Introduction to the formation and characteristics

Introduction to the formation and characteristics of PN junction

definition of PN junction:

in an intrinsic semiconductor, different impurities are doped to make one side become p-type and the other side become n-type. A PN junction is formed at the interface between P region and N region

formation of PN junction

(1) when p-type semiconductors and n-type semiconductors are combined, due to the difference of carrier concentration at the interface, electrons and holes must diffuse from the place with high concentration to the place with low concentration. However, electrons and holes are charged, and their diffusion results in the widespread use of the pet aerogel in the original electricity in the P and N regions. The negative ions that cannot move are left on the side of P region due to the loss of holes, and the positive ions that cannot move are left on the side of N region due to the loss of electrons. These immobile charged particles are usually called space charges, and they are concentrated near the interface between P region and N region, forming a very thin space charge region, which is what we call PN junction, as shown in Figure 1

(2) in this region, most carriers have either diffused to the other side, or have been compounded by most carriers diffused from the other side (becoming minority carriers after arriving in this region), that is, most carriers have been consumed, so this region is also called depletion layer, and its resistivity is very high, which is high resistance region

(3) there is a negative charge on one side of the p region and a positive charge on the other side of the N region, so there is an electric field in the space charge region from the N region to the p region. Because this electric field is formed by carrier diffusion rather than by applied voltage, it is the concern of customers that the product advantage is called internal electric field, as shown in Figure 2

(4) the internal electric field is caused by the diffusion movement of multipons. With its establishment, it will bring two kinds of effects: one is that the internal electric field will hinder the diffusion of multipons, and the other is that once the minority electrons in P and N regions are close to the PN junction, the internal electric field will drift to each other through strict tests before it can be put into use, narrowing the space charge region

(5) therefore, the diffusion movement widens the space charge region and enhances the internal electric field, which is conducive to the drift of minority electrons but not conducive to the diffusion of many electrons; The drift motion narrows the space charge region and weakens the internal electric field, which is conducive to the diffusion of many electrons but not conducive to the drift of few electrons. When the diffusion motion and drift motion reach dynamic equilibrium, the interface forms a stable space charge region, that is, the PN junction is in dynamic equilibrium. The width of PN junction is generally 0.5um

unidirectional conductivity of PN junction

when the applied voltage is not applied, the diffusion motion and drift motion are in dynamic balance, and the current passing through the PN junction is zero

(1) apply positive voltage (positive bias)

when the positive pole of the power supply is connected to the P area and the negative pole is connected to the n area, it is called adding positive voltage or positive bias to the PN junction, as shown in Figure 3. Since the PN junction is a high resistance region, and the resistance of P and N regions is very small, almost all the forward voltage is added to both ends of the PN junction. An external electric field is generated on the PN junction, and its direction is opposite to the internal electric field. Driven by it, the electrons in the N region will diffuse to the left and neutralize with the positive ions in the original space charge region, narrowing the space charge region. Similarly, the holes in the p region should also diffuse to the right and neutralize with the negative ions in the original space charge region, making the space charge region narrower. As a result, the internal electric field is weakened and the original dynamic balance of PN junction is destroyed. So the diffusion motion exceeded the drift motion, and the diffusion continued. At the same time, the power supply continuously adds positive charge to the p region and negative charge to the N region, resulting in a large positive current if in the circuit. And if increases with the increase of forward voltage

(2) apply reverse voltage (reverse bias)

when the positive pole of the power supply is connected to the n area and the negative pole is connected to the P area, it is called adding reverse voltage or reverse bias to the PN junction. The direction of the applied electric field generated by the reverse voltage is the same as that of the internal electric field, which strengthens the electric field in the PN junction. It pulls away the multipons (holes) in the p region and the multipons (free electrons) in the N region from the vicinity of the PN junction, further widens the PN junction, increases the resistance of the PN junction, breaks the original balance of the PN junction, and the drift motion under the action of the electric field is greater than the diffusion motion. At this time, the current passing through the PN junction is mainly the drift current formed by minority carriers, which is called the reverse current IR. Because the number of minority carriers is small at room temperature, the reverse current is very small, and when the applied voltage changes within a certain range, it hardly changes with the change of the applied voltage. Therefore, the reverse current is also called the reverse saturation current. When the reverse current can be ignored, the PN junction can be considered to be in the cut-off state. It is worth noting that because the intrinsic excitation intensifies with the increase of temperature, resulting in the increase of electron hole pairs, the reverse current will double with the increase of temperature. Reverse current is one of the main causes of circuit noise. Therefore, temperature compensation must be considered when designing the circuit

to sum up, when the PN junction is positive biased, the forward current is large, which is equivalent to the conduction of the PN junction. When the PN junction is reverse biased, the reverse current is small, which is equivalent to the cut-off of the PN junction. This is the unidirectional conductivity of PN and such phenomena have been common

volt ampere characteristic of PN junction

volt ampere characteristic curve: the relationship curve between the voltage applied at both ends of PN junction and the current flowing through diode is called volt ampere characteristic curve, as shown in Figure 4. u> The part of 0 is called positive characteristic, u