# Threshold voltage of a MOSFET

## Main Question or Discussion Point

Hi,

I recently conducted a lab experiment involving several mosfets of varying channel length and width and have plotted Ids against Vds for different values of Vgs (below is one of the graphs).

I am now trying to find the threshold voltage of each mosfet using their graphs although I am having trouble figuring out how to do that and cannot find much help that is already online (I'm probably just bad at using google). Any help concerning this would be greatly appreciated.

Thanks,
Alex

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phyzguy
If you look up the equations for the current of a MOSFET, you will find that in the saturation region:
$$I_d \propto (V_{GS} - V_T)^2$$

So, I would choose a value of Vds where the device is in the saturation region (Vds = -7V or -8V looks good), and the plot √(-Id) vs Vgs. The intercept should give you Vt.

vk6kro
The threshold voltage is the voltage needed from gate to source to make the FET conduct current from drain to source.

You can estimate this from your graph. It is about -6 volts, because no current flows at any collector voltage unless the gate voltage is at least -6 volts.

phyzguy
The threshold voltage is significantly less (in magnitude) than 6V, which you will see if you make the graph I recommended.

vk6kro
The question asked was, using THAT graph, how would you estimate the cutoff voltage.

Given that the lowest value of gate voltage given is -7 volts, you could just say " something less than 7 volts".

There may be some non linearity in the turn-on behaviour of the FET but we don't know at this stage.

The graph of drain current vs gate voltage would be much more useful, as you suggest.

phyzguy