In saturation region, the depletion layer decreases by decreasing the emitter voltage drop. This can be explained as follows:

Depletion Layer in Saturation Region:
The saturation region is one of the operating regions of a transistor, specifically a bipolar junction transistor (BJT). In this region, both the base-emitter junction and the base-collector junction are forward biased.

Depletion Layer and Emitter Voltage:
The depletion layer is the region around the junction where the mobile charge carriers (electrons or holes) are absent due to the formation of an electric field. The width of the depletion layer depends on the voltage applied across the junction.

When the emitter voltage decreases, it means that the voltage drop across the base-emitter junction decreases. This reduction in voltage causes a decrease in the width of the depletion layer.

Explanation:
To understand why the depletion layer decreases when the emitter voltage drops, we need to consider the behavior of the transistor in the saturation region.

In the saturation region, the base-emitter junction is forward biased. This means that the emitter voltage is higher than the base voltage by a certain amount, typically around 0.7 volts for a silicon transistor. This forward biasing causes a flow of majority carriers (electrons in an NPN transistor) from the emitter to the base.

As the emitter voltage decreases, the voltage drop across the base-emitter junction decreases. This reduction in voltage causes a decrease in the width of the depletion layer. The decrease in the width of the depletion layer allows for a larger flow of majority carriers from the emitter to the base.

In other words, when the emitter voltage drops, the electric field across the base-emitter junction weakens, allowing more carriers to cross the junction. This increased carrier flow leads to a higher collector current, which characterizes the saturation region.

Conclusion:
In the saturation region, the depletion layer decreases by decreasing the emitter voltage drop. This is because the reduction in voltage across the base-emitter junction weakens the electric field and allows for a larger flow of majority carriers.