1. diode--PN junction
The PN junction diode is the smallest unit of semiconductor analysis. P-type semiconductors will have a large number of holes through doping, which can be filled with electrons. N-type semiconductors have more active electrons.
Let us first understand the working process of a diode with only one P-N junction.
When a forward voltage is applied to the diode (P terminal is connected to the positive pole, N terminal is connected to the negative pole), the diode is turned on, and its PN junction has current flowing through it. This is because when the P-type semiconductor terminal is at a positive voltage, the negative electrons in the N-type semiconductor are attracted and flock to the P-type semiconductor terminal to which the positive voltage is applied, while the positrons in the P-type semiconductor terminal are directed towards the N-type semiconductor terminal. movement, thereby forming a conduction current.
In the same way, when a reverse voltage is applied to the diode (P terminal is connected to the negative pole, N terminal is connected to the positive pole), the voltage at the P-type semiconductor terminal is negative at this time, the positive electrons are gathered at the P-type semiconductor terminal, and the negative electrons are gathered at the N terminal. Type semiconductor terminal, electrons do not move, no current flows through its PN junction, and the diode is cut off.
2. MOSFET working principle
MOS field effect transistor is also called MOS FET, which is the abbreviation of Metal Oxide Semiconductor Field Effect Transistor (Metal Oxide Semiconductor Field Effect Transistor). It generally has two types of depletion and enhancement. Here we take the enhanced MOS as an example for analysis.
The field effect transistor is composed of source, drain and gate. Due to the different doping of the substrate, it can be divided into N-channel and P-channel field effect transistors. (Channel: As the name implies, two fillable channels are dug on the substrate to fill N-type or P-type semiconductors)
We connect the drain to the positive power supply and the source to the negative power supply.
For a field effect transistor, when there is no voltage at the gate, the source and the drain are equivalent to two back-to-back diodes, and no current will flow. At this time, the field effect transistor is in the cut-off state.
As shown in the right figure above, when a voltage is applied to the gate, when the voltage is less than a threshold VGS(th), the electric field between the gate and the substrate P will push the holes in the P-type semiconductor due to the action of the electric field. On, at this time, the negative electrons of the source and drain of the N-type semiconductor are attracted and rushed to the gate, but due to the blocking of the oxide film, the electrons are concentrated in the P-type semiconductor between the two N-channels.
As the gate voltage increases, the electron concentration near the gate increases. When a threshold VGS(th) is exceeded, the N-type semiconductor between the source and drain forms an electron channel. At the same time, since a positive voltage is applied to the drain, a current from the drain to the source can be formed, and the MOS transistor is turned on.
We can also imagine that there is a trench between two N-type semiconductors, and the establishment of the gate voltage is equivalent to building a bridge between them. The size of the bridge is determined by the size of the gate voltage. This is also the reason why MOS tubes are voltage-controlled transistors.
We can think of the MOS tube as a valve, and the gate is like a switch that controls whether the flow of water can pass through.
3. Working principle of CMOSFET
C-MOS field effect transistor (enhanced MOS field effect transistor), the circuit uses an enhanced P-channel MOS field effect transistor and an enhanced N-channel MOS field effect transistor together.
When the input terminal is low level, the P-channel MOS field effect transistor is turned on, and the output terminal is connected to the positive pole of the power supply.
When the input terminal is at a high level, the N-channel MOS field effect transistor is turned on, and the output terminal is connected to the power supply ground.
In the circuit, the output terminal is judged as 1 and 0 through the high point of the voltage, and a logic control is realized. Then all the logic can be combined with different logic rules to realize different logic functions.
In the chip design, the smallest unit of final production is the MOS tube. After the MOS tube is combined and packaged, it is the chip we want.
