For MOSCAP: why is surface potential constant after Vg = Vth

[jaus tail]

Hi,
I'm studying MOSCAP at university and there are 3 regions based on Vg.
Accumulation
Depletion
Inversion

For P substrate the surface potential increases as I increase the gate voltage (positive). The books say that at Vg = threshold voltage the surface potential is maximum. But why does it not increase further?
Book says that at inversion the surface is as N-type as the bulk is P-type.

What if I further increase Vg? Where will this charge go?

 

[BigJDubs]

When Vg is greater than the threshold voltage, the surface potential will start to decrease as more charge is injected into the channel. This creates a depletion region near the surface that acts as a barrier and prevents any further increase in the surface potential. As Vg increases, the depletion region becomes larger in size and the surface potential continues to decrease.