# Fixed effective charge in flat band voltage

• A
• anhnha

#### anhnha

I am having a hard time understanding flat band voltage in 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 ψox = -Q0/Cox.
So we need to use an external voltage to cancel this voltage ψox.
What I don't get here is the direction of external voltage for cancelling ψox.
As you can see from the picture, it has the same direction as ψox.
Should it be in the opposite direction so ψox can be cancelled?
Thank you.

#### Attachments

• Flat band voltage.PNG
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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.