How the diode works

The main principle of the diode is to use the unidirectional conductivity of the PN junction, adding a lead and a package to the PN junction to become a diode.

The crystal diode is a PN junction formed by a P-type semiconductor and an N-type semiconductor, and a space charge layer is formed on both sides of the interface, and a self-built electric field is built. When there is no applied voltage, the diffusion current caused by the difference in the carrier concentration on both sides of the PN junction and the drift current caused by the self-built electric field are equal and in an electrical equilibrium state.

When there is a forward voltage bias in the outside world, the mutual suppression of the external electric field and the self-built electric field increases the diffusion current of the carriers and causes the forward current. When there is a reverse voltage bias outside, the external electric field and self-built electric field are further strengthened, forming a reverse saturation current that is independent of the reverse bias voltage value within a certain reverse voltage range.

When the applied reverse voltage is high to a certain level, the electric field intensity in the space charge layer of the PN junction reaches a critical value, and the multiplication process of carriers is generated, a large number of electron-hole pairs are generated, and a large reverse breakdown current is generated. , Known as the breakdown phenomenon of the diode. The reverse breakdown of PN junction can be divided into Zener breakdown and avalanche breakdown.

 Principle of PN junction formation P-type semiconductor is an intrinsic semiconductor (a completely pure and complete semiconductor crystal) doped with a small amount of trivalent element impurities, such as boron.

Because the boron atom has only three valence electrons, it forms a covalent bond with the surrounding silicon atoms. Because of the lack of an electron, a vacancy is created in the crystal. When the electrons on the adjacent covalent bond gain energy, it may fill this The vacancy makes the boron atom an immovable anion, while the original covalent bond of the silicon atom lacks an electron to form a hole, but the entire semiconductor is still neutral. In this type of P-type semiconductor, holes are the main conduction, holes are majority carriers, and free electrons are minority carriers. The principle of N-type semiconductor formation is similar to that of P-type. Intrinsic semiconductors are doped with five-valent atoms, such as phosphorus. After incorporation, it forms a covalent bond with silicon atoms, generating free electrons. In N-type semiconductors, electrons are majority carriers, and holes are minority carriers.

Therefore, doping trivalent and pentavalent impurity elements in two different regions of the intrinsic semiconductor forms a P-type region and an N-type region. According to the characteristics of the N-type semiconductor and the P-type semiconductor, it can be seen that there is a There is a difference in the concentration of electrons and holes. Both electrons and holes diffuse from the high-concentration area to the low-concentration area. Their diffusion destroys the electrical neutrality at the original junction. PN junction unidirectional conductivity

A forward voltage V is applied to the PN junction. Under the action of this applied electric field, the equilibrium state of the PN junction is broken. The holes in the P area and the electrons in the N area move toward the PN junction, and the holes and the PN junction P area The negative ions are neutralized, and the electrons are neutralized with the positive ions in the N region of the PN junction, which narrows the PN junction. With the increase of the applied electric field, the diffusion motion is further enhanced, and the drift motion is weakened. When the applied voltage exceeds the threshold voltage, the PN junction is equivalent to a resistor with a small resistance value, that is, the PN junction is turned on.