In a MOSFET the channel is formed between gate and the body it would sense to use gate body voltage to form the channel. Why is gate source voltage used to form the channel?
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1\$\begingroup\$ It is just laziness. Get used to it.. we want the operation of mosfet to be simple, so we assume the body and source are shorted, and "pretend" that Vgs controls channel. But this simplification breaks quickly - for example, in a simple NAND gate where we stack two nmosfets, the threshhold voltage of stacked nmos increases. Then we rationalize this increase by saying the threshhold voltage of stacked nmos increased because source was at higher potential compared to body. Google body effect.. \$\endgroup\$– acrossCommented Mar 8, 2022 at 5:40
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\$\begingroup\$ Why don't we actually short the body and the source then ? As the source voltage will always be different from the body voltage \$\endgroup\$– mahmoud esmailCommented Mar 8, 2022 at 6:03
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\$\begingroup\$ In integrated circuits, body/substrate is shared by all the nmos transistors. This means the body terminals of all the nmosfets are at the same potential, usually 0V. Think of body as some common surface on which nmos transistors are grown. Now if you connect source and body you essentially ground the source. So you connect source to body only when you want the source of nmos grounded.. \$\endgroup\$– acrossCommented Mar 8, 2022 at 6:15
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\$\begingroup\$ Another way to put it is, you can't control body of each nmosfet independently. Because of the way an integrated circuit is manufactured, all the nmosfets have the same body terminal. (However, when it is needed, isolating bulk terminals of individual nmosfets is possible, but this takes up lot of area and never done in digital circuits.) \$\endgroup\$– acrossCommented Mar 8, 2022 at 6:26
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