Channel width tapering of serially connected MOSFET´s with emphasis on power dissipation

Transistor channel width tapering in serial MOSFET chains is shown in this paper to simultaneously decrease propagation delay, power dissipation, and physical area of VLSI circuits. Tapering is the process of decreasing the size of each MOSFET transistor width along a serial chain such that the largest transistor is connected to the power supply and the smallest is connected to the output node. A detailed explanation of the effects of tapering on the output waveform is presented with specific emphasis on the power dissipation of tapered chains. It is demonstrated that in many cases tapering decreases delay and changes the shape of the output waveform such that the time during which a load inverter is conducting short-circuit current is reduced. This decrease in short-circuit current also occurs in many cases where tapering may not offer a speed advantage. In addition, dynamic CV/sup 2/f power dissipation of the serial chain is reduced. In those circuits where tapering does not decrease propagation delay, tapering permits a designer to tradeoff speed for a reduction in both short-circuit and dynamic power dissipation, a tradeoff not normally available with untapered chains. Thus the total power consumed by a serial chain of MOSFET´s, as well as its propagation delay and area, can be reduced by channel width tapering. A design system for determining when tapering is appropriate, selecting the amount of tapering, and synthesizing the physical layout is presented. Physical layout issues unique to tapering are discussed, and fabricated test structures are described.<>