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Just for joking, if LDMOS (lateral diffusion MOS) had vertical diffusions, they'll be called VDMOS;-)
As sunkin said, the D region must be contacted someway. If the diffusion would be vertical, somewhere it has to bend up in order to be copntacted at the Si surface. Remember: CMOS is a plannar technology. It is always easier to make 2D (with some depth evidently) structures than to do them in 3D. Also keep in mind that most of CMOS wafer are just a bulk of Si. If you want to make 3D structures you should have some epi layers or something similar.
Vertical DMOS in IC technology forms with buried layer, that is main current flows through base in vertical direction, then by means of buried layer current flows horizontal into drain.
In horizontal DMOS the resistance of bulk into vertical direction too large and
main current flows in horizontal direction.
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