MOSFETs can be classified into two types based on the conductivity of their channels: N-channel and P-channel. In the process of forming the conductive channel in a MOSFET, two concepts are involved: the depletion layer and the inversion layer. So, what are the differences between them?

The depletion layer, as the name suggests, refers to the region in a MOSFET where electrons are depleted.

In an N-channel MOSFET, when a positive voltage is applied to the gate, electrons near the gate are repelled, creating a region of electron deficiency, which is the depletion layer. Conversely, in a P-channel MOSFET, when a negative voltage is applied to the gate, holes near the gate are repelled, creating a region of hole deficiency, which is also the depletion layer.

It's important to note that the carrier concentration in the depletion layer is very low, almost zero, so it has a high resistance.

On the other hand, the inversion layer refers to the region in a MOSFET where the carrier type is reversed due to the application of gate voltage.

In an N-channel MOSFET, when a positive voltage is applied to the gate, in addition to repelling holes, it also attracts free electrons in the substrate. These electrons are attracted to the region between the depletion layer and the insulating layer (SiO2), forming an n-type thin region called the inversion layer. The carrier type in the inversion layer is opposite to that of the substrate, so in an N-channel MOSFET, the inversion layer is p-type. Similarly, in a P-channel MOSFET, when a negative voltage is applied to the gate, in addition to repelling electrons, it also attracts holes in the substrate.

These holes are attracted to the region between the depletion layer and the insulating layer, forming a p-type thin region called the inversion layer. The carrier type in the inversion layer is opposite to that of the substrate, so in a P-channel MOSFET, the inversion layer is n-type.

So, what are the differences between the depletion layer and the inversion layer in a MOSFET?

In simple terms, they are both special regions formed in a MOSFET due to the application of gate voltage, but they differ in carrier type. The carrier concentration in the depletion layer is almost zero, while the carrier type in the inversion layer is opposite to that of the substrate. Additionally, the depletion layer changes with the gate voltage in the operation of the MOSFET, while the inversion layer is relatively stable.

Furthermore, the roles of the depletion layer and the inversion layer in a MOSFET are different: the depletion layer mainly isolates the source and drain, preventing current from flowing; whereas the inversion layer forms the conductive channel between the source and drain, responsible for current transmission.

In summary, the difference between the two can be summarized in one sentence: the depletion layer is an "electron desert," while the inversion layer is an "electron channel."