Working principle of MOSFET

Working principle of MOSFET

MOS Field Effect Transistor is also called MOS FET, which is the abbreviation of Metal Oxide Semiconductor Field Effect Transistor (Metal Oxide Semiconductor Field Effect Transistor). It generally has two kinds of depletion and enhancement. Here we take the enhanced MOS as an example.

Field effect transistors are composed of source, drain, and gate, and can be divided into N-channel and P-channel field effect transistors due to different substrate doping. (Channel: As the name implies, two fillable trenches are dug on the substrate to fill N-type or P-type semiconductors)

We connect the drain to the positive pole of the power supply, and the source to the negative pole of the power supply.

For field effect transistors, when there is no voltage on the gate, the source and drain are equivalent to two back-to-back diodes, and no current will flow. At this time, the field effect transistor is in the cut-off state.

When a voltage is applied to the gate, when the voltage is less than a threshold VGS(th), the holes in the P-type semiconductor will be pushed away due to the action of the electric field between the gate and the substrate P, and at this time, the N-type semiconductor The negative electrons of the source and drain are attracted to the gate, but due to the barrier of the oxide film, the electrons gather in the P-type semiconductor between the two N channels.

As the gate voltage increases, the electron concentration near the gate increases. When a threshold value VGS(th) is exceeded, the N-type semiconductor between the source and drain forms an electron channel. At the same time, due to the positive voltage applied to the drain, a current from the drain to the source can be formed, and the MOS transistor is turned on.

We can also imagine that there is a trench between two N-type semiconductors. The establishment of gate voltage is equivalent to building a bridge between them. The size of the bridge is determined by the magnitude of the gate voltage. This is why the MOSFET is said to be a voltage-controlled transistor.