We have demonstrated the highest RF output power density of 12.88 W/mm, to our knowledge, at 2.14 GHz in GaN high electron mobility transistor on silicon (Si) substrate at high voltage operation. This highest record was achieved by reducing the parasitic loss at the conductive interface layer between the epitaxial structure and the Si substrate, and thinning the Si substrate thickness. The parasitic loss evaluation by a capacitance–voltage method proved to be effective since the epitaxial wafer selection is possible without fabricating RF devices. Si substrate as thin as 60 µm results in good thermal resistance and contributes to the large RF output power density.

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This paper proposes a scheme to obtain location and vital health information using ZigBee system. ZigBee systems are wireless communication systems defined by IEEE 802.154. In the proposed scheme, location information is obtained using the Link Quality Indication (LQI) function of a ZigBee system, which represents the received signal strength. And, the vital health information are collected from the electrocardiogram monitor, the pulse and blood pressure device, attached to the patient's body. This information is then transmitted to an outside network by ZigBee systems. In this way, vital health information can be transmitted as ZigBee sensor data while patients with the ZigBee terminal are moving. In the experiments using actual ZigBee devices, the proposed scheme could obtain accurate location and vital health information from the sensor data. Moreover, to achieve high reliability in the actual service, the collected amount of sensor data was confirmed by the theoretic calculation, when a ZigBee terminal passed through ZigBee routers. These results indicate that the proposed scheme can be used to detect the accurate location of the ZigBee terminal. And over 99% of the sensor data on vital health information was obtained when the ZigBee terminal passed through approximately four ZigBee routers.

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