Toshiharu Marui

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.

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.

Influence of NH 3 -Plasma Pretreatment before Si 3 N 4 Passivation Film Deposition on Current Collapse in AlGaN/GaN-HEMTs

In AlGaN/GaN high electron mobility transistors (HEMTs), Si 3 N 4 passivation film brings effective improvements in the current collapse phenomenon, however, the suppression of this phenomenon in a high voltage operation can not be achieved in only the Si 3 N 4 deposition process. In order to solve this problem, we have demonstrated an NH 3 -plasma surface pretreatment in the chamber of plasma enhanced chemical vapor deposition (PE-CVD) just before Si 3 N 4 deposition process. We found that the optimized NH 3 -plasma pretreatment could improve the current collapse as compared with only the Si 3 N 4 deposition and an excessive pretreatment made it worse adversely in AlGaN/GaN-HEMTs. It was confirmed by Auger electron spectroscopy (AES) analysis that the optimized NH 3 -plasma pretreatment decreased the carbon contamination such as hydrocarbon on the AlGaN surface and the excessive pretreatment degraded the stoicheiometric composition of AlGaN surface.