Bipolar transistor is a current control device (controlling a larger collector current through a smaller base current), and MOS transistor is a voltage control device (controlling the on resistance between source and drain through a gate voltage).

    Under the on voltage drop of MOS transistor (field effect transistor), the on resistance is small, the gate drive does not need current, the loss is small, the drive circuit is simple, the self-contained protection diode has good thermal resistance characteristics, which is suitable for high-power parallel connection. The disadvantage is that the switching speed is not high and it is expensive.

    The switching speed of triode is high, and the IC of large triode can be very large. The disadvantages are large loss, large base driving current and complex driving.

    Generally speaking, for low-cost applications, triodes are considered first for ordinary applications, and MOS transistors are considered if not.

    In fact, it is wrong to say that current control is slow and voltage control is fast. To truly understand, we must understand how bipolar transistors and MOS transistors work.

    Triode works by the movement of carriers. Take the emitter follower of NPN tube as an example. When the base is applied with or without voltage, the PN junction composed of the base region and the emitter region prevents the diffusion movement of multiple electrons (the base region is holes and the emitter region is electrons). At this PN junction, an electrostatic field (i.e., an built-in electric field) pointing from the emitter region to the base region will be induced. When the base is applied with a positive voltage, the base region points to the emitter region, When the electric field generated by the applied voltage at the base is larger than the built-in electric field, the carriers (electrons) in the base region may flow from the base region to the emitter region. The small Zui value of this voltage is the forward on voltage of the PN junction (generally 0.7V in Engineering). However, at this time, there will be charges on both sides of each PN junction. At this time, if a positive voltage is applied to the collector emitter, the electrons in the emitter region will move to the base region under the action of the electric field (in fact, they all move in the opposite direction). Because the base region is very narrow, the electrons can easily cross the base region to reach the collector region and compound with the PN holes here (near the collector). To maintain balance, Under the action of the positive electric field, the electrons in the collector region accelerate the movement of the outer collector, while the holes move at the PN junction, which is similar to an avalanche process. The electrons at the collector return to the emitter through the power supply, which is how the transistor works.

    When the triode works, both PN junctions will induce charges. When the switch is in the on state, the triode is in the saturated state. If the triode stops at this time, the charges induced by the PN junction will return to the equilibrium state. This process takes time. However, MOS triodes work in a different way and do not have this recovery time, so they can be used as high-speed switches.

    (1) The field effect transistor is a voltage control element and the transistor is a current control element. When only a small current is allowed from the signal source, the field effect transistor shall be selected; Under the condition that 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 use most carriers to conduct electricity, so they are called unipolar devices. Transistors use both majority carriers and minority carriers to conduct electricity. Which is called a bipolar device.

    (3) The source and drain of some field effect transistors 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 transistors can work under very low current and voltage, and their manufacturing process can easily integrate many field effect transistors on one silicon chip, so field effect transistors are widely used in large-scale integrated circuits.

    (5) Field effect transistor (FET) is widely used in various electronic devices because of its high input impedance and low noise. In particular, using field effect transistors as the input stage of the entire electronic equipment can achieve performance that is difficult to achieve with ordinary transistors.

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