Table (6.2) A comparative study of GaAs, Si and Graphene.
As seen from the table, mobility of electrons is five times higher in GaAs than in Si. Also ni (intrinsic concentration) is much lower in GaAs as compared to Si. Hence GaAs based I.C. chips are going to be five times faster than Si based I.C. chips and by proper dimension sealing very high packing density can also be achieved. Thus GaAs I.C.’s seen to hold very good promise for Gigabit/Gigahertz applications. Defense department of U.S.A. is going in a big way for GaAs based Very High Speed Integrated Circuits (VHSIC) which will realize real time signal processors for weapon systems of the next decade.
As seen from Table (6.2), Silicon has indirect band gap while GaAs has direct band gap. As a result, electron-hole pair recombination is accompanied with phonon emission in Si. That is direct transitions are non-radiative. Hence GaAs based systems are indispensible for Opto-Electric applications. Moreover by proper compensation of GaAs very high resistivity substrate 10^8 ohm-cm can be achieved which is directly applicable in Microwave Integrated Circuits (MIC).
From Table (6.2), we also see that graphene will emerge as the material of the future. It is 2D carbon layer where carbon are bonded in honeycomb chicken net structure. It is a semi-metal or zero-gap semiconductor . Theoretically the mobility is very high at 200,000cm 2 /V-sec but in practice the substrate phonon scattering limits it to 20,000cm 2 /(V-sec).
In the era of discrete component based Electronic Systems there were three distinct and non-interacting disciplines: device design, circuit design and system design. But with the advent of I.C. based Electronic Systems the traditional boundaries between the three disciplines have been obliterated. Today an electronic circuit designer or I.C. chip designer must be knowledgeable in semiconductor fabrication technology, in the physical electronics and circuit modeling of semiconductor devices and in the methods and techniques of circuit analysis and design. Thus a chip design must be aware of the constraints and opportunities of device technology and circuit theory as well as must have a sufficient knowledge of Systems Engineering so that he is aware of the requirements of the users of his products. By the same token the systems Engineer must be aware of the capabilities and limitations of the present day State of Art of I.C. technology and accordingly limit his requirements expected from I.C. functional blocks currently available in the market.
Today the system’s design is fully Computer-Aided Design. In Digital System Design, most complex systems can be described by Hardware Description Language and implemented on Field Programmable Gate Array. Similarly there are softwares for designing Analog and Mixed Signal Systems.
Section (6.2) Two Broad Categories of Integrated Circuits
Integrated circuits broadly fall in two groups:
Monolithic Circuits and Hybrid Circuits.
A monolithic circuit is fully realized on a single silicon chip. Through Planar Fabrication Processes all circuit elements are simultaneously fabricated and interconnected on the same silicon chip. This monolithic implementation is very convenient for batch production hence is very suitable for low cost mass production. The range of passive component values, their tolerances and their temperature coefficients is more limited than with thin film processes but quite adequate for most digital circuits and some analog circuits. By CMOS technology, digital system as well as analog system implementation has been made feasible. CMOS enables the realization of resistances and capacitances by proper configuration of NMOS itself. If a greater variety of resistances are required then as is required in case of Active Filters we resort to switch-capacitor filter technique. This enables us to achieve a wide range of time-constants.In fact a large number of off-the-shelf-digital integrated circuits as well as numerous "custom" (special purpose) circuits for large-quantity production contracts are manufactured in this manner.