When using field-effect transistors, these precautions should be remembered

Field Effect Transistor (FET), abbreviated as Field Effect Transistor. Conducted by majority carriers, also known as unipolar transistors. It belongs to voltage controlled semiconductor devices. The new generation of amplification components developed based on the principle of the transistor has three polarities: gate, drain, and source. Its characteristic is the extremely high internal resistance of the gate, which can reach several hundred megaohms using silicon dioxide material. It belongs to voltage controlled devices.

1. Classification of Field Effect Transistors

According to the structure of field-effect transistors, they can be divided into two categories: junction field-effect transistors (JFETs) and insulated gate field-effect transistors (MOSFETs).

According to the channel material, there are two types: N-channel and P-channel, for junction type and insulated gate type.

According to the conduction mode: depletion type and enhancement type, junction field-effect transistors are both depletion type, and insulated gate field-effect transistors have both depletion type and enhancement type.

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

2. Comparison between Field Effect Transistor and Bipolar Transistor

(1) Field effect transistor is a voltage control element, while transistor is a current control element. Field effect transistors should be used when only a small amount of current is allowed to be drawn from the signal source; When the signal voltage is low and more current is allowed to be drawn from the signal source, transistors should be selected.

(2) Field effect transistors utilize both majority and minority carriers for conduction, and due to the sensitivity of minority carrier concentration to external conditions such as temperature and radiation, it is more suitable to use field effect transistors for situations with significant environmental changes.

(3) The source and drain of field-effect transistors are symmetrical and can be used interchangeably. The gate source voltage of depletion MOS transistors can be positive or negative, making them more flexible than transistors.

(4) Field effect transistors can operate under very low current and voltage conditions, and their manufacturing process can easily integrate many field effect transistors on a single silicon wafer. Therefore, field effect transistors have been widely used in large-scale integrated circuits.

(5) Field effect transistors can be used not only as amplifying devices and controllable switches like transistors, but also as voltage controlled variable linear resistors.

(6) Transistors have high on resistance, while field-effect transistors have low on resistance, only a few hundred m Ω. The efficiency of using field-effect transistors as electronic switches is higher than that of transistors.

 3. Precautions for using field-effect transistors

(1) In order to safely use field-effect transistors, the dissipated power of the transistor should not be exceeded in the design of the circuit. The limit values of parameters such as maximum drain source voltage, maximum gate source voltage, and maximum current.

(2) When using various types of field-effect transistors, they must be strictly connected to the circuit according to the required bias, and the polarity of the field-effect transistor bias must be observed. There is a PN junction between the gate source drain of a junction field-effect transistor. N-channel transistor gate cannot be biased positively, P-channel transistor gate cannot be biased negatively, and so on.

(3) Due to its extremely high input impedance, MOSFETs must short-circuit their lead pins during transportation and storage. Metal shielding packaging should be used to prevent the gate from being broken down by external induced potentials. Special attention should be paid to not placing MOS field-effect transistors in plastic boxes. It is best to store them in metal boxes and also pay attention to the moisture resistance of the diodes.

(4) In order to prevent gate induction breakdown of field-effect transistors, it is required that all testing instruments, workbenches, soldering irons, and circuits themselves must have good grounding. When soldering the pins, the source should be soldered first; Before connecting to the circuit. All lead terminals of the diode should be kept in a short circuited state, and the short circuiting material should be removed after welding. When removing the diode from the component rack, appropriate measures should be taken to ensure human grounding, such as using grounding rings; Of course, it is more convenient to use advanced gas thermal soldering iron to solder field-effect transistors, and it can ensure safety. When the power is not turned off, the diode should never be inserted or removed from the circuit. The above safety measures must be taken into account when using field-effect transistors.

(5) When installing field-effect transistors. Pay attention to the installation location and try to avoid being close to the heating element as much as possible; To prevent vibration of the pipe fittings, it is necessary to tighten the pipe shell.

(6) For power type field-effect transistors, good heat dissipation conditions are required. Because power type field-effect transistors are used under high load conditions. Sufficient heat sinks must be designed to ensure that the shell temperature does not exceed its rated value. Ensure the long-term, stable, and reliable operation of the device.