Gallium-nitride (GaN) substrates hold great promise for high-power transistors at microwave frequencies. While numerous devices are available for applications at typically around 5 GHz and below (WiMAX bands), the surface has barely been scratched in terms of the real high-power/frequency capabilities of GaN. The latest transistor development from Toshiba Corp. provides a closer look at what the future might hold. The firm’s X-band (8 to 12 GHz) GaN field-effect transistor (FET) is capable of81.3 W output power at 9.5 GHz—the highest output power yet reported for such a device at this frequency (see figure).

Earlier this year, Toshiba offered samples and small quantities of 50-W GaN devices. The company has optimized the epitaxial layer and device structure of this transistor for the best performance at X-band frequencies. It also claims that the device has about six times the power density of an equivalent-frequency GaAs FET. Details of the device were announced at the IEEE Compound Semiconductor IC Symposium (CSISC), held this past November 12-15 in San Antonio, TX.

Prior to this, the firm offered GaAs FETs capable of 90 W output power at 6 GHz and 30 W output power at 14 GHz for both radar and satellite-communications (satcom) amplifiers. But GaN offers higher saturation electron velocity, higher dielectric breakdown voltage, and a higher operating temperature range than GaAs. Toshiba had focused research efforts on C-band (4 to 8 GHz) devices and had achieved as much as 174 W output power at 6 GHz (see Microwaves & RF, April 2006). By performing work on structural optimization and enhancing the epitaxy for higher-frequency operation, the success at C-band was translated to higher Xband frequencies.

Higher-power transistors translate into less output stages in an amplifier for a given output-power level, less combining stages (and lost power from power-combining losses), less size, and less complexity (resulting in higher inherent reliability). Of course, heat generation is still an issue, depending upon the efficiency of the transistor. Judicious thermal design must be used with transistors of this power rating—especially when one device is capable of such output power.

The new X-band transistor features a high-electron-mobility-transistor (HEMT) structure for high power at high frequencies. The firm was able to achieve good performance by optimizing the composition and the thickness of the AlGaN and GaN layers and by carefully selecting the device gate length (subhalf micron) and the distance between the source and drain electrodes. Heat treatment technology achieves low contact resistance at the source and drain electrodes, allowing maximization of the GaN material characteristics. The firm employs a unique gate electrode structure and overcoat process to minimize gate leakage, backed by stepper lithographic techniques for effectively defining device features in a mass-production environment.

So far, the firm has succeeded in producing devices with uniform characteristics across a GaN wafer with well-behaved thermal dissipation. The heat management is also controlled by the use of low-loss packages. The company is exploring even higher power levels at higher frequencies for GaN.

Toshiba America Electronic Components, Inc., 19900 MacArthur Blvd., Suite 400, Irvine, CA 92612; (949) 6232900, FAX: (949) 474-1330,