Difference Between BJT and FET (How to Choose)

Author: Release time:2022-12-13 Source: Font: Big Middle Small View count:319

What is a BJT (Bipolar Junction Transistor)

 

A BJT, also known as a semiconductor triode, is a semiconductor device that controls current. Its function is to amplify a weak signal into an electrical signal with a larger amplitude, and it is also used as a non-contact switch. Usually, the BJT has the function of current amplification. Its structure is to make two PN junctions very close to each other on a semiconductor substrate. The two PN junctions divide the whole semiconductor into three parts, the middle is the base area, and the two sides are the emitters. The area and the collector area are usually arranged in two ways, PNP and NPN.

 

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The Function of the BJT

 

  • Amplify the weak signal: a small signal input to the base will cause a large current change in the collector, which is the most important and core part of the electronic circuit.


  • Use as a non-contact switch: use the cut-off and conduction characteristics of the BJT to control or drive the load; such as a gate circuit composed of a BJT, a switching power supply, etc.

 

What is a FET (Field Effect Tube)

 

Field-effect transistors are referred to as field-effect transistors, which are voltage-controlled semiconductor devices with high input resistance (107~1015Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, and safe working area. Wide and other advantages.

 

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The Function of the Field Effect Tube:

 

  • Field effect tubes can be used for amplification. Since the input impedance of the FET amplifier is very high, the coupling capacitor can have a small capacity, and it is not necessary to use an electrolytic capacitor.


  • The high input impedance of the field effect tube is very suitable for impedance transformation, and it is often used in the input stage impedance transformation of multi-stage amplifiers.


  • Field effect tubes can be used as variable resistors, electronic switches, and constant current sources.

 

12 Difference Between BJT and FET 2022 Updated

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1. Ordinary triodes that participate in conduction have both majority carriers and minority carriers, so they are called bipolar triodes; and in field effect transistors, only many electrons participate in conduction, so they are also called unipolar triodes.

 

Because the minority carrier concentration is greatly affected by factors such as temperature and radiation, the FET has better temperature stability, stronger radiation resistance, and lower noise figure than the triode. Field effect transistors should be selected when environmental conditions (temperature, etc.) vary greatly.

 

2. The BJT is a current control device, which can control the output current by controlling the base current. Therefore, the base always has a certain current, so the input resistance of the triode is low; the field effect transistor is a voltage control device, and its output current is determined by the voltage between the gate and the source, and the gate basically does not take current, therefore, Its input resistance is very high, up to 109 ~ 1014Ω. High input resistance is the outstanding advantage of FET.

 

3. The drain and source of the field effect transistor can be interchanged (some), and the gate voltage of the depletion-type insulated gate transistor can be positive or negative, and the flexibility is stronger than that of the triode. But it should be noted that for discrete field effect transistors, sometimes the substrate and the source have been short-circuited in the tube, and the source and drain cannot be used interchangeably.

 

4. Both field effect transistors and triodes can be used for amplification or as controllable switches. However, field effect tubes can also be used as voltage-controlled resistors, which can work under micro-current and low-voltage conditions. They have the advantages of low power consumption, good thermal stability, easy to solve heat dissipation problems, wide working power supply voltage range, etc., and the manufacturing process is simple. , easy to integrated production, so in the current large-scale and ultra-large-scale integrated circuits, MOS tubes occupy a dominant position.

 

5. The MOS tube has a very low interstage feedback capacitance, generally 5-10pF, while the collector junction capacitance of the triode is generally about 20pF.

 

6. The voltage amplification factor of the amplification circuit composed of field effect tubes is smaller than that of the amplification circuit composed of triodes.

 

7. Since the insulating layer between the gate and source of the MOS concept is very thin, the capacitance between the electrodes is very small, and the resistance between the gate and source is very large. When a charged object is close to the gate, a small amount of charge is induced on the gate to generate a high voltage. If the voltage is too high, it is difficult to release it, so that the insulating layer between the gate and the source breaks down, causing permanent damage. Therefore, when the tube is stored, the gate and source should be short-circuited to prevent the gate from being suspended. Especially when soldering MOS tubes, the shell of the electric soldering iron should be well grounded.

 

8. BJT uses the change of small current to control the change of large current; JFET uses the reverse voltage of PN junction to control the thickness of the depletion layer to change the width of the conductive channel, thereby controlling the size of the drain current; MOSFET uses The magnitude of the gate-source voltage is used to change the amount of charge induced on the surface of the semiconductor, thereby controlling the magnitude of the drain current.

 

9. The noise figure of the field effect tube is very small, and the field effect tube should be selected in the input stage of the low noise amplifier circuit and the circuit requiring a high signal-to-noise ratio.

 

10. The on-resistance of the triode is large, and the on-resistance of the FET is small, only a few hundred milliohms. In the current electrical devices, the FET is generally used as a switch, and its efficiency is relatively high.

 

11. The G pole of the field effect tube must have a discharge resistor to the ground, otherwise it will burn when it is powered on, and the base of the triode does not need

 

12. The field effect tube can work under the condition of very small current and very low voltage, and its manufacturing process can easily integrate many field effect tubes on a silicon chip, so the field effect tube is used in large-scale integrated circuits Has been widely used.

  

In general, whether in discrete component circuits or integrated circuits, it is a major trend for FETs to replace BJTs.

 

Summary

 

Field effect transistors(FET) are developed on the basis of triodes. The triode controls the output through the magnitude of the current, and the input consumes power. The field effect tube controls the output through the input voltage and does not consume power. The difference between FETs and BJT is voltage and current control, but these are all relative.

 

Voltage control also needs current, and current control also needs voltage, but it is relatively small. As far as its performance is concerned, FETs are significantly better than ordinary triodes. Regardless of the frequency or heat dissipation requirements, as long as the circuit design is reasonable, the use of FETs will significantly improve the overall performance.


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