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Wikipedia has several pictures of MOSFETs and various types of them. Here is one.

enter image description here

Notice that the Source and Body are connected. I understand this is to ensure there is no forward-bias or reverse-bias between them.

What I don't understand is, what is that "P+" region where the body connection exists? What is it and what is it's purpose? If it's a more highly-doped p-region then I don't understand why.

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1 Answer 1

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In a semiconductor, you can't generally make a resistive (ohmic) contact to a lightly doped region (as in the p substrate or bulk here). Such a contact would likely be very resistive, or act as a Schottky diode.

So (just like the N+ in the drain), a very heavy P+ diffusion is made for the metallic contact. This creates a degenerate (i.e. metallic or conductive) junction between the metal and the semiconductor, thus giving a good electrical contact to the bulk.

A secondary advantage of this is that the P+ bulk and N+ source can be quite close together. Having a low resistance connecting shorting these makes the FET more robust during high VDS voltages.

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  • \$\begingroup\$ Thank you, this explains a lot. But a follow-up question, is the same thing required on the opposite mosfet? So an n-substrate and p-type channels, do you need an N+ region for the ohmic contact? \$\endgroup\$
    – DrZ214
    Sep 5, 2021 at 22:18
  • \$\begingroup\$ Yes; you need a ++ contact for all lightly doped regions. \$\endgroup\$
    – jp314
    Sep 6, 2021 at 0:22

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