IMPACT OF LDD STRUCTURES ON THE OPERATION OF SILICON MOSFETS AT LOW-TEMPERATURE

The impact of Lightly Doped Drain structures (LDD) on the electrical c haracteristics of Si MOS transistors is investigated from liquid heliu m up to room temperature. It is found that the presence of LDD regions strongly alters the ohmic drain current characteristics at low temper atures (less than or equal to 100 K). This feature is attributed to th e increase of the source-drain series resistance due to the impurity F reeze-out which takes place in the LDD regions. In contrast, it is als o 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 functio n of the longitudinal electric field. The above LDD resistance depende nce with electric field is interpreted in term of field assisted impur ity ionization which occurs in the LDD regions. The mechanisms respons ible for the field assisted impurity ionization are discussed with reg ard to the Poole-Frenkel effect and the shallow level impact ionizatio n process. Moreover, a simple model for the field dependent LDD resist ance is proposed and makes it possible to explain quantitatively the c hange of the output conductance with temperature for MOS devices opera ted at low and very low temperatures.