How to identify transistor VT from simulation?

[abuelmaatti]

transistor vt

Folks, have got a very simple yet very difficult question.

How do I identify (accurately) the threshold voltage of a small-scale (l=180nm, w=100um) NMOS transistor from DC simulation? We all know that the threshold voltage is the voltage which if Vgs was higher than, current will start following in the channel and the transistor will be considered ON. The question is though, at what point do we say, current is now following? (This is like when somebody says large but doesn’t specify how large is large) because even at very small gate voltages there is still current in the channel but it is very small. So what is the ACCURATE point at which we say now current is following in the channel, so that is VT?

I carried out a VG sweep at a fixed VDD on the transistor and plotted the current against my swept gate voltage. We all know how this will look like, the current will be zero at very small gate voltages then when the gate voltage exceeds VT the current will start increasing in a square-law (I think?!) manner. Now, if I zoom into the graph near where the current starts (visibly) increasing I would notice that at these small voltages the current is not actually zero but has a small value and is increasing gradually, so it all depends on how much you zoom in. So how can I identify that ACURATE point where I would say this is a small current and this is a large current so that must be VT?

Thanks in advance for your help
Ali

 

[alpeshchokshi]

how to identify a transistor

if you will go for short length device like less than .18um the several short channel effects becomes prominent... like GIDL,DIBL, punchthrough... due to that the leakag current will start to flow even with vgs=0,,,
u just refer Tsividis-textbook thats really very good

 

[nxing]

vt transistor

there is not a sharp transistion from sub-threshold operation to moderate inversion. Usually, when you have the Id ~ Vg curve with the fixed vds, you will get a somehow constant slop curver after some Vg, just extend that curve toward you x-axis and the intercept point will be quite accute of your Vth

 

[dipswitch]

what is vt of a transistor

As many definitions of "Vt", there are also as many methods of measuring Vt. The most standard method we use in industry is called Maximum-gm extrapolation. It is good for most practical purposes:

(1) Set Vds = 0.1V (this ensures device is in linear region)
(2) Plot Ids versus Vgs.
(3) Also plot gm (derivative of Id) versus Vgs.
(4) Find the slope of Id curve at the point of maximum gm.
(5) Extrapolate that slope down to Id=0.
(6) The Vt is equal to the Vgs intercept at Id=0, minus Vds/2

Mathematically, this is equivalent to using the Id equation in linear region, and setting Id=0. Solving, you get Vt = Vgs - Vds/2

Good luck!

 

[jamesqjf]

vt from id-vg curve

If you use spectre for the simulation, you can do the DC simulation and save the bias point information, the you can click Print->DC operation points, then you choose the MOS you want to see, the Vt is automatically showed up in the window