Qualitative Understanding of MOSFET in Depletion

[stn0091]

Homework Statement

No actual problem statement. I'm just having trouble understanding what is happening during the depletion and inversion stages.

See the attached picture. During the depletion stage, what does it mean by "immobile acceptor ions?"

Homework Equations

None

The Attempt at a Solution

Accumulation stage
There is a negative bias on the metal electrode that attracts positively charged hole carrier atoms to the oxide-substrate interface. Makes sense.

Delpletion Stage
Voltage on gate is increased to some positive value that is still less than the threshold. The text states that holes are repelled from the oxide-substrate interface. Hole density near that interface is reduced below the doping levels.

Here's where I'm not sure about things.

The positive charge on the gate needs to be balanced. Is this balanced by free electrons in the p-type substrate that are pulled to the oxide-substrate interface? Do these electrons fill in the holes near the oxide-substrate surface, resulting in the "immobile acceptor ions" labeled in (b) of the picture?

Inversion Stage
Gate voltage is increasingly positive resulting in more electrons being pulled to the oxide-substrate interface. An electron inversion layer is created.

Why is the electron inversion layer created? Why don't the electrons that are pulled to the interface just ionize more holes, resulting in a larger depletion layer?

 

[nilesh_pat]

Is the inversion layer created because the gate voltage is so strong that it pulls more electrons to the interface than what can be ionized?