In the image, threshold voltage (Vt) lies at point 4 which is a point where weak inversion has just started, why is Vt located there and not at the point of Cmin? Why is the weak inversion region shown to be starting after the C-V curve starts increasing?
(Image source: http://transport.ece.illinois.edu/ECE340S14-Lectures/ECE340Lecture34-MOS_CapII-Dist.pdf)
In MOS devices the threshold voltage is when the surface potential is twice the bulk potential. Also with CV you should think about if it's high or low frequency and what charges are responding. If you were to do a high frequency CV the curve would correspond better to your hypothesis.
When you first turn the device on the majority carriers are moving away so the capacitance decreases. Further increased bias accumulates the minority carriers and this causes the capacitance to increase.
Adam Weidling Thanks for your answer.
When the surface potential is twice the bulk potential, doesn't that then denote the onset of strong inversion? Also I understand since this is a low frequency CV curve, inversion layer formation by the thermally generated charge carriers occurs at the oxide-semicomductor interface to give a high capacitance. How is that related to defining the threshold voltage at point 4 in the graph? If capacitance shows initial trends of increasing, doesn't that denote the MOS has entered inversion which is where the threshold voltage should be defined?
Rushikesh Jadhav Ah. I see your doubt. Is there a reason you're trying to quantify Vt from the CV curve? The way that I always think about CV is what charges are responding at the given test condition. Basically when you first turn your device on you'll be conducting in the sub-threshold region. The threshold condition won't occur until you accumulate sufficient charge which I guess lies at that point on the CV curve.
This may help here they show it a little differently. (https://inst.eecs.berkeley.edu/~ee130/sp03/lecture/lecture22.pdf)
Adam Weidling Not really going into quantifying Vt, learning how to extract Dit and Qit at SiN-Si interface. This question cropped up as I was reading about threshold voltage and was confused at what point should the threshold voltage be defined. As you mentioned, what I can interpret is that the ambiguity comes in because of the time dependency of inversion layer formation since thermal generation and inversion layer formation is not a spontaneous process. So we define Vt at a point where there is significant inversion layer formation and not immediately where C starts to increase.
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