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MOS in series with resistor - Question

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srikanth1986

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I have a N-MOSFET(take Vt=0.5V, Vg=1V) in series with a resistor(say 1K).
I apply a voltage of 3V across the structure, with both possible polarities.

1. Which case would see a larger current and why?
2. What if Vg=3V with nothing else changed?

Thanks in advance...
 

The absolute value of the current will be the same for both cases. This happens because the MOS structure is symmetrical in general. This means that the bias determines which terminal is the source and which is the drain (for NMOS source at the lower voltage terminal, drain at the higher voltage terminal) and so, when you change the polarity of the 3V, the two terminals are interchanged.

Added after 6 minutes:

If Vg is increased then the current through the MOS/resistor series combination will rise. Also, if the MOSFET was operating in the saturation region for Vg=1 then with Vg=3 it may enter the triode region, depending on both the value of the resistor and the transistor parameters.
 

This sounds like a hw problem...

First of all, its unclear how the body of the nmos is biased...is it always tied to gnd or is it tied to one of the terminals?

If we assume for now that the body is always tied to ground. Then we need to consider the transistor's resistance in the two configurations. When the transistor is placed closer to the ground node, its Vgs will be 1 V. In the other case, the Vgs will be less than 1V, making the transistor more resistive. The branch with lower total resistance will conduct more current. same in the 3V case.
 

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