What are the benefits of using GaN HEMTs in place of silicon LDMOS for Doherty amplifiers?

Simon M. Wood, Manager RF Product Development, Cree

Currently, the prevailing trend in the telecommunications market is the ability to access ever increasing amounts of data, such as video and web browsing, while on the move. There are two primary solutions to this problem at the system level: more complex modulation schemes and/or more signal bandwidth.

Complex modulation schemes are used in all cellular networks that place extraordinary efficiency demands on the transmitter and the power amplifier. However, complex modulations schemes require a high peak to average ratio (PAR) signal, which forces the power amplifier to run at a lower power level and perform less efficiently. Consequently, Doherty amplifiers have become the most common architecture used in these high data demand telecom applications.

Doherty amplifiers are inherently non-linear amplifiers that boost the efficiency of a power amplifier under backed-off power conditions. Using silicon LDMOS transistors, a transmitter power amplifier may only be 35 to 40 percent efficient in a typical LTE system.

Advanced gallium nitride (GaN) transistor technology has been tested and proven over the past five years and is recognized for its ability to provide significant benefits to modern telecom systems. Cree has developed a 50 V process that is capable of delivering a power density of 8 W/mm. This high power density is well matched to high efficiency amplifier techniques, such as the Doherty amplifiers, in which thermal considerations are mitigated.

The typical efficiency of a GaN-based Doherty amplifier is 45 to 50 percent, which effectively reduces power consumption by more than 15 percent.

The Cree GaN process also enables the design of very low output capacitance transistors, which directly impacts the bandwidth that can be realized in Doherty amplifiers. For example, the output impedance of the CGHV27100F is approximately four times higher than a comparable LDMOS transistor with about 10 times less capacitance. Additionally, since a single Cree GaN HEMT transistor can cover multiple bands instantaneously (i.e. enable carrier aggregation), GaN can provide particularly significant improvements to LTE communications system design. Cree’s GaN on silicon carbide (SiC) materials system also allows smaller transistors to be realized in smaller packages, enabling typical form factor reductions of greater than 50 percent. GaN has a higher cut-off frequency (FT) than silicon LDMOS, enabling the employment of harmonic terminations or waveform engineered approaches to further boost efficiency and allowing high power Doherty amplifiers to be designed at even higher frequencies.

Consequently, GaN HEMTs enable more cost competitive, compact, and efficient solutions with broader bandwidth capabilities than existing silicon LDMOS technologies, which proves beneficial when employed in Doherty amplifiers utilized in LTE communications systems.

Mario Bokatius, Product Manager, Freescale

The clear benefit of using GaN based Doherty power amplifiers over the incumbent Si LMDOS is the potential for higher system efficiency, in particular at higher frequency bands within the cellular communication spectrum.

Increasing power amplifier efficiency by five percentage points, for example, could result in a power consumption savings of up to 30 Watts, depending on the configuration of the equipment. 

Reduced power consumption not only helps the environment and operating expenses, but also allows for reduced cooling requirements, which result in smaller and lighter equipment. With significant improvements in reliability and linearizability of GaN power transistors over the last couple years, the main question for the widespread adoption of GaN in cellular base station equipment remains: Are manufactures ready to pay the still higher price tag of GaN compared to Si LDMOS? 
Either way, Freescale is positioned to remain the leading supplier of RF power transistors to the cellular infrastructure market.

This article originally appeared in the November/December print issue. Click here to read the full issue.