Working principle of MOSFET

Working principle of MOSFET

MOS FET is also called MOS FET, which is the abbreviation of metal-oxide-semiconductor field-effect transistor. It generally has two types: depletion type and enhancement type. Here we take enhanced MOS as an example.

FET is composed of source, drain and gate, which can be divided into N-channel and P-channel FET due to different doping of substrate. (Channels: As the name implies, two fillable channels are dug on the substrate and filled with 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 FET, when there is no voltage at the gate, the source and drain are equivalent to two back-to-back diodes, and no current will flow. At this time, FET is in the off state.

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

With the increase of gate voltage, the electron concentration near the gate will increase. When a threshold value VGS(th) is exceeded, an electron channel will be formed in the N-type semiconductor between the source and the drain. At the same time, because the positive voltage is applied to the drain, the current from the drain to the source can be formed, and the MOS diode is turned on.

We can also imagine that there is a trench between two N-type semiconductors, and the establishment of gate voltage is equivalent to building a bridge between them, and the size of the bridge is determined by the size of the gate voltage. This is why MOS transistors are voltage-controlled transistors.