Using different doping processes, through diffusion, the P-type semiconductor and N-type semiconductor are fabricated on the same semiconductor (usually silicon or germanium) substrate, and a space charge area called PN junction is formed at the interface between them. : PN junction). The PN junction has unidirectional conductivity, which is a characteristic used by many devices in electronic technology, such as the material basis of semiconductor diodes and bipolar transistors.
It can be seen from the formation principle of the PN junction that if the PN junction is to be turned on to form a current, the resistance of the internal electric field in the space charge region must be eliminated. Obviously, adding a larger electric field in the opposite direction to it, that is, the P area is connected to the positive electrode of the external power supply, and the N area is connected to the negative electrode, which can offset its internal self-built electric field, so that the carriers can continue to move, thus forming a linear Forward current. The applied reverse Voltage is equivalent to greater resistance of the built-in electric field, the PN junction cannot be turned on, and there is only a very weak reverse current (formed by the drifting movement of minority carriers, and the current is saturated due to the limited number of minority carriers). When the reverse voltage increases to a certain value, because the number and energy of minority carriers increase, they will collide and destroy the internal covalent bonds, causing the originally trapped electrons and holes to be released, increasing the current, and finally The PN junction will be broken down (become a conductor) and the reverse current will increase sharply.
This is the characteristic of PN junction (unidirectional conduction, reverse saturation leakage or breakdown conductor), and it is also the most basic and important physical principle of transistors and integrated circuits. It is inseparable from the analysis of all complex circuits based on transistors. . For example, the diode works based on the unidirectional conduction principle of the PN junction; and a PNP structure can form a triode, which contains two PN junctions. Diodes and transistors are the most basic components in electronic circuits.