI process, the performance of this RF switch is steadily improving.

The RF SOI process can meet the current frequency band and performance requirements, but it also meets some new challenges. RF switch itself contains field effect transistor (FET), which will be affected by unnecessary channel resistance and capacitance. Generally, 10 to 14 FETs are stacked in RF switch. As the number of FETs increases, devices may encounter a series of problems caused by insertion loss and resistance.

In addition, parasitic capacitance is also a problem. Skyworks believes that 30% or more of the parasitic capacitance in RF switches comes from the interconnects in devices, including metal layers or bonding lines. These are also problems in the RF SOI process. At present, the mainstream manufacturing process of RF switches is based on the 180nm and 130nm processes of 8-inch wafers. Many interconnection layers are based on aluminum, which is cheap, but has high parasitic capacitance. Therefore, copper is used in some specific layers of RF switches. Copper is a better conductor and has less resistance than aluminum.

In recent years, RF SOI wafer foundries have migrated from 8-inch wafers to 12 inch wafers, and process nodes have also migrated from 130 nm to 45 nm. Typically, 12 inch fabs use only copper interconnects, which allows RF switch manufacturers to reduce capacitance. However, the 12 inch wafer increases the manufacturing cost, which forms a contradiction. That is, cost sensitive mobile phone OEMs need RF switches to keep prices low, but RF switch manufacturers and wafer foundries want to keep profits.

Compared with RF SOI, some excellent features of RF MEMS are highlighted, which broadens the development path of RF switches.

There are many kinds of RF MEMS switches, which can be driven by different mechanisms. Because of its low power consumption and small size, electrostatic drive is commonly used in RF MEMS switch design. MEMS switches can also be turned on or off using inertial, electromagnetic, electro thermal, or piezoelectric forces.

RF MEMS can provide very low Ron (on resistance), which also means lower insertion loss. However, RF MEMS has not yet achieved mass production. Considering the risks, mainstream OEMs are hard to choose new technologies and small suppliers, unless compared with RF SOI, RF MEMS switches can provide attractive prices, and their reliability and supply stability are sufficiently guaranteed.

The rise of Chinese local manufacturers

In 2020, the global RF switch market will be more than US $2 billion, and the Chinese market will account for at least 50%. In addition to the traditional split RF switch, the development trend of RF front-end modularization makes more and more switches integrated into the module, especially the mobile phone PA module. In the future, more and more switches will be integrated into the PA module. This is the development direction of international large factories, and Chinese local manufacturers are also rapidly following up.

At present, internationally renowned RF switch suppliers mainly include Skyworks, Broadcom, Murata, Zhuoshengwei, Weir, etc.

Due to the relatively low technical difficulty and high localization rate of RF switches, Zhuoshengwei has occupied about 15% of the global market share of RF switches (including switches and Tuner, switches in separate types and modules), and manufacturers such as Ziguang Zhanrui, Angruiwei, Aiwei, Feixiang Technology, Weijie Chuangxin, Weier Shares, and Jamaicin also have Switch production capacity, with an overall localization rate of about 20%. However, few companies take switches as their main products, such as Zhuoshengwei, Jamaican Xinxin, Weir Co., Ltd. (switches/LNA are the main products of the company's RF product line) and Aiwei Electronics (switches/LNA are the main products of the company's RF product line). Most other manufacturers take switches as their secondary products.

The strength of Zhuoshengwei is an important reason. Its gross profit margin of RF switch exceeds 50%, and its annual sales volume exceeds 5 billion yuan, making it an absolute industry leader.

Driven by the market demand, the number of local RF switch manufacturers in China has been growing, and now there are more than 15. However, in addition to Zhuoshengwei, there is no company whose annual sales of discrete switches exceed 100 million yuan. The OEM manufacturers represented by Xiaomi, among the local RF switch enterprises in China that supply for them, except Zhuoshengwei, all others have a small share. At present, the strategy of big customers to regard Zhuoshengwei as the first supplier will not change, and the rest will be shared by more than 10 companies.

Zhuoshengwei's RF switch performance began to grow rapidly in 2017. Before 2015, RDA, Feixiang Technology and Weijie Chuangxin had already shipped RF switch products. Until today, these companies' RF switch products are still being promoted in the market, but the growth of the separate RF switch industry is weak.

To sum up, in the field of RF switches, Chinese enterprises have a high market share. On this basis, local enterprises should make persistent efforts to increase investment in chips and modules with higher technology content and profits in RF front-end, in order to achieve higher level of development.