Electrical – 4-terminal MOSFET in this CMOS circuit

cmos

The question is which logic function does this circuit implement if A and B are input and Y is output?

I'm not familiar how 4-terminal MOSFET works but trying to solve Y using A=0 B= 0

The abovest transistor wouldn't be turned on because \$V_{SG} = 0\$

The belowest transistor also cannot be turned on because \$V_{GS}\$ is negative.

For the transistor having \$V_{DD}\$ at base, which side should we choose as a source?

Edit: I still don't know where to expect source when \$V_{DD}\$ and \$GND\$ are ignored as it is said in the comments. I would appreciate an explanation in solving this.

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Best Answer

The 4 terminal MOSFET is a symmetrical device. Drain and source are interchangeable. For a 3-terminal MOSFET the source is the one that's connected to the body.

What you have here is called a transmission gate. Notice that the top MOSFET is a PMOS while the bottom is an NMOS, so they will both be 'on' at the same time (when \$\bar{A}\$).

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