Basic principle of MOS tube and important knowledge points of single chip microcomputer

This is a N Ditch MOS tube , It can realize such functions , When there is no voltage at the grid , It's cut-off , When we apply voltage to the grid , It turns on .

Let's talk about how it works , This is a MOS The semiconductor structure of a tube , The two blue areas are N Type semiconductor , It is doped in pure silicon crystal 5 Valence phosphorus , At this point, the outermost layer of the phosphorus atom has a free electron , Because free electrons are negatively charged , So we call it N Type semiconductor ,N Taken from negative The first letter of .

If we give N Type a semiconductor connection , It is conductive , Because it has extra free electrons inside , Whether it is connected in the forward direction or in the reverse direction, it can be conducted , The yellow area is P Shape semiconductor , Or in pure silicon crystal 3 Valence boron , here , One electron is missing from the outermost layer of the boron atom .

We replace the missing electron with a hole , Holes attract electrons to be positively charged , Let's call it alpha P Type semiconductor ,P Taken from the positive The first letter of , It should be noted that ,P T-type semiconductors are not full of holes , There are also free electrons in it , Just not N There are so many in type a semiconductors , hold P The type and N Type semiconductors are bonded together , It forms a diode .

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Power it up , There is an electric field , The direction of the force on the electron is opposite to the direction of the electric field line , Let's just throw away the electric field lines , It's easier to understand , When P When terminating the positive pole , The direction of the force is like this , Free electrons can move to the left , Compound with holes , Under the action of power supply , This kind of movement keeps going , This creates an electric current .

When N When terminating the positive pole , The direction of the force is like this , The free electron is forced by an electric field to the right , Cannot compound with holes , here PN The end of the knot , No current is generated , This is the unidirectional conductivity of the diode , Give me two N Type a semiconductor leads out two metal electrodes , Respectively as MOS The drain and source of the tube , We connected it to electricity , At this time it is cut-off .

Because two diodes are formed between them , And in the opposite direction , If there is a conduction, there will be a cutoff , So at this time MOS Pipe cut off , In order to make it work , stay P A thin layer of silicon dioxide insulation is added to the area , Then a metal plate is made on the insulating layer , The grid set is formed .

Let's connect the grid , Now , There is an electric field on the metal plate , It can put P The electrons in the region are attracted near the insulating layer , And drive the hole away , The higher the voltage , More cushions attracted .

The whole process is like this , When free electrons attract enough , Formed N Ditch , So-called N Ditch , Namely N A channel between type semiconductors , Because of this time N The channel replaces the original P Holes in semiconductor , Make the original semiconductor PN The knot no longer exists .

For the sake of understanding , At this point, we can think of this area as a whole N Type semiconductor , So after the grid voltage is applied , It turns on , When we remove the grid voltage ,N The ditch disappeared , here MOS The tube must cut off , Here are two MOS A very important characteristic of a tube .

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The first characteristic is MOS The gate input impedance of the tube is very high , This is because there is an insulating layer , It almost completely closes the electronic channel , Its input resistance can reach hundreds of millions of ohms , So its input takes almost no current , This is why almost all chips are integrated inside MOS tube .

The second feature is MOS The grid of is easily broken down by static electricity , Because the grid input impedance is very large , Induced charge is difficult to release , The high voltage it generates easily breaks through this thin insulating layer , cause MOS The pipe is permanently damaged , Because after the insulation layer is broken down , Charges can no longer be aggregated as before N Ditch .

Instead, it creates a current between the grid and the source , There is an electric field effect , Thus, a metal oxide semiconductor field effect transistor is formed , Finally, let's look at N Circuit symbols for channels , You may have more understanding , These are the source and drain metal plates respectively , This is a grid level metal plate , In the middle is the insulating layer , And this arrow represents N The flow direction of channel electrons .

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