Design Con 2015

Gallium Nitride (GaN) technology overview

Alex Lidow, Johan Strydom, Michael de Rooij, Yanping Ma -October 03, 2012


Editors note: This book, "GaN Transistors for Efficient Power Conversion", published by Power Conversion Publications helped me learn a great deal more about the benefits of the GaN process as well as the history of the silicon MOSFET. The book is a complete tutorial for designers who use MOSFETs in their power designs and want to learn about this newer technology begun almost a decade ago and now greatly refined by Lidow, an industry veteran in the power MOSFET arena.

Here is the first of a number of chapters to come on EDN: I hope you enjoy them as much as I have enjoyed the book. Chapter five, "Buck Converters" and Chapter six, "Isolated full bridge converters", has been also published on EDN -- Steve Taranovich

CHAPTER 1:

Gallium Nitride (GaN) Technology Overview

Silicon Power MOSFETs from 1976-2010

For over three decades power management efficiency and cost showed steady improvement as innovations in power in power MOSFET (metal oxide silicon field effect transistor) structures, technology, and circuit topologies paced the growing need for electrical power in our daily lives. In the new millennium, however, the rate of improvement slowed as the silicon power MOSFET asymptotically approached its theoretical bounds.

Power MOSFETs first started appearing in 1976 as alternatives to bipolar transistors. These majority carrier devices were faster, more rugged, and had higher current gain than their minority-carrier counterparts. As a result, switching power conversion became a commercial reality. AC-DC switching power supplies for early desktop computers were among the earliest volume consumers of power MOSFETs, followed by variable speed motor drives, fluorescent lights, DC-DC converters, and thousands of other applications that populate our daily lives.

One of the earliest power MOSFETs was the IRF100 from International Rectifier Corporation, introduced in November 1978. It boasted a 100 V drain-source breakdown voltage BVDSS and a 0.1 Ω on-resistance; the benchmark of the era. With a die size over 40 mm2, and with a $34 price tag, this product was not destined to broadly replace the venerable bipolar transistor immediately.

Many generations of power MOSFETs have been developed by several manufacturers over the years. Benchmarks were set, and fell, every year or so for 30 plus years. As of the date of this writing, the 100 V benchmark is arguably held by Infineon with the IPB025N10N3G. In comparison with the IRF100’s resistivity figure of merit of 4 Ω – mm2, the IPB025N10N3G has figure of merit of less than 0.1 Ω – mm2 [1]. That is almost at the theoretical limit for a silicon device [2].

There are still improvements to be made. For example, superjunction devices and IGBTs have achieved conductivity improvements beyond the theoretical limits of a simple vertical majority carrier MOSFET. These innovations may still continue for quite some time and will certainly be able to leverage the low cost structure of the power MOSFET and the well-educated base of designers who, after many years, have learned to squeeze every ounce of performance out of their power conversions circuits and systems.

The GaN Journey Begins

HEMT (High Electron Mobility Transistor) gallium nitride (GaN) transistors first started appearing in about 2004 with depletion-mode RF transistors made by Eudyna Corporation in Japan. Using GaN on silicon carbide (SiC) substrates, Eudyna successfully brought transistors into production designed for the RF market [3].

The HEMT structure was based on the phenomenon first described in 1975 by T. Mimura et al. [4] and in 1994 by M. A. Khan et al. [5], which demonstrated unusually high electron mobility described as a two-dimensional electron gas (2DEG) near the interface between an AlGaN and GaN heterostructure interface. Adapting this phenomenon to gallium nitride grown on silicon carbide, Eudyna was able to produce benchmark power gain in the multi-gigahertz frequency range. In 2005, Nitronex Corporation introduced the first depletion-mode radio frequency (RF) HEMT transistor made with GaN grown on silicon wafers using their SIGANTIC® technology [6]. 

GaN RF transistors have continued to make inroads in RF applications as several other companies have entered in the market. Acceptance outside this market, however, has been limited by device cost as well as the inconvenience of depletion-mode operation.

In June 2009 Efficient Power Conversion Corporation (EPC) introduced the first enhancement-mode gallium nitride on silicon (eGaN) field effect transistor (FET) designed specifically as power MOSFET replacements. These products were to be produced in high volume at low cost using standard silicon manufacturing technology and facilities.

The basic requirements for power semiconductors are efficiency, reliability, controllability, and cost effectiveness. Without these attributes, a new device structure would have no chance of economic viability. There have been many new structures and materials considered as a successor to silicon; some have been economic successes, others have seen limited or niche acceptance. In the next section we will look at the comparison between silicon, silicon carbide, and gallium nitride as candidates for the dominant platform for next-generation power transistors.


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