Fujitsu Develops Gallium-Nitride HEMT For Power Supply

June 24, 2009 by Jeff Shepard

Fujitsu Laboratories Ltd. announced the development of a new structure for gallium-nitride high electron-mobility transistors (GaN)(HEMT) that can minimize power loss in power supplies, thus enabling reduced power consumption of electronic equipment such as IT hardware and home electronics. The new technology blocks the flow of current from power supplies in stand-by mode and produces high-density current when turned on (on-state current), and has the potential to cut power consumption of electronic equipment by one-third. If applied to data centers, Fujitsu states that its new GaN HEMT would be able to reduce total power consumption by 12%.

GaN HEMTs are one type of transistor featuring high breakdown-voltage that has attracted much attention in recent years. GaN HEMTs have less than one-fifth (1/5) the on-state loss of silicon transistors and have excellent high-speed characteristics, so that switching losses are less than 1% of those of silicon transistors.

According to the company, conventional GaN HEMTs have required a negative gate voltage to be applied when in stand-by mode. In 2008, Fujitsu Laboratories developed a new 3-layer cap structure for GaN HEMTs – by sandwiching an aluminum-nitride (AlN) layer between n-type GaN layers – which can suppress the current when in stand-by. The new GaN HEMT structure was originally developed for wireless transmission amplifiers; however, because the voltage that was applied at the gate electrode in order to switch between on and off states ("turn-on voltage") was in the range of 0.5V, it could not be used for power supplies, which need to apply +2 to +4V in order to apply hundreds of volts at the drain. In addition, power supplies effectively need to have an on-state current density of at least 600mA/mm.

In addition to its GaN HEMT with the three-layer (3-layer) cap structure, Fujitsu made the following advances in GaN HEMT technology. A technology was developed for precise removal of the cap layers and a part of the AlGaN layer only below the gate electrode. By leaving a thin n-type AlGaN layer on the GaN carrier layer, while suppressing damage to the GaN layer, on-state voltage can be increased beyond +2 V while preserving the total interruption in stand-by, enabling high-speed performance when turned on. Furthermore, a gate structure was developed that uses an insulated gate structure with an atomic layer-controlled oxide film having atom-level flatness. Because this suppresses gate leak current in which travelling electrons flow to the gate electrode when turned on, a positive voltage can be applied to the gate electrode, resulting in high on-state current density.

The on-state voltage of the new transistor reaches +3V, which can easily be applied to power supplies while achieving a current density of 829mA/mm - double that of the transistor design on which the new transistor is based – results in high current values. Among transistors that can achieve an on-state voltage of at least +2V and completely interrupt current when off, Fujitsu’s new transistor features what is described as the world’s highest on-state current density, making it the first GaN HEMT in the world that has the characteristics required for power supply. With the new transistor in power supplies, power loss can be reduced to one-third that of power supplies based on conventional silicon transistors.

Additionally, the high-frequency performance of Fujitsu’s new transistors would enable more compact power supplies. High-speed transistor operation would allow for more compact coils and transformers, which have been particularly difficult to miniaturize in conventional power supplies with low-frequency operation: the size of ac adapters for notebook PCs, for example, could be reduced to one-tenth current sizes. Smaller power supplies would contribute to reducing pace requirements for data centers, as well.

Fujitsu is progressing with practical implementations of GaN transistors featuring high breakdown-voltages, with the aim of producing power supplies based on them by approximately 2011.