When we do circuit design, what is the difference between transistors and MOS transistors for switching?

When we do circuit design, what is the difference between transistors and MOS transistors  for switching?

　　Nature of work:

　　, triode current control, MOS diode belongs to voltage control.

　　Cost problem: triode is cheap, MOS diode is expensive.

　　Power consumption problem: transistor loss is large.

　　Drive capability: MOS diodes are commonly used in power switches and high-current switching circuits.

　　In fact, the transistor is relatively cheap, convenient to use, and commonly used in digital circuit switch control.

　　Diodes are used in high-frequency and high-speed circuits, high-current occasions, and places that are sensitive to base or drain control current.

　　Generally speaking, in low-cost situations, ordinary applications should first consider using transistors , and then consider MOS diodes if they are not possible.

　　In fact, it is wrong to say that current control is slow and voltage control is fast. It takes a real understanding of how bipolar transistors  and MOS transistors  work. Triode is by the movement of carriers to work, taking the NPN diode emitter follower as an example, when the base plus no voltage, the base region and the emission region composed of the pn junction to prevent the diffusion movement of multiple tons (the base region is a hole, the emission area is an electron), in this pn junction will induce an electrostatic field (that is, the built-in electric field) pointed by the emission area to the base area, when the base plus positive voltage is pointed to the base area to the transmitting area, the electric field generated by the base applied voltage is greater than the built-in electric field, Only then can carriers (electrons) in the base region flow from the base region to the transmit region, and a small value of this voltage is the forward voltage of the PN junction (generally considered to be 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 plus a positive voltage, under the action of the electric field, the electrons in the emission region move to the base region (in fact, they are all moving in the opposite direction of the electrons), because the width of the base region is very small, the electrons easily cross the base region to reach the collector region, and recombine with the hole of the PN here (close to the collector), in order to maintain equilibrium, the electrons in the collector region accelerate the outer collector movement under the action of the positive electric field, and the holes move at the PN junction, this process is similar to an avalanche process.

　　The electrons of the collector are fed back to the emitter through the power supply, which is how the transistor works. When the triode is working, both PN junctions will induce a charge, when the switch diode is in the on state, the triode is in a saturated state, if the transistor is cut off at this time, the charge induced by the PN junction will return to the equilibrium state, this process takes time. MOS transistors  work differently and do not have this recovery time, so they can be used as high-speed switching diodes.

　　(1) FETs are voltage control components, while transistors  are current control components. In cases where only less current is allowed from the signal source, FETs should be selected; In the case of low signal voltage and more current from the signal source, transistors  should be selected.

　　(2) FETs use majority carriers to conduct electricity, so they are called unipolar devices, while transistors are both majority carriers and minority carriers to conduct electricity, which is called bipolar devices.

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

　　(4) FETs 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 FETs 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, the use of field effect diodes as the input stage of the entire electronic device can obtain performance that is difficult to achieve with ordinary transistors.

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