transmission gate

Why is transmission gate connected this way (source of NMOS connected with drain of PMOS, and source of PMOS connected with drain of NMOS)? enter image description here

Why don't we connect source of NMOS to source of PMOS and drain of NMOS to drain of PMOS?

Is there a difference?

  • \$\begingroup\$ How do you think the current will flow then? \$\endgroup\$ – a concerned citizen Dec 1 '20 at 9:39

Look at a sideview of how a MOSFET is made:

enter image description here

Note how the Source and Drain are completely the same!

For such a MOSFET (the ones that are used in ICs, not power MOSFETs, those are different) what is called the Source and what is called the Drain depends on how the current flows.

In a transmission gate we might not know yet in what direction the current flows. It could be that the current is AC and changes direction all the time. In that case you just choose the Drain and Source connections, it does not matter.

The MOSFET symbols that you use have a small arrow in the source:

enter image description here

Not all symbols have that and it is also not needed, as I wrote: Drain and Source are identical so the Source having an arrow doesn't mean that it is different. The arrow only indicates the direction of current.

This transmission gate circuit is the same as the ones you show but uses MOSFET symbols without the arrow at the Source:

enter image description here


Note how in this schematic you cannot tell what the Source or Drain is of these transistors. And it does not matter. The circuit works anyway!

  • \$\begingroup\$ I assume you mean "Note" in your last paragraph \$\endgroup\$ – slebetman Dec 1 '20 at 22:02
  • \$\begingroup\$ While a MOSFET can be made symmetrical, using a non-symmetric layout may reduce drain-gate capacitance at the expense of increasing source-gate capacitance. A lot of chips use mostly symmetric MOSFETs, but in some cases one orientation will yield better performance than the other. That having been said, I can imagine situations where a CMOS transmission gate might, depending upon usage, benefit from having the two sources connected together, or from having each transistor's source connect to the other's drain. \$\endgroup\$ – supercat Dec 1 '20 at 23:37

A conventional MOS device is bidirectional. It does not know which terminal is its source or drain, and only the voltages on those terminals will determine it.

In PMOS, the source has the higher voltage, while in an NMOS the drain has the higher voltage. Therefore it is a meaningful way to draw a transmission gate to connect the NMOS source to the PMOS drain and vice versa, but it usually does not matter, which terminal is which in this application case.

An exception could be LDMOS or other non-symmetrical devices, where the source and the drain are well defined.


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