Fixed effective charge in flat band voltage

[anhnha]

I am having a hard time understanding flat band voltage in MOS capacitor.
Please see the picture below about MOS capacitor.
Q₀ is a fixed effective charge at the surface between oxide and semiconductor and it is positive here.
The voltage across the oxide caused by the fixed charge ψ_ox = -Q₀/C_ox.
So we need to use an external voltage to cancel this voltage ψ_ox.
What I don't get here is the direction of external voltage for cancelling ψ_ox.
As you can see from the picture, it has the same direction as ψ_ox.
Should it be in the opposite direction so ψ_ox can be cancelled?
Thank you.

 

[BigJDubs]

No, the external voltage should be in the same direction as ψox for it to be cancelled. In other words, the voltage of the MOS capacitor is the sum of the fixed charge voltage (ψox) and the external voltage. When the two voltages are equal and opposite, they cancel each other out and the total voltage across the capacitor is zero, which is known as the flat band voltage.