Gallium nitride (GaN) is often regarded more or less as the younger brother of gallium arsenide (GaAs), and to some extent this is an accurate view. Lately, GaN has been receiving a great deal of attention, for the most part because its strong points - handling high frequencies, high power levels and higher operational linearity - are required by technologies that are now, or will soon become, significant economically.
The market for GaN devices can be divided into two main sectors: LEDs, where GaN enables much higher efficiency in green, blue and ultraviolet applications; and communication devices, where power and linearity become paramount. GaN has been used in LEDs for some years, but its application in volume to solving communication problems is really just getting under way. Two of the most important applications are in mobile telephone base stations and in military radar systems. In particular, the military sees a vital role for GaN in X-band radar systems. This is the high-end application area that GaN should excel in.
Although the need for GaN devices is motivating a great deal of exciting research and development, the volume of GaN devices being sold into all markets will, in the next few years, amount to no more than a small fraction of the volume of GaAs devices sold. At the moment, there is no reason to suppose that this volume relationship will change much if one looks ahead five or even ten years. As exciting as GaN is, there are simply many more mobile phone handsets and satellite television receivers that use GaAs chips.
One of the most active areas of research and development in GaN is the matter of native substrates for GaN. At least six different substrate materials are currently being used, at least experimentally (and in most cases commercially), for GaN. The reason for such a diversity of substrates is that there are no GaN ingots, as there are ingots of silicon. If GaN is melted to the required very high temperature in an attempt to grow a crystal of the material, the liquid simply dissociates into gallium and nitrogen.
There are, however, GaN substrates onto which GaN is epitaxially grown. The substrates themselves are grown, and then sliced into wafers. Most GaN-related epitaxial growth processes are limited in thickness (the active layer of GaN on a wafer of any material is generally 1 to 2 microns), but one firm, Kyma, has succeeded in growing GaN substrates that reach 1-centimetre in thickness.
At the moment, the native substrates in use for GaN, for both LED and high-frequency, high-power applications, include bulk GaN, diamond, silicon carbide, sapphire, aluminium nitride, and silicon. Additional substrates, such as AlN for LED substrates, are beginning to look for market share. The governing factors in the selection of the substrate type for a given application include the degree of lattice mismatch between the active GaN layer and the substrate, the thermal conductivity of the substrate, the difference in coefficient of thermal expansion between the substrate and the epitaxial layer, and the overall cost of the substrate.