Why in MOSFETs Halo doping is done near source and drain junction only?

[amitrt]

Somewhere I read that the reason of halo doping in the MOSFETs is to increase the average doping of channel as the channel length is scaled to lower values. But I think there should be more about why is it done only at the source drain junction, and not somewhere else randomly like at the center.
Awaiting for your thoughtful reply.

 

[cmos]

In the short channel MOSFET, there is a possibility for the space charge regions (SCR) associated with the source and the drain to come into close proximity/contact with each other. This gives rise to the so-called effect of punch through. By performing the halo implant, you suppress the width of the SCRs.

Note that the halo is not only near the source/drain but it is also underneath the inversion channel. By doing this (as opposed to doing it "randomly like at the center"), the affect on threshold is minimal.

 

[amitrt]

Hi,

thank you for the very informative and wonderful reply.

amit