How Inversion Layer form in a MOSFET?

Imagine you have a MOSFET with a P-type substrate, meaning the majority carriers in the substrate are holes (positively charged). The MOSFET also has N-type regions called the source and drain, where electrons (negatively charged carriers) are prevalent.

Step-by-Step Formation of the Inversion Layer

At the beginning, there's no voltage applied to the gate, so the source and drain are separated by the P-type substrate, which doesn't conduct because it forms a PN junction with the N-type regions.

Applying the Gate Voltage

When you apply a positive voltage to the gate (relative to the substrate), something interesting begins to happen. This positive voltage creates an electric field that influences the semiconductor below the gate. The electric field from the gate repels the positively charged holes (majority carriers) in the P-type substrate, pushing them deeper into the substrate, away from the surface. As the holes move away, a depletion region forms near the gate. This region lacks free-charge carriers (holes), creating a barrier.

Creating the Inversion Layer

If you keep increasing the positive gate voltage, the electric field becomes strong enough to attract the minority carriers, which are electrons in the P-type substrate. These attracted electrons gather near the silicon/oxide interface, just beneath the gate. As more electrons accumulate, they create a thin, conductive layer called the inversion layer.

Example

Imagine you have a room filled with people (holes) and a magnet (positive gate voltage) above the ceiling. As you increase the magnet's power, the people (holes) move away from the ceiling. Eventually, the magnet is strong enough to pull metal filings (electrons) from the floor towards the ceiling, forming a thin layer of metal filings right below the magnet. This layer allows a pathway for electric current, just like the inversion layer in a MOSFET.