High-power high-frequency device technology is a key technology for a wireless communication system, which is the information network infrastructure of the 21st century. Widegap semiconductors, such as GaN and SiC, are strongly expected as high-power high‐frequency devices owing to their material properties. In this paper, the present status and future prospect of these widegap semiconductor high-frequency devices are reviewed.

The polarization effect of the AlGaN／GaN HEMT structure and its v‐E characteristics were explained. The effective electron velocity of the HEMT channel and its temperature dependence were studied by delay time analysis. The off‐state breakdown and current collapse were also studied in detail and their mechanisms were clarified.

Although significant progress has been achieved in GaN-based high-power/high-frequency electronic devices, surface-related problems still need an immediate solution. In particular, the so-called current collapse and gate leakage effects not only degrade microwave-output performance but also impede the reliable operation of the devices. This paper reviews surface-related issues in GaN FETs and AlGaN/GaN HFETs, particularly focusing on the correlation with surface disorder, surface defects and surface states. The surface-passivation and insulated-gate structures are also discussed for the reliability improvement of GaN-based devices.

Metal/insulator/semiconductor field-effect transistors based on a surface p-type conductive layer of hydrogen‐terminated diamond exhibits a transconductance of more than 100mS/mm and a cutoff frequency (fT) of 20 GHz. By the reachable reduction of parasitic factors such as source resistance, gate resistance, and substrate capacitance, the fT and the maximum frequency of oscillation are expected to be 40 GHz and 100 GHz, respectively, in the 0.2 mu m gate FET. Carrier transport characteristics have been mainly estimated by fT at different applied electric fields and different gate lengths. The channel conductance is about 200 cm²/Vsec and the mean carrier velocity is 5x10⁶cm/sec which is one half of the saturated velocity has been obtained. A new type of interfacial control technology which diminishes the scattering centers at the surface and enhances the channel mobility as high as the bulk mobility is highly desired.

This paper gives an overview of recent research activities on high-frequency electron devices using III-nitride semiconductors. The advantages of utilizing nitride semiconductor heterojunctions as high-voltage and high-power electron devices at microwave frequencies are discussed. The device processing and structural design are described, focusing on the importance of improved breakdown characteristics enabled by the introduction of a field plate. The DC, small-signal and power performance of the fabricated devices is presented. Finally, the future subjects and prospects of nitride heterojunction devices and their applications are discussed.

Silicon carbide (SiC) is a promising material for fabricating high-power and high-frequency devices because of its superior properties. SiC high-frequency devices are expected to be used for applications such as power amplifiers for base station transmitters and power modules for radar systems. We developed SiC-MESFETs and evaluated their DC and RF characteristics. In this paper, I will give the current status of the SiC high-frequency devices-SIT, JFET, BJT, and IMPATT diodes-that were developed by other institutions, and describe the experimental results obtained from the MESFETs that were developed by us.

Recently, research studies on three-dimensional (3D) displays have been very active. These studies involve not only the improvement of the conventional method but also the development of some new technologies. This report introduces some new technologies from recent research studies on 3D display, such as the hologram and multiview methods, which use an array of two-dimensional images to obtain a natural stereoscopic effect.

Gallium Nitride (GaN material) has high breakdown voltage and high saturation velocity characteristics. Moreover, two-dimensional electron gas (2DEG) with high electron mobility can be generated at the AlGaN/GaN heterointerface. For these reasons, an AlGaN/GaN HEMT has been expected as a new-generation high-power and high-frequency device. For the development of a high performance AlGaN/GaN-HEMT, both 2DEG with high electron density and low parasitic resistance characteristics is needed. We have been proposing that a recessed gate structure can realize these demands. In this paper, we report that the recessed gate AlGaN/GaN-HEMT fabricated on the SiC substrate showed extremely high transconductance and high frequency characteristics.

Using high-quality diamond homoepitaxial thin films, we fabricated diamond short-gate MESFETs (metal-semiconductor field-effect transistors). FETs exhibited the highest RF characteristics among diamond devices, namely, a transient frequency (f_T) of 25GHz, maximum oscillation frequency for a maximum available gain (MAG) (f_max(MAG)) of 63GHz and Mason's unilateral gain (U) (f_max(U)) of 81GHz. Thus, we have achieved amplification in the millimeter wave range for the first time. The power measurements of the A class at 1GHz showed a linear gain of 14dB in a wide range and a maximum output power level of 0.35W/mm. The first RF noise measurements showed a minimum noise figure of 1dB at 3GHz.

The focus margin of lithography becomes increasingly critical as the LSI design rule decreases. Chemical mechanical polishing (CMP) is an essential process for planarization and is widely applied in ULSI processing. As the slurry characteristics greatly influence the polishing performance, slurry optimization is important in achieving a high degree of planarization. To study the polishing mechanism in detail, the tests simulating the dielectric CMP process have been performed using an atomic force microscope. The phenomena occurring between the slurry and the wafer surface were evaluated.

The two-stage vapor-phase method has been developed for the synthesis of gallium nitride (GaN) powder, which is a promising phosphor material. This novel method consists of the stages of seed formation and particle growth, at which two appropriate chemical reactions are preceeded in a gas flow, and is expected to allow the efficient synthesis of GaN powder and high controllability of its properties.

The AMS technique has contributed markedly to the research topic that the Yayoi period started much earlier than has been generally believed, probably by about 500 years. Recently, AMS, known as an ultrasensitive and high-throughput technique, has gained also high precision, which must promote application to archaeology in the coming many years. In this article, I describe the results from our current research programs that make best use of high-precision 14C dating using the AMS technique including the Yayoi issue.

The current status of optoelectronic integrated circuits (OEICs) and its technological issues to be solved are described in the viewpoints of material science and technologies. Compound-semiconductor-based OEICs are reviewed. New types of OEIC, multimaterial OEIC, are discussed toward an integrated optical interconnection between Si-LSI chips. Si-based OEICs are presented as an example of commercial OEICs. Finally, I show technological issues for future OEICs.

This article reviews the principles behind the operation of a magnetic random access memory (MRAM) and the advantage of using such a memory in comparison with other nonvolatile memories. Although MRAM is expected to be a universal memory in the future, it has not been realized yet. The problems to be tackled are described. Also, in this paper, we report on the efforts to develop an MRAM with low power, high speed and high density.