The bipolar tube is a current control device (controlling the larger collector current through the current with a smaller base), and the MOS tube is a voltage control device (controlling the source-drain conduction resistance through the grid voltage).

    Under the conduction voltage drop of MOS tube (field effect tube), the conduction resistance is small, the gate drive does not need current, the loss is small, the drive circuit is simple, the protection diode is provided, the thermal resistance characteristic is good, and it is suitable for high-power parallel connection, but the disadvantage is that the switching speed is not high, and it is relatively expensive.

    The switching speed of the triode is high, and the Ic of the large triode can be done very well. The disadvantages are large loss, large base drive current, and complex drive.

     Generally speaking, for low cost applications, triodes should be considered first for common applications, and MOS tubes should be considered if not.

     In fact, the understanding that current control is slow and voltage control is fast is wrong. To truly understand the working mode of bipolar transistor and mos transistor, we must understand.

    The triode works by the movement of carriers. Take the emitter follower of npn tube as an example. When the base is applied without voltage, the pn junction composed of the base area and the emitter area is used to prevent the diffusion movement of multiple carriers (the base area is a hole, and the emitter area is an electron). At this pn junction, the electrostatic field from the emitter area to the base area (that is, the built-in electric field) will be induced. When the direction of the base plus positive voltage is the base area to the emitter area, When the electric field generated by the base applied voltage is greater than the built-in electric field, the carrier (electron) in the base region can flow from the base region to the emission region. The small value of this voltage is the forward conduction voltage of the pn junction (generally considered as 0.7v in engineering). But at this time, there will be electric charges on both sides of each pn junction. At this time, if the collector-emitter is applied with a positive voltage, the electrons in the emission area will move towards the base area (in fact, they move in the opposite direction) under the action of the electric field. Because the width of the base area is very small, the electrons can easily cross the base area to reach the collecting area, and compound with the holes of the PN here (close to the collector), in order to maintain balance, Under the action of positive electric field, the electrons in the collecting area accelerate the movement of the outer collector, while the holes move at the pn junction, which is similar to an avalanche process. The electrons of the collector return to the emitter through the power supply, which is the working principle of the transistor.

    When the triode is working, both pn junctions will induce charges. When the switch is on, the triode is saturated. If the triode is off, the charge induced by the pn junction will return to the equilibrium state. This process takes time. The mos triode works in a different way and has no recovery time, so it can be used as a high-speed switch.

    (1) Field effect transistor is a voltage control element, while transistor is a current control element. In the case that only a small amount of current is allowed to be taken from the signal source, the field effect transistor should be selected; When the signal voltage is low and it is allowed to take more current from the signal source, the transistor should be selected.

    (2) Field-effect transistors use most carriers to conduct electricity, so they are called unipolar devices. Transistors use both most carriers and a few carriers to conduct electricity. It is called bipolar device.

    (3) The source and drain of some FETs can be used interchangeably, and the gate voltage can also be positive or negative. The flexibility is better than that of transistors.

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

     (5) Field-effect transistors have the advantages of high input impedance and low noise, so they are also widely used in various electronic devices. In particular, using field-effect transistors as the input stage of the entire electronic equipment can achieve the performance that is difficult to achieve by ordinary transistors.

    (6) Field-effect transistors are divided into junction type and insulated gate type, and their control principles are the same.