Primary analog circuits: overview of 6-1 FET


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1. Comparison between FET and BJT

The full name of FET isField effect transistorDifferent from BJT, the core working principle of FET is to change the conductivity of semiconductor by applying an electric field on the semiconductor. In different electric fields, semiconductors exhibit different conductive properties, so they are called “field effect” transistors.

Recall BJT’s most basic formula: IC=βIBIts core working mechanism is through the base current IBControl collector current ICIt is a bit like a CCCS (current controlled current source), while FET controls the electric field inside the FET by applying different voltages to the gate of the FET, so as to control the current flowing through the FET, which is a bit like a voltage controlled current source.

B in BJT refers to “bipolar”, which means that there are both electron and hole conductive carriers in BJT. However, FET is a unipolar element, and its conduction depends on only one carrier: n-channel FET only has electrons, p-channel FET only has holes.

Although the invention time of FET is about 10 years later than BJT, various performance indexes of FET are better than BJT, such as high input impedance, low power consumption, good thermal stability and so on. Especially for high input impedance, the input impedance of general FET can reach 1 to several hundred megohm, far exceeding the typical input impedance of BJT, which is very useful in the design of amplifier circuit. In the early years, because of the high price of FET, BJT was used more in discrete component circuits. FET’s are usually used at low cost. BJT can only be used in some occasions where extreme squeezing cost is required, or when large current amplification is required, and some UHF circuits are used.

2. Classification of FET

According to its structure, there are two kinds of MOSFET: Junction FET and metal oxide semiconductor FET.

The structure of JFET is relatively simple, which is suitable for explaining the working principle of FET. However, its performance is not as good as that of MOSFET, and the working current is relatively small, so it is used less now. The performance of MOSFET is better, but its structure is slightly more complex, and it can be divided into depletion type and enhanced type. Each type has n-channel tube and p-channel tube, as shown in the figure below

Figure 6-1.01

Although there are so many types of FET, the most commonly used one is “enhanced n-channel” FET. However, no matter which type of FET, n-channel is used more than p-channel. This is because the n-channel tube is electron conductive (electrons move in the conduction band, the resistance is small), while the p-channel tube is hole conductive (the hole moves in the valence band, and the resistance is large). NMOS with the same driving ability usually occupy less area than PMOS. Therefore, n-channel transistors are preferred in circuit design. Only when designing some special counterpart circuits that require N-P pairing, The p-channel is used.

In addition, there is a variant of JFET: metal semiconductor FET, which is generally used in RF and high-speed computing, as long as you know it.

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