The doping profile of the source and drain region has an important impact on
the device characteristics. On the one hand, it is desirable to have shallow
junctions to reduce the influence of the drain on the channel and to improve
the gate control over the inversion charge. On the other hand, a deep and
heavily doped source and drain region reduces the series resistance. One
possibility to achieve both is to introduce lightly doped drain (LDD)
regions, where a deep implant is used at the contact and connected via a
shallow implant to the channel. Another approach is to use raised
source/drain contacts which are formed at a higher elevation than the
channel [22].
A. Gehring: Simulation of Tunneling in Semiconductor Devices
