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CMOS transistor nodes Source and Drain same or different?

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nschutten

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interchangeable drain source

I just started custom IC design and was wondering about the following: when looking at the layout of a simple CMOS transistor, it seems that source and drain are fully interchangeable. Is this correct or are there things to worry about / take care off when deciding between source and drain connections?
 

drain source interchangeable

Both the source and drain are same. However it is best to have more contacts at the source side rather than drain.
 

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    nschutten

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Can you please tell me reason for this more contacts on source side?
 

Re: CMOS transistor nodes Source and Drain same or different

nschutten said:
when looking at the layout of a simple CMOS transistor, it seems that source and drain are fully interchangeable. Is this correct or are there things to worry about / take care off when deciding between source and drain connections?
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For schematic simulation this is totally correct. For layout, it depends: For a simple CMOS transistor, this is also correct. But you also need a bulk contact, and this one should be close to the source, as not only Vgs controls the MOSFET current, but also Vgb (to a lesser extent).
With more complex transistor structures (e.g. interdigitized or "fingered"), one usually has a source stripe at both sides of the transistor structure (i.e. one more source than drain stripes), and in this case of course more source than drain contacts.
 

The function of the diffusion area in your MOSFET depends on potentials connected to them. For a nMOSFET, the drain has higher potential: VD>=VS.
 

i think they are interchangable in layout. however terminal near to bulk connection is taken generally considerred as source.
 

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    nschutten

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For general low voltage CMOS, the drain and the source are interchangeable while for many HV CMOS, the drain is different from the source.
 

Re: CMOS transistor nodes Source and Drain same or different

For schematic simulation this is totally correct
 

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