Development of World’s First Gallium-Nitrate HEMT for power supply by Fujitsu
Power loss as heat in current power supplies used in today’s electronic hardware accounts for 30% or more of the total power consumed by the device. Furthermore, wasted heat creates the need for additional cooling equipment resulting in a ripple effect of increased power consumption.
The power supplies take the alternating-current (AC) power from a wall outlet and convert it to the stabilized direct-current (DC) power that electronics require. The AC power from the public utility companies features an erratic behavior making it necessary to first stabilize and then reduce the voltage before converting it to DC power. The devices that are responsible for this voltage reduction are transistors which are capable of switching between a state that passes current and one that blocks the current using the resulting high-frequency AC power. The power supply typically use silicon transistors but such transistors suffer from some power loss which is known as “on-state loss” when passing current as well as considerable amount of power loss during switching between ON and OFF states known as “switching loss” contributing to 30% of total power loss in a power supply. Normally, “on-state loss” caused when a transistor is ON and the “switching loss” occurs when switching from an ON state to an OFF state contributing to a power loss which increases when switching speed is slow. One effective solution to over come the “on-state loss” is use of transistors which are made of high breakdown-voltages such as Gallium-Nitrate HEMT (High electron mobility transistor). GaN HEMTs transistors have less than one-fifth the on-state loss of silicon transistors plus an excellent high-speed characteristic resulting in switching losses of less than 1% of those of silicon transistors. Current power supplies used in products such as PCs, home appliances and automobiles demand a complete current-interruption in stand-by mode, in which no voltage is being applied to the gate electrodes.
Based on this need Fujitsu has made an announcement in development of new structure for gallium-nitride high electron-mobility transistors Gallium-Nitrate (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 newly developed technology is capable of blocking the flow of current from power supplies in stand-by mode and at the same time produces high-density current when turned on, and has the potential to cut power consumption of electronic equipment by one-third. The new technology if applied to data centers could potentially contribute to reduction of total power consumption by 12% resulting in the effect of removing 330,000 tons of CO2 from Japan as a whole according to Fujitsu. The new technology was developed based on already announce 3-layer cap structure for GaN HEMTs by Fujitsu on 2008, 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, since the voltage that was applied at the gate electrode to switch between ON and OFF states was in the range of 0.5 V, it could not be used for power supplies, which need to apply +2 to +4 V 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 600milli-amps/millimeter (600 mA/ mm). Therefore Fujitsu had to make two improvements in order to make the technology suitable for power supplies. First was in developing a technology for precise removal of the cap layers and a part of the AlGaN layer only below the gate electrode, and 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. Second was in development of gate structure 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 traveling 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. These two improvements resulted in on-state voltage of the new transistor which reaches +3 V, which can easily be applied to power supplies while achieving a current density of +3 V, which can easily be applied to power supplies while achieving a current density of 829 mA/ mm. With the new transistor in power supplies, power loss can be reduced to one-third (1/3) 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 space requirements for data centers, as well.
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