Impact of GaAs and Si technologies on adder characteristics

Abstract

Two radiation-hard technologies, E/D-MESFET GaAs and SOS-CMOS Si are compared, from the point of view of propagation delay and VLSI layout area, for different types of adders. The major intention was to evaluate the impact of two technologies on the adder design. However, the overall results are more general. They show how the adder design trade-offs change when the dependency of gate delays on fan-in and fan-out changes. The major conclusion of this research is that traditionally slow adders, like ripple-carry or similar, although still slower than traditionally fast adders, like carry-lookahead or similar, may be better suited for incorporation into GaAs microprocessors with word lengths of up to 32 bits. This is because the delay gap between the two adder groups has decreased and the VLSI area gap between the same two adder groups has increased. Consequently, if simpler adder is incorporated, the remaining area could be ‘invested’ into resources that would speed up the execution of compiled HLL code more than the incorporation of a faster (but area-consuming) adder and a faster system clock.