The pursuit of economical hardware is a key R&D theme in wireless communication equipment including space applications. One solution is compactness and high-functionality using integrated circuits (IC). In particular, the introduction of highly functional IC technology is essential for key high-power or low-noise amplification circuits.
Our research group successfully built a highly functional small module using compound semiconductor monolithic microwave integrated circuit (MMIC) technology. In this article, I will outline our research and the results by showing examples of small-scale devices and electrical equipment using the devices including space application.
Compound semiconductor and microwave IC
In addition to the condenser, resistor, coil, etc., the transistor is a critical component for data communication using radio waves or cables in smartphones, computers, etc. A device aggregating these transistors in small size is called an Integrated Circuit (IC). In the past, silicon (Si) has been widely used as a material for transistors and ICs. However, high-speed devices using ICs are needed to transmit moving pictures or a huge volume of data. The material used in such devices is a compound semiconductor. Some famous examples are gallium arsenide (GaAs), indium phosphorus (InP) and gallium nitride (GaN).
The electrons in the devices using such compound semiconductors move at very fast speeds, allowing us to operate at high-speed and high-frequency and, eventually, to realize high-performance devices and circuits. The compound semiconductors, however, are made of a combination of different types of molecules, unlike single molecule Si. Thus, advanced technology is required to fabricate them and integrate with other elements such as resistors.
By producing devices such as resistors, condensers and coils on Si or compound semiconductors and connecting them with lines for signal transmission, we can obtain small MMICs that enable very-high-frequency radio-wave or ultra-high-speed communication. The MMIC has a base of semi-insulating substrate (not full insulation but with higher resistivity than metal) made of pure Si or compound semiconductor with extremely low impurity. On this base, we added a layer with impurity called the epitaxial layer where electrons run (i.e. conduct); a layer to provide electrons necessary for electron conduction; a metal pad for electrodes and a layer of thin insulation film.
By fabricating lines of circuit pattern width of around 0.1?m and square metal pads of around 100?m on the epitaxial layer by electron-exposure technology, we can integrate low-frequency resistors, condensers and coils. With these technologies, we produced field-effect transistors (FET) of less than 1mm square, microwave amplifier circuits of 5mm square, etc. Recently, we also even attempted to integrate a very small antenna (e.g. patch antenna) on MMIC. This is the so-called one-chip transceiver.