Impact of LDD structures on the operation of silicon MOSFETs at low temperature

Abstract

The impact of Lightly Doped Drain structures (LDD) on the electrical characteristics of Si MOS transistors is investigated from liquid helium up to room temperature. It is found that the presence of LDD regions strongly alters the ohmic drain current characteristics at low temperatures ( ≤ 100 K). This feature is attributed to the increase of the source-drain series resistance due to the impurity freeze-out which takes place in the LDD regions. In contrast, it is also found that, for sufficiently high drain and gate voltages, the drain current is only weakly affected by the presence of LDDs. The analysis of the drain current and output conductance characteristics allows us to demonstrate that the LDD resistance is indeed a non-linear function of the longitudinal electric field. The above LDD resistance dependence with electric field is interpreted in term of field assisted impurity ionization which occurs in the LDD regions. The mechanisms responsible for the field assisted impurity ionization are discussed with regard to the Poole-Frenkel effect and the shallow level impact ionization process. Moreover, a simple model for the field dependent LDD resistance is proposed and makes it possible to explain quantitatively the change of the output conductance with temperature for MOS devices operated at low and very low temperatures.