MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor), that is, metal-oxide-semiconductor field effect transistor, belongs to the insulated gate type. Its main feature is that there is a silicon dioxide insulating layer between the metal gate and the channel, so it has a very high input resistance (up to 1015Ω). It is also divided into N-channel tube and P-channel tube. Usually, the substrate (substrate) and the source S are connected together. According to different conduction methods, MOSFETs are divided into enhancement type and depletion type. The so-called enhanced type refers to: when VGS=0, the tube is in the cut-off state. After adding the correct VGS, most carriers are attracted to the gate, thereby "enhancing" the carriers in this area and forming a conductive channel. . The depletion type means that when VGS=0, the channel is formed, and when the correct VGS is added, the majority of carriers can flow out of the channel, thus "depleting" the carriers and turning the tube to cut off.
Taking the N-channel as an example, two highly doped source diffusion regions N+ and drain diffusion regions N+ are formed on a P-type silicon substrate, and then the source S and the drain D are respectively drawn out. The source and the substrate are connected internally, and the two are always kept at the same potential. When the drain is connected to the positive pole of the power supply, the source is connected to the negative pole of the power supply and VGS=0, the channel current (that is, the drain current) ID=0. As VGS gradually increases, attracted by the positive gate voltage, negatively charged minority carriers are induced between the two diffusion regions, forming an N-type channel from the drain to the source. When VGS is greater than the tube When the turn-on voltage VTN (generally about +2V) is reached, the N-channel tube starts to conduct, forming a drain current ID.