With the increasing down-scaling of integrated circuits, Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) became increasingly smaller, with increasingly shorter gates. This requires the junctions, particularly the junctions between lightly doped source/drain regions and pocket regions, to be shallower. However, due to the diffusion of the implanted impurities, it is very difficult to reduce the depth of the junction to about 50 Å for Short Channel Effect (SCE) control.
What makes the reduction of the junction depths more difficult is that for smaller MOSFETs, the pocket regions need to have higher impurity concentrations. However, with heavier pocket implantations, several adverse effects may be resulted. For example, although the SCE control and the Drain-Induced Barrier Lowering (DIBL) may be better, the carrier mobility of the carriers in the channel region is degraded, resulting in smaller device drive currents. To solve these problems, Silicon-On-Nothing (SON) MOSFETs have been proposed. SON MOSFETs have air-gaps under channel regions. With the air-gaps, the SCE of the SON MOSFETs is improved, and leakage currents can be reduced.