Classification, Structure, and Principle of MOSFET

Classification, Structure, and Principle of Field Effect Transistors

Field Effect Transistor (FET), abbreviated as FET, is a relatively new type of semiconductor material that uses the electric field effect to control the current of the transistor, hence its name. It is a semiconductor device that uses only one type of carrier to participate in conduction, which is a semiconductor device that controls the output current with input voltage. Divided by the carriers involved in conduction, it has N-channel devices with electrons as carriers and P-channel devices with holes as carriers. From the structure of field-effect transistors, they can be divided into junction field-effect transistors and insulated gate field-effect transistors.

1. Junction field-effect transistor

(1) Structure of junction field-effect transistor

It involves manufacturing PN junctions on both sides of an N-type semiconductor silicon wafer, forming a structure with two PN junctions sandwiching an N-type channel. The two P regions are the gate, and one end of the N-type silicon is the drain and the other end is the source.

(2) Working principle of junction field-effect transistor

Using N-channel as an example to illustrate its working principle.

When VGS=0, when a certain voltage is applied between the drain and source, multi carrier drift motion will be formed between the drain and source, resulting in drain current. When VGS is zero, the PN junction is reverse biased, forming a depletion layer. The channel between the drain and source will narrow, and the ID will decrease. VGS continues to decrease, the channel continues to narrow, and the ID continues to decrease until it reaches zero. The gate source voltage VGS corresponding to zero drain current is called the pinch off voltage VGS (off).

(3) Characteristic curve of junction field-effect transistor

There are two characteristic curves for junction field-effect transistors,

One is the output characteristic curve (ID=f (VDS) | VGS=constant),

The second is the transfer characteristic curve (ID=f (VGS) | VDS=constant).

Characteristic curve of N-channel junction field-effect transistor

2. Working principle of insulated gate field-effect transistor

Insulated gate field-effect transistors are divided into:

Depleted type → N-channel, P-channel
Enhanced → N-channel, P-channel