Difference Between Field Effect Transistor and Thyristor

Both field effect transistors and thyristors are switching devices commonly used in electronic circuits, but there are essential differences between the two. Field effect transistors include junction field effect transistor JFET and metal-oxide semiconductor field effect transistor MOSFET. Thyristor generally refers to thyristor, which can be divided into unidirectional thyristor SCR and bidirectional thyristor Triac according to the conduction direction.

What is a Field Effect Transistor

The MOSFET is mainly introduced here. MOSFET has three electrodes, namely gate G, source S and drain D, wherein gate G is the control terminal, and source S and drain D are output terminals. From the aspect of semiconductor composition, it can be divided into NMOS and PMOS. The circuit symbols of these two MOSs are shown in the following figure:

The substrate of PMOS is an N-type semiconductor. When VGS<0, a P-channel will be formed, so it is called a P-channel MOS; while the substrate of NMOS is a P-type semiconductor, when VGS<0, an N-channel will be formed. So it is called N-channel MOS.

MOS tube is a voltage-driven device, mainly used for controllable rectification, power switch, signal amplification, etc., and is widely used. The channel of the MOS tube depends on the level of VGS. For NMOS, when VGS > 0, NMOS is turned on, otherwise NMOS is turned off; for PMOS, VGS < 0, PMOS is turned on, otherwise it is turned off.

Thyristors, also known as thyristors, are semiconductor devices like field effect transistors, and their shape and packaging are basically the same, but their uses in circuits are different.

▲ BT136 triac in TO-220 package.

▲ N-channel MOS field effect transistor in TO-220 package.

Thyristor can be divided into unidirectional thyristor and bidirectional thyristor. They can be used as electronic switches in electronic circuits to control the on-off of the load; they can be used to adjust the AC voltage, thereby realizing dimming, speed regulation, and temperature regulation. In addition, unidirectional thyristors can also be used for rectification.

Thyristors are widely used in daily life. Like voice-activated lamps used at home, low-power one-way thyristors such as BT169 and MCR100-6 are generally used as electronic switches to drive light bulbs. In the stepless dimming of incandescent bulbs or the stepless speed regulation of electric fans, high-power bidirectional thyristors such as BT136 are often used to achieve dimming or speed regulation.

Field effect transistors belong to unipolar semiconductor devices, which can be divided into junction field effect transistors and MOS field effect transistors. Each type of field effect transistor is divided into P channel and N channel. Field effect transistors can be used as amplifiers in electronic circuits to amplify signals and as switching devices to control the on-off of loads, so field effect transistors are more widely used than thyristors.

In the power amplifier circuit, using VMOS field effect transistor as the power amplifying element can improve the sound quality; in the switching circuit, when driving a large current load such as a motor, using a MOS field effect transistor as an electronic switch can reduce the burden of the pre-drive circuit ( If a thyristor is selected, a larger drive current needs to be drawn from the front-end circuit).

To sum up, the difference between FET and thyristor is: FET can not only amplify the signal, but also can be used as a high-speed electronic switch to control the on-off of the load, and can also realize functions such as speed regulation, dimming, temperature regulation, etc. It is used to form an amplifier to amplify the signal. When used as an electronic switch, the operating frequency is not as high as that of the field effect tube, and it can only be used for low-speed control.

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