The motivations behind the development of GaAs integrated circuits (IC) are two-fold: to integrate high speed logic with optical sources and to meet the increasing demand of realising LSI/VLSI with higher speed and lower power dissipation for large scale computer applications. GaAs gigabit circuits have been growing in complexity to more than 3000 gates on a single chip. Although this is encouraging, more efforts are needed to improve production yield. By far the most work on GaAs digital IC has been done using MESFET as the active devices. MOSFET technology is yet to mature from the practical IC point of view. The logic gate types used in circuits are predominantly of the enhancement-mode driver and depletion-mode load configuration (E/D).

A brief survey of the state-of-the-art of GaAs digital IC is presented. Implemented circuits are described and compared with those achieved through various technologies. GaAs gate arrays, multipliers, accumulators and memories are discussed. At liquid N₂-temperature, a switching time of 5·8 ps/gate has been achieved for 0·35µm gate devices. This and similar other results lead to the conclusion that at the VLSI level of future Gbit circuits, GaAs devices in the form of HEMT operated at 77 K can outperform Si-devices. At′ LSI complexities, experimental GaAs MESFET and 300 K HEMT have a lead on Sicircuits—it is then this range in which Gbit/GaAs should find their application.

The magnetic response of YBa₂Cu₃O_7−x−5 mol% Ag composite to low-frequency magnetic field and its microstructure have been studied. Microstructural analysis shows evidence of platelet-type grain growth and silver fills the intergranular regions. The granular nature of the sample is revealed from the strong decrease in a.c. response in the presence of d.c. magnetic field. The intergranular shielding current estimated from the complex response and using the Bean’s model sharply increases with temperature below transition temperature.