Catalog

1、 Doped semiconductors

2、 Carrier diffusion and drift

3、PN Switching characteristics of the junction

4、 The temperature is right PN Effect of knot

5、 Zener breakdown and avalanche breakdown

Reference material ：

Distance from last write semiconductor , It's been a long time , Last time, I shared the basic concept of intrinsic semiconductor ： 【 Electronic microscopic 】 Valence band , conduction band , Forbidden band , Fermi level , Carrier migration …… - You know

Today, I want to talk about the foundation of semiconductor industry ：PN Junction and diode

1、 Doped semiconductors

If the intrinsic semiconductor is doped , We can get P Type a semiconductors and N Type semiconductor . Pictured 1-1,P Type a semiconductors use holes as most carriers ,N Type a semiconductors use electrons as most carriers . If the doped semiconductor is simply energized , We will find that the conductivity of semiconductors has been greatly enhanced （ Relative to intrinsic semiconductors ）, In semiconductors with the same doping concentration ,N Type a semiconductor has stronger conductivity （ The mobility of electrons is about that of holes 3 times ）.

chart 1-1 Intrinsic semiconductors and doped semiconductors

2、 Carrier diffusion and drift

If you take a P Type a semiconductor and a N Type semiconductors together , Then due to the diffusion movement ,N Electrons in type a semiconductors will go P Neutralizing hole recombination in type a semiconductor ;P The holes in type a semiconductors will go N Type semiconductor internal and electronic recombination . At this time, it is near the joint surface P The empty orbital of A-type semiconductor is filled with electrons to form negatively charged particles , and N Type a semiconductors lose free electrons , Form particles with punctuality , The diffusion movement forms the internal electric field constructed by positive and negative particles . As the diffusion depth increases , The field strength of the internal electric field is increasing , At this time, the carrier entering the electric field is accelerated ： Electrons move against the electric field , Holes move in the direction of the electric field ,P Type a semiconductor gains holes ,N Type a semiconductor acquires electrons , Thus, the built-in electric field strength is weakened to form “ Negative feedback ”. Final PN The built-in field strength of the junction is in a dynamic balance , Pictured 2-1.

stay PN There are almost no free moving carriers in the built-in electric field region of the junction , The positive and negative particles are bound by the lattice and cannot move freely . This space region is called space charge region , Also called depletion layer , Also known as barrier area . When I first learned analog circuits , We are often confused by these three nouns , Here we might as well take another look ： The space charge region is divided according to the charge characteristics , This part is a charge that cannot move freely ; From the perspective of electric field , The built-in electric field hinders the diffusion of carriers , Like a wall , If the carrier wants to pass through this region, it must obtain the energy to overcome the barrier ; From the perspective of carriers , Here the electrons and holes are combined , As if the nearby carriers were consumed , Therefore, it is also called depletion layer . therefore , Space charge region = Barrier region = Depletion layer .

chart 2-1 PN The structure of the junction and the built-in electric field

3、PN Switching characteristics of the junction

PN Depletion layer formed by junction , It can be said to be the infrastructure of the whole semiconductor industry ： Because of its single-phase conductivity . Pictured 3-1, If we were P Type semiconductor plus positive voltage ,N Type semiconductor plus negative voltage ,P Type semiconductor electrons are pumped away , There are holes left ;N Type semiconductors get electronic supplements . Electrons and holes provided by external voltage are recombined in the depletion layer , The space charge region decreases , Form a continuous flow of current . contrary , Pictured 3-2, If in P A negative voltage is applied to the type a semiconductor ,N A positive voltage is applied to the type a semiconductor ,P Type a semiconductor gains excess electrons ;N Type semiconductor electrons are pumped away , Leave a positive hole , The space charge region increases , The built-in field strength increases , Hinder the drift motion of electrons ,PN The knot remains in the cut-off state .

