CMOS transistor nodes Source and Drain same or different?

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nschutten

Joined: 18 Jan 2009
Posts: 47
Helped: 5

19 Jan 2009 9:21 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?

sandeep_torgal

Joined: 02 Jan 2008
Posts: 121
Helped: 7

19 Jan 2009 10:09 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.

shitansh

Joined: 06 Jan 2009
Posts: 135
Helped: 12

19 Jan 2009 11:30 CMOS transistor nodes Source and Drain same or different?

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

erikl

Joined: 09 Sep 2008
Posts: 735
Helped: 138
Location: Germany

19 Jan 2009 23:55 Re: CMOS transistor nodes Source and Drain same or different

nschutten wrote:

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?

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.