Noise in short channel MOSFETs

For long-channel MOSFETs, the power spectral density of wideband noise in the drain current is predicted by an expression derived from thermal noise in the MOSFET channel. For short channel MOSFETs, observed noise can be much higher than predicted from thermal noise analysis of long channel MOSFETs. While the cause of this excess noise is the subject of some controversy, it can be understood by considering the fundamental difference between shot noise (carrier motions are independent events) and thermal noise (carrier motions are dependent due to thermal equilibration). This paper reviews the literature on noise in short channel MOSFETs and shows that the increased noise can be seen as resulting from the current noise approaching a shot noise limit as carrier transit time in the MOSFET channel becomes so small that thermal equilibration does not have time to occur.

I. Introduction

Designers of analog and mixed-signal integrated circuits face both challenges and opportunities as CMOS device dimensions enter the nanoscale region. For mixed-signal design, scaling offers the possibility of increased speed but also difficulties associated with reduced supply voltage, degraded transistor characteristics, and increased variability. An additional issue in short channel devices is an increase in wideband noise over the noise observed in long-channel devices.