chart 3-1 PN The positive conduction of the junction

chart 3-2 PN Knot reverse cutoff

4、 The temperature is right PN Effect of knot

In the use of semiconductor devices , I often come across a concept , It is a certain parameter of this device （ Such as equivalent impedance , Breakdown voltage ） Is it a positive temperature coefficient or a negative temperature coefficient . Temperature has a crucial influence on the characteristics of semiconductors , So at the micro level, how does temperature affect PN What about the knot ? On the one hand, the increase of temperature can improve the intrinsic carrier excitation of semiconductors , The carrier concentration is increased to a certain extent , More carriers participate in the reduction of conductive resistivity ; On the other hand , The rise of temperature leads to the increase of lattice vibration , The average free path of carriers becomes shorter , The mobility of carriers decreases , Resistivity increases .

Due to different processes and application scenarios , The positive conduction voltage drop of different types of diodes presents positive temperature coefficient or negative temperature coefficient . Pictured 4-1, Two identical diodes are connected in parallel . If the diode characteristics are shown in the left figure , One of the diodes has a higher temperature , The greater the current shunted by the diode , The higher the current, the higher the temperature , Further aggravate the shunt of the diode , As a result, the diode will bear most of the current , May cause thermal failure of diode ; If the diode characteristics are shown in the right figure , Under high current conditions , The tube voltage drop with positive temperature coefficient can automatically equalize the current of parallel diodes , Only diodes with this characteristic can meet the conditions for parallel use .

chart 4-1 Diode forward conduction temperature curve

For diode reverse withstand voltage , The decrease of temperature means the decrease of lattice vibration , Carriers are more likely to drift through the barrier region , Form reverse breakdown current . As the temperature goes up , The reverse withstand voltage of diode is reduced .

5、 Zener breakdown and avalanche breakdown

If we use two heavily doped P Type a semiconductors and N Type semiconductor makes diodes , Then the carrier concentration will be very high ,PN The depletion layer of the junction will be very thin . At this time, adding the reverse voltage can easily help the carrier pass through the barrier region , So as to obtain a certain reverse current , This breakdown is called “ Zener breakdown ”, This diode is called a zener diode . Pictured 5-1, Enter the reverse breakdown region （Reverse breakdown）, The current increases rapidly , The voltage remains constant , So it's also called “ Voltage stabilizing diode ”.

Simply speaking , Use highly doped semiconductors to form a thin depletion layer , In the process of carrier migration, the scattering effect of lattice and impurity ions on carriers is relatively limited , You can ignore . Carriers can easily cross the depletion layer , The breakdown that forms the depletion layer . But the breakdown energy is not high , It is a recoverable breakdown . As the temperature goes up , The carrier activity in the depletion layer is enhanced , Breakdown voltage decreases . Leakage current increases , This needs attention in circuit design .

chart 5-1 Zener diode characteristic curve

For ordinary doped diodes , The same increase in temperature will enhance the carrier activity in the depletion layer , It's easier to be inspired . However, due to the long distance of the depletion layer, it needs to pass through more lattices , Lattice vibration is enhanced , The average free path of carriers becomes shorter , Thus, the withstand voltage of the diode is improved . After diode reverse pressure, more energy is needed to accelerate the carrier , To cross the barrier , Form a reverse current . In the accelerated electronic process , Because the electric field is very strong , After accelerating electrons, it is easy to bombard other particles that are close to ionization （ Imagine bombarding billiards vigorously and miraculously ）, So the avalanche effect happened . Avalanche effect ionizes more and more carriers , Finally, the current is getting bigger and bigger , Form breakdown current , Burn the diode .

Compare zener breakdown and avalanche breakdown , Pictured 5-2 It can be seen that , The inflection point of avalanche breakdown curve is slow , The inflection point of zener breakdown is steep . And zener breakdown occurs first , Avalanche breakdown occurs again , Avalanche breakdown energy is much larger than zener breakdown energy .

chart 5-2 Zener breakdown and avalanche breakdown

Reference material ：

1. https://www.ednchina.com/news/5615.html

2. https://zhuanlan.zhihu.com/p/77910253