What is the Field Effect Transistor(FET)?

Field Effect Transistor, FET. For short, the field effect tube is expressed by the letter Q, PO,MN. The FET belongs to the voltage controller, and the output current is controlled by the electric field effect caused by the input voltage.

The FET is mainly divided into junction FET and insulated gate FET. The junction FET is divided into N channel and P channel. Insulated gate FET is also called metal oxide semiconductor FET (MOS FET), which is divided into depleted MOS transistor and enhanced MOS transistor.There are also N channel and P channel. The symbol is shown in figure 1.In the circuit diagram, the N channel and P channel MOS tube are distinguished by looking at the middle arrow of the MOS tube. The cut head is an N channel MOS tube and the arrow outward is a P channel MOS tube.

The majority of FET in mainboard power supply circuit are insulated gate enhanced FET with N channel, as shown in Fig. 2.

（a）N-channel MOS tube  （b）P-channel MOS tube

Fig.1 MOS tube symbol

Fig.2   Physical drawing of MOS Tube in Common use on main Board

One FET has three poles like the Triode, which are grid (expressed in G, also called

control pole), drain (expressed in D, also called input pole) and source (expressed in S, also known as output pole). The shape and pin arrangement of the MOS tube are the same, the left pin is G pole, the middle pin is D pole (same as above), and the right pin is S pole.