How Field Effect Transistors (MOS) Work

  The working principle of the FET is in one sentence, that is, "the ID flowing through the channel between the drain and the source is used for the connection between the gate and the channel. The reverse biased gate voltage formed by the pn junction controls ID". To be more precise, the width of the path through which ID flows, that is, the cross-sectional area of the channel, is controlled by the change in the reverse bias of the pn junction, resulting in a change in the expansion of the depletion layer. In the unsaturated region of VGS=0, because the expansion of the transition layer is not very large, according to the electric field of VDS applied between the drain and the source, some electrons in the source region are pulled by the drain, that is, from the drain A current ID flows to the source. The transition layer extending from the gate to the drain forms part of the channel as a plugging type, and the ID is saturated. This state is called pinch-off. This means that the transition layer blocks part of the channel, not the current being cut off.

Because there is no free movement of electrons and holes in the transition layer, it has almost insulating properties under ideal conditions, and it is usually difficult for current to flow. But at this time, the electric field between the drain and the source is actually two transition layers in contact with the lower part of the drain and the gate, and the high-speed electrons pulled by the drift electric field pass through the transition layer. The saturation phenomenon of ID occurs because the intensity of the drift electric field is almost constant. Secondly, VGS changes in the negative direction, and let VGS=VGS(off), at this time, the transition layer roughly covers the whole area. Moreover, most of the electric field of VDS is applied to the transition layer, and the electric field that pulls electrons to the drift direction is only a short part close to the source, which makes the current unable to flow.

Field effect transistors are divided into junction field effect transistors (JFET) and insulated gate field effect transistors (MOS transistors).

According to the channel material type and the insulated gate type, there are two types of N-channel and P-channel; according to the conduction mode: depletion type and enhancement type, junction field effect transistor All are depletion type, and insulated gate FETs have both depletion and enhancement types.

Field effect transistors can be divided into junction field effect transistors and MOS field effect transistors, and MOS field effect transistors are divided into N-channel depletion and enhancement; P-channel depletion There are four types: type and enhanced type.