A SMALL-SIGNAL MODEL FOR THE FREQUENCY-DEPENDENT DRAIN ADMITTANCE IN FLOATING-SUBSTRATE MOSFETS

This paper examines the frequency-dependent drain admittance of silicon-on-sapphire (SOS) MOSFET's from the perspective of the circuit designer. Measurements of small-signal drain characteristics as a function of frequency, bias conditions, and device geometry are presented, which have major implications for analog circuit design. These are explained in terms of a new small-signal circuit model. Physical explanations for the observations are given and the poles and zeros of the model identified to assist designers carrying out hand calculations with easily manipulated expressions. Frequency-dependent thermal effects are also discussed. Similar effects can be expected in other SOI technologies.