Title :
A comprehensive delay model for CMOS inverters
Author :
Dutta, Santanu ; Shetti, Shivaling S Mahant ; Lusky, Stephen L.
Author_Institution :
Dept. of Electr. Eng., Princeton Univ., NJ, USA
fDate :
8/1/1995 12:00:00 AM
Abstract :
A method to accurately calculate the delay and the output transition-time of a CMOS inverter for any input ramp and output loading is considered. This paper is an extension of Sakurai´s work (1990) on delay modeling of inverters for fast input ramps. We observed that two different mechanisms, that can be adequately modeled analytically, govern the delay and the output transition-time of an inverter in two extreme cases: infinitely fast and infinitely slow inputs. These extreme points are joined by a curve that can predict the delay and the output transition-time for any input. We found that the delay and the output transition-time for an inverter with small fanouts are similar to those for large input transition-times. This behavior is explained by the use of I-V trajectories. We describe a method to generate parameters to model delay and output transition-time for different fanouts and input transition-times; this method can be generalized to add parameters for different temperatures and supply voltages. Given a new process technology and its corresponding SPICE-model parameters, our delay calculation scheme comprises characterizing a minimal number of coefficients for each new technology (a one-time process) and evaluating the analytical forms thereafter to obtain the delay and the transition-time. Our delay equations also explain negative delays that arise in case of slow input rise-times. A program incorporating the above idea has been implemented in C. Delay and transition-time values obtained from the program have been found to be typically within 3% of SPICE
Keywords :
CMOS logic circuits; SPICE; delays; integrated circuit modelling; logic gates; C language; CMOS inverters; I-V trajectories; SPICE model; computer program; delay model; fanouts; input ramp; output transition-time; process technology; Delay; Delay effects; Equations; Integrated circuit interconnections; Inverters; MOSFETs; Resistors; SPICE; Semiconductor device modeling; Temperature; Voltage;
Journal_Title :
Solid-State Circuits, IEEE Journal of