NEC Corp. (NEC) and NEC Electronics Corp. (NECEL) have developed a gallium nitride (GaN) semiconductor power transistor on a silicon (Si) substrate that has improved the control and suppression of electrical currents when electrical power is turned off (normally-off characteristics), a necessary feature for the safe operation of consumer electronics and IT devices. The transistor features a new structure for the layer beneath the gate electrode, which improves the controllability of threshold voltage that intercepts electrical currents and enables the realization of low-power losses, high speed switching and high temperature operations.
Power transistors that use silicon semiconductors occupy an important role in the advancement of energy conserving machines, as they convert electrical power and serve as a controlling element for a wide range of applications, from consumer electronics to industrial machinery. Future expectations are high for the implementation of Nitride semiconductor transistors, which demonstrate lower-level losses, higher speeds and higher temperature operations when compared to current silicon transistors. However, it is important to suppress variations of threshold voltage and improve reproducibility of their normally-off characteristics.
Current transistors are composed of a two-layer structure that consists of an AlGaN electron supply layer and GaN channel layer. A great 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 20nm to 30nm to achieve normally-off characteristics. A high precision etching process is specifically required in order to reduce the variation of threshold voltage and stabilize 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 electron supply layer. This is accomplished by introducing an electric charge neutralization layer, the "piezo" neutralization layer, within the electron supply layer, while at the same time introducing a buffer layer, located beneath the channel layer, with the same composition as the piezo neutralization layer. This structure enables uniform manufacturing of nitride semiconductor power transistors that realize normally-off characteristics at a low cost. These transistors also exhibit excellent performance such as low power loss and high breakdown voltage.
NEC
NECEL
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