Dialogue with Roger Hall: GaN technology will continue to receive attention

Roger Hall -- General Manager, Qorvo High Performance Solutions

It has become an industry consensus that gallium nitride (GaN) technology is becoming increasingly popular in RF and power applications.

GaN devices are divided into RF devices and power electronic devices . RF device products include PA, LNA, switch, MMIC, etc., which are aimed at base station satellite, radar and other markets; power electronic device products include SBD, normally closed FET, normally open FET, cascade FET and other products, which are aimed at wireless charging, power switch, envelope tracking, inverter, converter and other markets. According to the process, GaN devices are divided into two categories : HEMT, HBT RF process and SBD, Power FET power electronic device process .

Currently, there are three main processes in the RF market: GaAs , Si-based LDMOS , and GaN processes . The disadvantage of GaAs devices is that the device power is low, usually less than 50W. The disadvantage of LDMOS devices is that there is a limit to the operating frequency, and the maximum effective frequency is below 3GHz. GaN makes up for the defects of the two traditional technologies, GaAs and Si-based LDMOS, while reflecting the high-frequency performance of GaAs, it combines the power handling capability of Si-based LDMOS.

In the RF PA market, the bandwidth of LDMOS PA will decrease significantly with the increase of frequency and is only effective within a frequency range of no more than about 3.5GHz. The frequency of GaN devices using 0.25-micron process can reach 4 times that of LDMOS PA, the bandwidth can be increased by 20%, the power density can reach 6~8 W/mm (LDMOS is 1~2W/mm), and the trouble-free working time can reach 1 million hours. It is more durable and has obvious comprehensive performance advantages.

Traditionally, LDMOS technology has dominated the wireless infrastructure space, but is this changing? The answer to this question is a resounding yes.

Since 5G requires massive MIMO and Sub-6GHz deployment, the use of millimeter wave (mmWave) spectrum will be required, which will face a series of challenges. GaN technology can play an important role in sub-6GHz 5G applications, helping to achieve goals such as higher data rates. High output power, linearity and power consumption requirements are driving the PA deployed by base stations and network OEMs to switch from LDMOS technology to GaN. GaN provides multiple advantages for 5G sub-6GHz massive MIMO base station applications:

• GaN performs well at frequencies of 3.5 GHz and above, while LDMOS is challenged at these high frequencies.

• GaN has high breakdown voltage, high current density, high transition frequency, low on-resistance and low parasitic capacitance. These characteristics can be converted into high output power, wide bandwidth and high efficiency.

• The GaN in Doherty PA configuration achieves an average efficiency of 50%~60% at 100W output power, significantly reducing transmit power consumption.

• The high power density of GaN PAs enables small form factors that require less printed circuit board (PCB) space.

• Using GaN in a Doherty PA configuration allows the use of quad flat no-lead (QFN) plastic packages instead of expensive ceramic packages.

• GaN's efficiency at high frequencies and wide bandwidths means massive MIMO systems can be more compact. GaN can operate reliably at higher operating temperatures, which means it can use smaller heat sinks. This allows for more compact form factors.

Building the RF front end (RFFE) to support these new sub-6GHz 5G applications will be a challenge. The RFFE is critical to the system's power output, selectivity, and power consumption. Complexity and higher frequency ranges drive the need for RFFE integration, size reduction, lower power consumption, high output power, wider bandwidth, improved linearity, and increased receiver sensitivity. In addition, the coupling requirements between the transceiver, RFFE, and antenna are tighter.

Some of the goals of 5G sub-6GHz RFFE, and how GaN PA can help achieve them, include:

• Higher frequencies and higher bandwidths : 5G uses higher frequencies than 4G and requires wider component carrier bandwidths (up to 100 MHz). GaN-on-Silicon-carbide (GaN-on-SiC) Doherty PAs achieve wider bandwidths and higher power-added efficiency (PAE) than LDMOS at these frequencies. The higher efficiency, higher output impedance, and lower parasitic capacitance of GaN devices allow for easier broadband matching and scaling to very high output powers.

• High power efficiency at higher data rates : GaN has soft compression properties, making it easier to pre-distort and linearize. Therefore, it is easier to use in digital pre-distortion (DPD) high-efficiency applications. GaN is able to operate on multiple cellular bands, helping network operators deploy carrier aggregation to increase spectrum and create larger data pipes to increase network capacity.

• Minimizing system power consumption : How do we meet the high data rate requirements of 5G? We need more infrastructure, such as data centers, servers, and small cells. This means an overall increase in network power consumption, driving the need for system efficiency and overall power savings, which seems difficult. Again, GaN can provide a solution by providing high output power as well as improving base station efficiency.

The above content is from: Semiconductor Industry Observation Translated from "Microwaves & RF"

The figure above shows a block diagram of an exemplary sub-6GHz RFFE that uses a Doherty PA design to achieve high efficiency. In terms of new products, in December 2018, Qorvo released the industry's first 28GHz GaN front-end module, the QPF4001, expanding its 5G business scope. After base station equipment manufacturers get involved in 5G, this new FEM can help them reduce overall system costs.

The 28 GHz band is the preferred band for early 5G-based fixed wireless access (FWA) deployments, enabling operators to meet 5G requirements for speed, latency, reliability, and capacity. The QPF4001 FEM integrates a high-linearity LNA, a low-loss transmit/receive switch, and a high-gain, high-efficiency multi-stage PA in a single MMIC. The compact 5x4 mm air-cavity surface-mount package is optimized for phased array elements spaced at 28 GHz in 5G base station architectures.

Qorvo's new GaN FEM makes millimeter wave phased array systems smaller, more powerful and more efficient, and can direct signals to areas where more bandwidth is needed. This application uses Qorvo's high-efficiency 0.15 micron GaN-on-SiC technology, allowing users to more efficiently achieve higher EIRP levels while minimizing array size and power consumption, thereby reducing system costs.

*Qorvo currently has a broad range of GaN process technologies for manufacturing products for 5G applications