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Exploring the gallium nitride technology

Posted: 16 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:gallium nitride? eGaN FETs? Envelope Tracking?

It has been three years since the commercialisation of gallium nitride (GaN) devices as MOSFET replacements in a commercial DC-DC application. It has since set a new path for GaN early adopters. They are the experimenters, universities' researchers, semiconductor industry experts and customers at large. Since that time, there has been major interest and rapid progress in the development and continuous commercialisation of this new technology.

Capturing GaN performance has unleashed a new world of emerging applications with successes that no one would deny. With the emergence of GaN devices, coupled with now attainable applications previously not achievable with MOSFET-based FETs, a favourable stage has been set for GaN-device developers to release emerging application potential largely unimagined and untapped.

No doubt gallium nitride has long been recognised as a promising material for power transistors. In 2013, several GaN-enabled applications have surfaced such as wireless power transmission, RF DC-DC "Envelope Tracking," radiation hard power supplies, and high-energy pulsed lasers.

GaN structure
eGaN FETs start with a silicon substrate. Since the lattice structures of GaN and silicon are not compatible, a thin layer of aluminium nitride is deposited as a seed layer for the GaN. The GaN is then grown and a thin layer of aluminium gallium nitride is added. This causes a strain in the crystal just under the AlGaN. Because GaN is piezoelectric, this strain creates a small electric field where electrons pool to form a two dimensional electron gas or 2DEG. The gate area is grown in a very special way such that it depletes the underlying 2DEG, causing the device to be normally OFF C or enhancement mode.

In an enhancement-mode device, which eGaN FETs are, a positive bias on the gate relative to the source turns the device on with a bidirectional channel, and shorting the gate to the source turns it off. This is a lateral device with a small gate area and small die area for its RDS(ON) as compared with a silicon MOSFET thereby enabling reductions to gate capacitance, output capacitance and overall switching speed.

LGA construction
For eGaN FETs, solder terminations are added to form a land grid array. This configuration has many advantages. It has both glass passivation and plastic dielectric to isolate the active area from the environment giving excellent reliability results with little wasted space. It also reduces the internal metal resistance and minimises stray inductance for optimal high frequency performance.

Emerging applications
Below is a partial list of the applications already using eGaN FETs around the world. In each of these applications, the eGaN FETs made the system perform in a way that power MOSFETs couldn't. The key characteristic that these customers would take most advantage of is the eGaN FET's ability to operate at higher frequencies or to switch with much less power loss.
???RF DC-DC "Envelope Tracking"
???Wireless Power Transmission
???Rad Hard
???Power Over Ethernet
???RF Transmission
???Network and Server Power Supplies
???Point-of-load Modules
???Solar Micro-inverters
???Energy-efficient Lighting
???Class D Audio

According to Yole Development's 2011 research report, the worldwide GaN market projection is for 250% year-on-year growth from 2011 to 2015. For the silicon carbide (SiC) market, this report projected 35% year-on-year growth during the same period

There is an estimated over USD 15B wireless power transmission market by 2020 where eGaN FETs enable higher efficiency and operation at safer frequencies. Wireless communications using LTE 4G infrastructure is forecasted to grow to USD 24B in 2013 with envelope tracking enabling a doubling of base station efficiency.

There is another $100M market for Rad Hard MOSFETs that can be replaced by eGaN FETs as they can withstand more than 10 x radiation exposure than MOSFETs and enable higher system efficiency.

Capturing envelope tracking application potentials
The concept of envelope tracking (ET) for radio frequency (RF) amplifiers is not new. But with the ever increasing need for improved cell phone battery life, better base-station energy efficiency, and more output power from very costly RF transmitters, the need for improving the RF Power Amplifier (PA) system efficiency through ET has become an intense topic of research and development. There are many papers on the basics and advantages of envelope tracking. The key to their ability to improve efficiency lies in the power amplifier's peak to average power requirements (PAPR).

It is possible to achieve peak PA efficiencies as high as 65% with a fixed supply, but given PAPRs as high as 10, the average efficiency is likely to be lower than 25%. Through modulation of the PA supply voltage, this can be improved to over 50%essentially doubling the efficiency and reducing PA losses by two-thirds! This will not only reduce power consumption, but also lower the cost of operation, reduce cooling requirements and shrink the size.

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