Bipolar tube is a current control device (large collector current is controlled by small base current), and MOS tube is a voltage control device (source drain conduction resistance is controlled by grid voltage).

    Under the conduction voltage drop of MOS tube (FET), the conduction resistance is small, the grid drive does not require current, and the loss is small. The drive circuit is simple, with its own protective diode, and the thermal resistance is good. It 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 the triode is high. Ic of the large triode can be made very large. The disadvantages are large loss, large base drive current, and complex drive.

    Generally speaking, in low cost occasions, triode is considered first for ordinary applications, and MOS tube is considered if not.

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

    The triode works by the movement of carriers. Take the emitter follower of the npn tube as an example. When the base is applied with or without voltage, the pn junction composed of the base area and the emission area is used to prevent the diffusion movement of multiple carriers (the base area is a hole, and the emission area is an electron). At this pn junction, an electrostatic field (i.e. an internal electric field) from the emission area to the base area will be induced. When the base is applied with a positive voltage, the base area points to the emission area, When the electric field generated by the applied voltage of the base is greater than the built-in electric field, it is possible for the carrier (electron) in the base region to flow from the base region to the emission region. The small value of zui 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 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 toward the base area under the action of the electric field (in fact, the electrons move in the opposite direction). Because the width of the base area is very small, the electrons can easily cross the base area to reach the collector area, and compound with the holes of the PN here (close to the collector). To maintain balance, Under the action of positive electric field, the electrons in the collector 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, the two pn junctions will induce charges. When the switch is in the on state, the triode is in the saturated state. If the triode is up to date at this time, the charges induced by the pn junction will return to the balanced state, which takes time. However, mos triodes work in different ways without this recovery time, so they can be used as high-speed switches.

    (1) FETs are voltage control elements, while transistors are current control elements. Field effect transistor shall be selected when less current is allowed to be taken from the signal source; When the signal voltage is low and more current is allowed to be taken from the signal source, transistors should be selected.

    (2) FET is a unipolar device because it conducts electricity by using a majority of carriers, while transistor conducts electricity by using a minority of carriers as well as a majority of carriers. It is called bipolar device.

    (3) The source and drain of some FETs can be interchanged, and the gate voltage can also be positive or negative, which is more flexible than transistors.

    (4) FET can work under the condition of very small current and very low voltage, and its manufacturing process can easily integrate many FETs on a silicon chip, so FET has been widely used in large-scale integrated circuits.

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

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