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Double Diffusion MOS transistor.

Use a n-doped semiconductor to build a n-channel-type device. Make a p-diffusion. Inside, put a n-diffusion. Put field oxide over p-diffusion, between epi and n-diffusion. Put gate polysilicium. The drain can be another n-diffusion at the side (lateral DMOS) or a n-buried layer (vertical DMOS).

        ________________
       |                     <-- field oxide
------------------------
source  |        | <----- channel
--------         |
                 |
p-well          /
---------------


n epi


--------------------------------------------
                  buried layer (drain)

Its vertical variant is usually used to take big voltages as its drain area can be very long and is usually only doped lightly.

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