Semiconductor devices (I) PN junction

（ One ） Intrinsic semiconductor Impurity semiconductor

Intrinsic semiconductor It is a semiconductor with pure crystal structure . The impurities 、 Have a stable structure .
At a certain temperature, the carrier concentration in the intrinsic semiconductor is certain , And the concentration of free electrons and holes is equal .
Impurity semiconductor There are two kinds of ：N Type semiconductor P Type semiconductor

N Type semiconductor ：

• Intrinsic semiconductor doping 5 After the valence element , Some silicon atoms in the original crystal will be replaced by impurity atoms . The outermost layer of impurity atoms has 5 A valence electron , among 4 A covalent bond with silicon , More than one electron is attracted only by its own nucleus , It can become a free electron at room temperature .

  Electrons are called majority carriers ( abbreviation children ),Negitive
  Holes are called minority carriers ( abbreviation Young son ).
  5 Valence impurity atoms are called Donor atom .
  
  P Type semiconductor
  In silicon or germanium crystal doped with a small amount of 3 Valence impurity element , Like boron 、 gallium 、 Indium, etc , Composition P Type semiconductor .

    The hole is ** Most carriers **,Positive 
    Electron is ** Minority carriers **
   3  Valence impurity atoms are called ** Acceptor atom **.
  

explain ：
1. The concentration of doped impurities determines the concentration of most carriers ; Temperature determines the concentration of minority carriers .
2. The number of carriers in impurity semiconductors is much higher than that of intrinsic semiconductors , Therefore, its conductivity is greatly improved .
3. Impurity semiconductors generally remain electrically neutral .

（ Two ） PN The formation of knots

PN junction Formation process ： Spread 、 Drift movement
Doping on one side of a semiconductor single crystal becomes P Type semiconductor , The other side is doped into N Type semiconductor , A special thin layer is formed at the junction of the two areas , be called PN junction .

original P Area and N All areas are electrically neutral , because children The spread of , It will inevitably lead to the destruction of the electrical neutrality inside the semiconductor , Thus in the conductor PN An internal electric field is formed at the junction ： Pay attention to the direction of the internal electric field


The width of the depletion layer is determined by the intensity of the diffusion movement . The resistivity of the depletion layer is very high , It is high resistance area . This is because the resistance reflects the conductivity , The depletion layer has reached dynamic equilibrium , Basically non-conductive , Natural resistance is very large .

Although whether it is diffusion motion or drift motion , The actual motion is all electrons . But in order to distinguish between diffusion motion and drift motion , Diffusion motion is often referred to as multiparticle motion , Because diffusion is the process of many children in this region entering the other region ; The corresponding drift motion becomes the minority carrier motion .

（ 3、 ... and ）PN The nature of the knot

3.1 Unidirectional conductivity

stay PN Add a small forward voltage to the junction , You can get a larger forward current , To prevent excessive current , Resistance can be connected R.
Reverse current is also called reverse saturation current I S. Very sensitive to temperature , As the temperature rises , IS Will increase dramatically .

3.2 PN The capacitance effect of the junction
When PN When the voltage on the changes ,PN The amount of charge stored in the junction will change accordingly , send PN The junction has a capacitive effect .
Capacitance effect includes two parts ： Barrier capacitance 、 Diffusion capacitance