ALLEVIATION OF SUBTHRESHOLD SWING AND SHORT-CHANNEL EFFECT IN BURIED-CHANNEL MOSFETS - THE COUNTER-DOPED SURFACE-CHANNEL MOSFET STRUCTURE

One of the important issues for the realization of deep, submicrometer CMOS is the choice between a buried channel or a surface channel for pMOS. Conventionally, buried-channel devices have been considered diff icult to miniaturize and operate at low voltage. This is because they have a distinct short-channel effect and a large S-swing (subthreshold swing). However, the conventional analysis considers only the case in which the buried layer is deep. The junction depth is reduced on mini aturization, but there has not been sufficient analysis for the case o f a shallow buried layer. Therefore, we have analyzed by simulation th e way in which the buried-channel S-swing and short-channel effect dep end on the depth of the buried layer and the source/drain junctions. W e have found that if the buried layer is shallow enough for complete d epletion, the S-swing becomes smaller than that for a surface channel and the subthreshold characteristics become sharper. In other words, w e have shown for the first time that the optimized structure for a min imum S-swing is not a surface channel but a buried channel. This struc ture is called CDSC (counter-doped surface-channel). It is also demons trated that the short-channel effect can be suppressed better than in a surface channel if the source/drain is shallower than the buried lay er. Therefore, CDSC is promising for deep, submicrometer pMOS.