Scaling Effects on Thermal and Gate Induced Noise of Small Geometry LDD Mosfets

An analytical model is developed to study the impact of scaling on the thermal and gate induced noise of the small geometry LDD MOSFETs. The analyses includes the short channel, narrow width, velocity saturation, hot carrier and LDD effects. The intricacy in analysis when considering all these effects simultaneously as in the small geometry devices is overcomed and a generalized expression valid in the submicrometer range of device operation is developed. The results so obtained for effective current noise gain are compared with the measured results of short channel LDD MOSFETs and are found to be in good agreement. The results show that at frequencies much below the cut-off frequency, the drain noise is just 1% or even less of the gate induced noise. A comparison of the effective current noise gain in conventional and LDD MOSFETs is also done for the first time and it is observed that the current noise gain is higher in conventional MOSFETs than in the LDD MOSFETs because of the n series resistance. It is also observed that while the thermal noise increases with the decrease in device length, the gate induced noise decreases with the decrease in device length. A reversal of trends is observed when the device width is scaled down.