Is GaN the New RF "Superman"?

Gallium Nitride (GaN) semiconductor technology offers designers unique advantages compared to both Gallium Arsenide and Silicon for RF power applications from HF to 20 GHz. Design engineers see its potential and are actively investigating commercial and defense GaN-based solutions. GaN has also seen rapid adoption as die, in packages and in modules; custom designs are supported by advanced foundry services including TriQuint’s 0.25 µm process.

GaN has in a sense achieved "celebrity" status as a new star on the global semiconductor stage. But like any new star, its potential is only somewhat realized. More will be learned about how to best leverage its inherent benefits, extend device lifetimes and lower costs. As a leading Gallium Arsenide manufacturer as well as a Gallium Nitride pioneer, TriQuint is uniquely qualified to guide designers in choosing the device or process technology that best matches their needs.

GaN on Silicon Carbide has inherent qualities that have helped earn it the spotlight it enjoys today. GaN offers the high frequency advantages of a typical compound RF semiconductor, but with significantly better thermal conductivity and tensile strength compared to other process technologies. Gallium Nitride’s high operating voltage makes it ideal for high power RF applications.

Silicon Carbide has about twice the thermal conductivity of Silicon and roughly five times that of GaAs. An RF designer new to GaN might wonder why this does not always translate into junction-to-case thermal resistance improvements of the same magnitude. This is because the semiconductor substrate is only one portion (although an important part) of several materials that, when taken together, create the total thermal resistance of the device. Primarily, GaN’s improved thermal conductance enables manufacturers to increase the amount of RF output power per square millimeter of semiconductor area, thus offsetting costs while enabling smaller device form factors.

In discrete form, GaN products provide a solution for a very broad frequency range, from HF to X-band and in MMIC form this is extended to well beyond Ku-band. Further, GaN on SiC devices operate well at high voltage because of the material’s inherent superior breakdown characteristics. While LDMOS has good breakdown characteristics with devices available at 28 and 48 V, products are limited to about 4 GHz in high power applications. GaAs can operate at impressive frequencies (well beyond K-band), but the technology presents greater challenges for high voltage, rugged operation and commercially available products of these types have been limited. However, TriQuint has developed GaAs-based high-power devices that offer excellent linearity and power handling in commercial wireless frequency ranges.

GaN on SiC combines high operating voltage, increased power density, and low parasitic capacitance resulting in higher device impedances per watt of RF output power. One of the primary challenges of RF amplifier design is the need to create a matching network to keep the RF power device operating optimally over a given frequency band. The higher impedance of GaN compared to its LDMOS and GaAs cousins make this task much easier. Manufacturers don’t need to employ MOSCAPS or GaAsCaps to transform the impedance to a higher level at frequencies below 6 GHz or so. Depending on the RF output level of the device, internal matching may be required at lower frequencies for large devices. Internal matching has consequences: it can double the amount of semiconductor material needed, thus increasing cost and size, while significantly increasing wire bond complexity and transforming the transistor into a narrowband device. GaN has therefore become the leading technology for emerging wideband applications.

Thanks to the superior impedance of GaN, devices with no internal matching can be used up to roughly 6 GHz at power levels around 30 W compared to only about 1.5 GHz for similar LDMOS devices. This benefit is extremely useful to RF designers because it means a single device at a given RF power level can be used across an extremely broad frequency range. This is not the case with internally matched devices, because the matching network limits the device to narrowband operation. Although there is a broad range of internally matched devices tuned for the most common commercial telecommunication frequency bands, designers who work at other bands—prior to GaN—have had limited or no standard product RF power transistor options.

Is a GaN device the best choice for your application? Like any RF design question, the answer depends on a number of variables. However, as evidenced by the growing number of GaN-based devices on the market, their power, bandwidth and efficiency, the future is bright. As this technology evolves, GaN has great potential for performance and cost-effectiveness. As a leader of the DARPA Phase III GaN Wide Bandgap Semiconductor program and DARPA’s new GaN high-power ‘NEXT’ development initiative, TriQuint’s wide range of die-level MMIC and packaged products and pioneering high frequency GaN foundry process offer solution options for high power, high frequency application needs.

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