When does the NMOS form the channel? VGS > VTH or VGB > VTH?
I see that textbook talks about VGS > VTH, channel formed, but the capacitor is formed between the gate and the body, so i think it should be VGB >VTH?
When does the NMOS form the channel? VGS > VTH or VGB > VTH
cmos
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Best Answer
An n-channel mosfet forms channel when $$V_{GS} > V_{TS}\tag1$$ or when $$V_{GB} > V_{TS} + V_{SB}\tag2$$ (Both conditions are actually same)
Where \$V_{TS}\$ is the threshold voltage with source as reference terminal and is given by: $$V_{TS} = V_{T0} + \gamma(\sqrt{\phi_t+V_{SB}}-\sqrt{\phi_t})$$
\$\gamma\$ and \$\phi_t\$ are device parameters (see body effect). \$V_{T0}\$ is the threshold voltage with source and bulk shorted (\$V_{SB}=0\$). The threshold voltage given in datasheet is \$V_{T0}\$ .
The point I want to make is that you can not say that a mosfet will form channel if either \$V_{GS}\$ is greater than a particular voltage or \$V_{GB}\$ is greater than a particular voltage without the knowing the voltage between source and bulk, \$V_{SB}\$.
For example, for an n-MOS of threshold voltage 3V, \$V_{GB} = 4V\$ does not ensure channel formation if source terminal is at a higher potential than bulk (or body) terminal.
One can actually defined two threshold voltages- \$V_{TS}\$ with respect to source and \$V_{TB} (= V_{TS} + V_{SB})\$ with respect to bulk. Text books and articles, especially circuit related, usually defines it with respect to source terminal.