Why is a Flat Band Voltage Used in MOS Capacitors?

[xhwubai]

Homework Statement

In mos capacitor, energy band will bend because of different work function between M&S.

But Why apply a "flat band voltage" to flat the bent band? what's the purpose?

Homework Equations

The Attempt at a Solution

I have been looking for the answer from many books, and can't find. I just wana know why.

 

[Defennder]

Hi I don't think there may be any "purpose" in doing so. It's just a concept we understand the MOS cap with. Some notes and textbooks may start off by considering an ideal case of a MOS cap where the bands are originally flat and then develop the concepts and equations from there. So it is inevitable that sooner or later they would have to introduce a "flat band voltage" concept to account for the case whereby the MOS cap does not originally start off with band level which is the same throughout.

It's just like in intro freshman physics of mechanics, we start off by assuming zero friction to simplify the setup. Only afterwords, once the basic concepts have been taught, would factors like friction and drag come into play.

 

[xhwubai]

Thank Defennder very much! I got it!