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what is the Cause of non-linearity in sub-threshold region

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simplsoft

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Hello

I have a question. what is the main cause of non-linearity in sub-threshold reason.

In saturation reason as we know ID= 0.5Kn'(W/L)(VGS-th)2, and the VGS square term is creating the non-linearity. What term is creating non-linearity in sub-threshold region?
 

It's Boltzman.
Boltzman distribution (of carrier concentration vs energy - where only carriers with energy larger than the barrier energy, on the path from source to channel and drain, are contributing to source-drain sub-threshold current)
Current ~ exp(-E/kT), and E=a-b*Vg, where Vg is the gate voltage.
 

Thanx for your reply.

Can you please explain it more as I didnt get you. Can you explain with equations too to define what term is creating non-linearity.

Sub-threshold region is the region when VGS is less than VT and ID=Ise(-vgs/nVT).(1-e-Vds/Vt).
What term is creating non-linearity. What term we have to improve to reduce non-linearity?
 

I thought you are talking about linearity / nonlinearity of Ids with respect to Vgs.
Since Vgs is in the exponent, the exponent is the root cause of the Ids(Vgs) nonlinearity.
The physical mechanism of the exponent is Boltzman distribution.
 

In op-amps, I have read in many research papers that It the linearity is very low around 75mV. the linearity has to be increased. It means if I want to improve linearity of op-amp and I am operating the transistors in sub-threshold region then I have to reduce vgs? M I right
 

simplsoft said:
Thanx for your reply.

Can you please explain it more as I didnt get you. Can you explain with equations too to define what term is creating non-linearity.

Sub-threshold region is the region when VGS is less than VT and ID=Ise(-vgs/nVT).(1-e-Vds/Vt).
What term is creating non-linearity. What term we have to improve to reduce non-linearity?
Click to expand...

Is it not obvious ?? Exponential term brings the non-linearity.If you expand this exponential term into series, you will see that the term consists of N order polynomial equation.
 

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