Toshiba Develops New Gallium Nitride Power Field Effect Transistor

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Toshiba has announced development of a gallium nitride power field effect transistor that far surpasses the operating performance of gallium arsenide FET widely used in microwave solid-state amplifiers for radar and satellite microwave communications in the 8GHz to 12GHz X-band frequency range. The new transistor achieves an output power of 81.3W at 9.5GHz, the highest level of performance yet reported at this frequency. Toshiba realized this breakthrough performance enhancement by optimizing the epitaxial layer and chip structures for X-band operation. The result is a GaN power FET with six times the power density of a GaAs FET and the world's highest output power at the 9.5GHz frequency level.

Toshiba has established manufacturing technology for GaN power FET in the range of 50W and started to release samples. The company expects to start mass production within the next six months.

Ever increasing communications flows are driving demand for higher output power in the amplifying devices used in radar and satellite microwave communications. Until now, Toshiba has met this demand with GaAs-based FETs offering 90W output power at 6GHz frequency and 30W at 14GHz. However, balancing heat dissipation and performance characteristics in high frequencies is a critical issue with GaAs, and is reaching to the point that the material is fast approaching the upper limits.

GaN shows great promise for application in high output power amplifiers that support higher frequencies above the microwave frequency band, since it offers higher saturation electron velocity, higher dielectric breakdown voltage and a higher operating temperature range than GaAs. Toshiba initially directed its efforts at developing a GaN power FET for the 4GHz to 8GHz (C-band), and last year announced a GaN power FET with a power output of 174W in the 6GHz band. The company has now built on its success in that area with structural optimization that has achieved a device supporting higher X-band frequencies and that achieves the highest power output yet obtained by a GaN power FET operating at 9.5GHz. This progress will allow for integration of devices for solid-state amplifiers and allow for their downsizing, even while they deliver higher output power.

Toshiba is confident that this breakthrough opens the way even higher frequency levels in the 12GHz to 18GHz (Ku-band). The company will continue its development activities toward this.

Key features;
1. Epitaxial layer structure - The FET adopts a High Electron Mobility Transistor (HEMT) structure. By optimizing conditions of the composition and the thickness of the AlGaN and GaN layers, Toshiba has achieved outstanding performance.
2. Chip structure - Working with the epitaxial layer structure, Toshiba processed and optimized the FET unit structure, including gate length and the distance between the source and drain electrodes. This not only assures heat dissipation but also high performance in the X-band frequencies (9.5GHz).
3. Process and package - Toshiba's heat treatment technology achieves low contact resistance at the source and drain electrodes, allowing maximization of the GaN material characteristics. In order to produce high performance at X-band, the FET requires a gate electrode under 0.5 micron meter. As a high voltage is applied, suppression of current leakage at the gate electrode is essential for achieving high level performance. A unique gate electrode structure and overcoat process contributes to suppressing gate leakage to 1/30 that of today's conventional technology.
4. Chip uniformity and output merge - The conventional approach to boosting output power in GaN devices has been to fabricate a large chip with high power capacity in a large package. Such devices ran very hot, which could cause components to deteriorate and also damage the amplifiers into which the devices were integrated. GaN is a difficult material to work with and achieve uniformity of chip characteristics, and tends to suffer power losses when combining the power of multiple chips in a package. Toshiba's unique process technology achieves chip uniformity across the wafer, while the company's advanced power management reduces power dissipation inside the package. These approaches successfully disperse heat and reduce potential component deterioration, and support achievement of a high power output of over 80W in the X-band.

5. Stepper lithography - While electron beam exposure technology has been commonly used in a lithography process for GaN power FET as the C-band and higher frequencies, Toshiba has adopted stepper exposure that is better suited to mass production for X-band FET, which require a gate length of less than 0.5 microns.

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