In AlGaN/GaN high electron mobility transistors (HEMTs), Si3N4 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 Si3N4 deposition process. In order to solve this problem, we have demonstrated an NH3-plasma surface pretreatment in the chamber of plasma enhanced chemical vapor deposition (PE-CVD) just before Si3N4 deposition process. We found that the optimized NH3-plasma pretreatment could improve the current collapse as compared with only the Si3N4 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 NH3-plasma pretreatment decreased the carbon contamination such as hydrocarbon on the AlGaN surface and the excessive pretreatment degraded the stoicheiometric composition of AlGaN surface.

This paper describes the architecture and the performances of a GaAs 88 self-routing switch LSI for ATM switching system. The communication system such as broadband integrated sevices digital network (B-ISDN) requires the hardware switch LSI which exchanges packet cells at a date rate up to several Gb/s. GaAs LSIs are suitable for such application because of its high speed operation and low power dissipation. To clarify the feasibility of GaAs LSI, an 88 self-routing switch LSI is fabricated using 0.5 µm gate GaAs MESFETs and its oerformances are examined. This LSI consists of a switching network for exchanging the packet cells and the "NEMAWASHI" network which detects the cell destined to the same output port. The basic network architecture is a self-routing switch using Batcher-Banyan network. This network consists of basic 22 switch element. Since each element switches the route accorging to the destination of the input cells, self-routing operation is performed without the external circuit for routing control. The LSI is fabricated using 0.5 µm gate GaAs MESFETs. 7003 logic gate are integrated on the chip of 8.2 mm7.4 mm. To reduce the impedance of ground line on the chip and to obtain the enough noise margin, the third level interconnection with low sheet resistance is implemented. As the results of functional evalution, the full function of switching network and "NEMAWASHI" network are verified. Maximum operation speed of 1 GHz is obtained.

Current collapse phenomenon is a well known obstacle in the AlGaN/GaN HEMTs. In order to improve the surface stability of HEMTs, we have investigated the SiN passivation film deposited by T-CVD, and we found that it improves both gate leakage current and current collapse phenomenon [1]. Moreover, we compared the T-CVD and PE-CVD passivation films, on high electric field DC and RF characteristics. We found that T-CVD SiN passivation film improves BVds-off by 30% because of the reduction of gate leakage current. It also improved ηd in the output power characteristics by load-pull measurement, which indicates the decrease of the current collapse phenomenon. Also we fabricated a multi-fingered 50 W-class AlGaN/GaN HEMT with T-CVD SiN passivation film and achieved 61.2% of high drain efficiency at frequency of 2.14 GHz, which was 3.6 points higher than that with PE-CVD SiN passivation film.

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.

Isolation characteristics, which are important factors in designing GaAs ICs, are investigated focusing on leak current between circuit elements on a semi-insulating substrate and on the sidegating effect that results from leak current between MESFETs. We have found that the large leak current comes from the projecting edge, located outside the channel, of the gate electrode and that this leak current is the main cause of the sidegating effect. By taking into account quantitatively evaluated isolation characteristics, we can improve LSI design rules to reproducible and reliable operation.