advancements of gallium nitride (GaN) HEMTs for next-generation high-speed wireless communication.

Fujitsu Laboratories Ltd today announced the development of a new high-reliability technology for high power gallium nitride (GaN), high electron-mobility transistors (HEMT)

Details of the new technology were presented at the 2007 IEEE MTT-S International Microwave Symposium (IMS)।

As wireless communication data-rates continue to become increasingly faster, power consumption in base stations - such as those for mobile phones – is also increasing GaN HEMTs are promising as devices to realize lower power consumption for high power amplifiers in next-generation high-speed wireless communication systems. However, in order for GaN HEMTs to be used as high power, high voltage-endurance devices, they must maintain high reliability - a long lifespan – as it anticipated that they must withstand harsh usage conditions, including high temperatures and high drain voltages.

Fujitsu's New Technology

In long-term reliability testing, Fujitsu analyzed and discovered that in GaN HEMTs a correlation exists between gate leakage current and reliability (Figure 1). Furthermore, Fujitsu observed that the increase in gate leakage current depends on the quality of crystal and the structure of GaN HEMTs. Fujitsu improved crystal quality and optimized the layer structure to mitigate the electric field in a gallium-nitride HEMT structure with few surface traps by using its proprietary n-type GaN cap layer (surface layer) (Figure 2).

Figure 1: Relationship between gate current and reliability

Figure 2: Structure of gallium-nitride (GaN) हेम्त

Results

Results of high-temperature pinch-off testing of these devices, operating at temperatures above 300°C confirmed long-term high reliability in excess of one million hours at channel temperatures of 200°C (Figure 3). This corresponds to a lifespan of more than 100 years, the most reliable in the world. Pinch-off testing is the most severe reliability test, with high channel resistance and strong electric fields at the drain side of the transistor gate electrode . This is the world's first result in which such a long-term lifespan was confirmed under pinch-off condition with such a harsh testing factor as 50 V drain, as the previous record was at 28V maximum.

Figure 3: High-temperature pinch-off test

Notes

1 Gallium nitride (GaN):

A wide band gap semiconductor that features an advantage in that it features strong resistance to voltage breakdown, compared to conventional semiconductor materials such as silicon (Si) or gallium arsenide (GaAs)।

2 High electron-mobility transistor (HEMT):

A type of field-effect transistor that utilizes the characteristic that, electrons located in the junction of two different bandgap materials move at higher speeds compared to electrons located in ordinary doped semiconductors। Fujitsu pioneered these transistors by developing the world's first HEMT in 1980. Currently, HEMTs are used in a wide range of infrastructure technologies such as satellite broadcasting receivers, mobile phones, global positioning system (GPS) navigation systems, and broadband wireless access systems.

3 WiMAX:

Worldwide Interoperability for Microwave Access, a collective term for wireless interface devices - embedded in notebook PCs or handheld terminals - that meet the standards IEEE802.16 and IEEE802.16e for wireless interfaces. The WiMAX Forum certifies WiMAX-related hardware and networks.