The effect of Mn-doping into a GaN buffer layer grown by metal organic chemical vapor deposition (MOCVD) on the reduction in the leakage current of high-electron-mobility transistors (HEMTs) was investigated. Both the surface morphology and crystallinity maintained their quality even after heavy Mn-doping. The sheet resistance of GaN films increased with increasing amount of Mn-doping. The origin of semi-insulating GaN layer is considered to be electron scattering and the carrier compensation mechanism involving deep levels generated by the Mn impurity. When using the Mn-doped GaN buffer layer for the HEMT structure, the leakage current was reduced to five orders of magnitude lower than that without Mn-doping. Although Mn-doping is an effective technique for reducing the buffer leakage current, it is found that current collapse is emphasized when using the Mn-doped GaN buffer layer. We suggest that Mn atoms, which diffused to the GaN channel layer, induce the current collapse.

NMIJ / AIST has been disseminating the national torque standard to Japanese industry by using deadweight type torque standard machines (DWTSMs). In general, DWTSMs can generate more precise torque than other types of TSMs. On the other hand, the calibration takes much longer time than others. One possible solution is to use a reference type torque calibration machine (RTCM). RTCMs have been developed in some national metrology institutes (NMIs). We have started the development of the first RTCM in the range of 100 mN · m to 10 N · m. In this study, we developed the automatic calibration system of the RTCM and investigated the calibration procedures for a low nominal capacity torque measuring device (TMD). It was found that the calibration could be realised by the RTCM, compared with the reliable DWTSM of rated capacity of 10 N · m at NMIJ.

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