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

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amitrt
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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.
 
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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.
 
Hi,

thank you for the very informative and wonderful reply.

amit