Isao Tamai

12.88 W/mm GaN High Electron Mobility Transistor on Silicon Substrate for High Voltage Operation

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.

Coplanar Waveguides on High-Resistivity Silicon Substrates With Attenuation Constant Lower Than 1 dB/mm for Microwave and Millimeter-Wave Bands

Coplanar waveguides (CPWs) with extremely low loss have been successfully developed on high-resistivity silicon (HR-Si) substrates as interposers of multichip modules for microwave and millimeter-wave bands. The attenuation constant of these CPWs on HR-Si is less than 1 dB/mm for frequencies up to 100 GHz, which is comparable with that of CPWs on semi-insulating compound semiconductor substrates. Conventional CPW structures show a larger attenuation constant due to the effects of the low-resistivity layer generated at the interface between the insulating layer of SiN and the HR-Si substrate, which has been detected through both experimental investigations and numerical calculations. A CPW structure that suppresses the effects of the low-resistivity layer is presented in this paper. The fabrication process is rather simple and can be smoothly integrated in conventional semiconductor device fabrication processes.

Effects of a Thermal CVD SiN Passivation Film on AlGaN/GaN HEMTs

In AlGaN/GaN high electron mobility transistors (HEMTs), drain current reduction by current collapse phenomenon is a big obstacle for a high efficient operation of power amplifier application. In this study, we investigated the effects of SiN passivation film quality on the electrical characteristics of AlGaN/GaN HEMTs. First, we conducted some experiments to investigate the relationship between electrical characteristics of AlGaN/GaN HEMTs and various conditions of SiN passivation film by plasma enhanced chemical vapor deposition (PE-CVD). We found that both gate current leakage and current collapse were improved simultaneously by SiN passivation film deposited by optimized condition of NH3 and SiH4 gas flow. It is found that the critical parameter in the optimization is a IN-H/ISi-H ratio measured by Fourier transforms infrared spectroscopy (FT-IR) spectra. Next, a thermal CVD SiN was applied to the passivation film to be investigated from the same point of view, because a thermal CVD SiN is well known to have good quality with low hydrogen content and high IN-H/ISi-H ratio. We confirmed that the thermal CVD SiN passivation could improve much further both of the gate leakage current and the current collapse in AlGaN/GaN-HEMTs. Furthermore, we tried to apply the thermal CVD SiN to the gate insulator in MIS (Metal Insulator Semiconductor) structure of AlGaN/GaN HEMTs. The thermal CVD SiN passivation was more suitable for the gate insulator than PE-CVD SiN passivation in a view of reducing current collapse phenomena. It could be believed that the thermal CVD SiN film is superior to the PE-CVD SiN film to achieve good passivation and gate insulator film for AlGaN/GaN HEMTs due to the low hydrogen content and the high IN-H/ISi-H ratio.