Principles of Field Effect Transistor Application

Working principle: The working principle of field-effect transistors can be summarized as follows: "The ID flowing through the channel between the drain and source is controlled by the reverse bias gate voltage formed by the pn junction between the gate and the channel. More precisely, the width of the ID flowing through the channel, i.e. the cross-sectional area of the channel, is controlled by the depletion layer expansion caused by the change in the reverse bias of the pn junction. In the unsaturated region of VGS=0, the extension of the transition layer is not very large. According to the electric field of VDS applied between the drain and source, some electrons in the source region are pulled away by the drain, that is, a current ID flows from the drain to the source. The transition layer extending from the gate to the drain forms a blocking type in the channel, causing ID saturation. This state is called a pinch off. This means that the transition layer blocks a part of the channel, rather than cutting off the current.

In the transition layer, due to the lack of free movement of electrons and holes, it has almost insulating properties in an ideal state, and current is usually difficult to flow. But at this time, the electric field between the drain and source is actually two transition layers in contact with the lower part of the drain and gate, and the high-speed electrons pulled away by the drift electric field pass through the transition layer. The saturation phenomenon of ID occurs due to the almost constant strength of the drift electric field. Secondly, VGS changes in the negative direction, making VGS=VGS (off), and the transition layer roughly covers the entire area. Moreover, the majority of the electric field in VDS is applied to the transition layer, pulling electrons towards the drift direction, with only a short portion near the source, which further hinders current flow.

effect:

1. Field effect transistors can be applied for amplification.

The high input impedance of field-effect transistors is very suitable for impedance transformation.

3. Field effect transistors can be used as variable resistors.

4. Field effect transistors can be conveniently used as constant current sources.

Field effect transistors can be used as electronic switches.