# How to measure Vth using gmax method?

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#### walker5678

##### Full Member level 3
There are two method of measuring Vth, constant current and gmax, anyone know the details of gmax method? thanks a lot!

#### kobeghost

##### Junior Member level 2
hspice vth

there is an exact formula in baker's book,from the exercise he did with hand calculation and spice simulation ,it is quite accurate,however,it is too complicated,you can turn to the book.another question: why do you need to calculate it ,why not see it in the spectre simulation

#### walker5678

##### Full Member level 3
measure vt

Because in fab foundry's PCM data, some use constant current method to measure Vth, some use gmax method. And i found the constant current (1uA) Vth has large difference with the Vth in model files and spectre simulation. However the gmax method Vth is close to the Vth in model files and spectre simulation. So i want to know why?

#### dachuan315

##### Junior Member level 2
hspice vth_cc

if you use hspice, you can use vth(m*) or lv9(m*)

#### engrak

##### Member level 3
vth at 1ua

i doing mosfet char from long time , do u need practical metod , as we mostly use gmamx metod to measur vth of atransistor. keep vds=0.1 v (linear region )
sweep vgs =0- 5v @0.05v plot both gm and Id , from point of max. value of gm draw a tangent , the point of X-axis where this tangent touches x-axis is known as vt of mosfet.

### anil.rawat

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### phatbt03112

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#### MikeR

##### Member level 4
hspice lv9

I think for practical implementation you can use next method: build diff amp with great W/L relation input transistors and measure Vgs.

#### engrak

##### Member level 3
constant current vth

can u explain it ??

#### dwayne22

##### Junior Member level 2
vth measure

If you are asking how to measure Vt in the lab to use in spice, engrak's answer is very good. In addition, you must subtract 0.5*Vds (0r 50 mV) to be most accurate.

### anil.rawat

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### phatbt03112

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