In MOSFETs, the formation of the depletion region is controlled by the gate voltage. The current in the depletion region of the MOSFET is primarily determined by the current from the drain to the source.

When the gate voltage is lower than or equal to the threshold voltage, a depletion region exists between the drain and the source, limiting the current flow.

However, when the gate voltage exceeds the threshold voltage, the MOSFET enters the amplification or saturation region, meaning that the gate voltage is sufficiently high to completely reverse the depletion region.

In the saturation or amplification region, the current from the drain to the source in the MOSFET is no longer significantly influenced by the depletion region. Instead, it is mainly determined by the gate voltage and the electric field within the transistor.

When the gate voltage exceeds the threshold voltage, the depletion region narrows and curves, and a reverse layer forms beneath the gate, allowing more electrons to pass through (from the drain to the source).

The reverse layer acts as a conducting channel from the drain to the source. The higher the gate voltage, the thicker the reverse layer, and the voltage increase at the gate is dropped across the reverse layer, preventing further enlargement of the depletion region.

Therefore, when the gate voltage of the MOSFET exceeds the threshold voltage, the depletion region no longer plays a dominant role and becomes very small. This leads to higher conduction currents, so the depletion region does not continue to enlarge.