NEC and NEC Electronics Develop High Threshold Voltage Control Gallium Nitride (GaN) Power Transistor on a Silicon (Si) Substrate
New Delhi, December 11, 2009: NEC Corporation (NEC) and NEC ElectronicsCorporation (NECEL) today announced the development of a nitride semiconductor (*1) power transistor on a silicon (Si) substrate thathas improved the control and suppression of electrical currents whenelectrical power is turned off (normally-off characteristics (*2)),a necessary feature for the safe operation of consumer electronics and IT devices. The transistor features a new structure for the layerbeneath the gate electrode, which improves the controllability ofthreshold voltage that intercepts electrical currents and enables the realization of low-power losses, high speed switching and hightemperature operations.
Power transistors that use silicon semiconductors occupy animportant role in the advancement of energy conserving machines, asthey convert electrical power and serve as a controlling element for awide range of applications, from consumer electronics to industrialmachinery. Future expectations are high for the implementation ofNitride semiconductor transistors, which demonstrate lower-levellosses, higher speeds and higher temperature operations when compared to current silicon transistors. However, it is important to suppressvariations of threshold voltage and improve reproducibility of theirnormally-off characteristics.
Current transistors are composed of a two-layer structure thatconsists of an AlGaN electron supply layer and GaN channel layer. Agreat deal of variation in threshold voltage occurs due to differences in the thickness of the AlGaN electron supply layer under the gate,which is etched down to a few nm from 20-30 nm to achieve normally-offcharacteristics. A high precision etching process is specificallyrequired in order to reduce the variation of threshold voltage andstabilize normally-off characteristics.
A five layer structure makes it possible for these new transistors to control threshold voltage with a high degree of precision by reducing the threshold voltage's dependence on the thickness of the electronsupply layer. This is accomplished by introducing an electric charge neutralization layer, the "piezo" neutralization layer, within theelectron supply layer, while at the same time introducing a bufferlayer, located beneath the channel layer, with the same composition asthe piezo neutralization layer. This structure enables uniformmanufacturing of nitride semiconductor power transistors that realize normally-off characteristics at a low cost. These transistors alsoexhibit excellent performance such as low power loss and highbreakdown voltage.
Looking forward, NEC and NEC Electronics aim to accelerate researchand development towards the design, evaluation and implementation of nitride semiconductor power transistors.
NEC will present transistor results at the International ElectronDevices Meeting (IEDM) held in Baltimore from December 7, 2009(announcement on December 7).
(*1) Nitride Semiconductor: Semiconductors composed of Gallium nitride (GaN)/aluminum nitride gallium (AlGaN) that have bandgaps larger than 3.4eV. These promising devices feature high breakdown field strength and high electron mobility, as well as components thatproduce high power output and resist high temperatures andvoltage. Th ese semiconductors are widely used by Blue LEDmaterials that capitalize on large bandgaps.
(*2) Normally-Off Characteristics: Characteristics that blockelectrical currents when voltage is not applied to a transistor. This is necessary for the safe operation of machinery and tools.
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