Useful information | Why does a diode conduct electricity in one direction? This article finally explains it
The diode is a very commonly used component in electronic circuits. It is very common. The diode has the characteristics of forward conduction and reverse cutoff.
When a positive voltage is applied to the positive end (positive pole) of a diode and a negative voltage is applied to the negative end (negative pole), the diode is turned on and current flows through the diode. When a negative voltage is applied to the positive end (positive pole) of a diode and a positive voltage is applied to the negative end (negative pole), the diode is turned off and no current flows through the diode. This is called the unidirectional conduction characteristic of a diode. The following explains why a diode conducts unidirectionally.
The diode is composed of a PN junction, that is, a P-type semiconductor and an N-type semiconductor. Therefore, the characteristics of the PN junction lead to the unidirectional conductivity of the diode. The PN junction is shown in Figure 1.
Figure 1 Schematic diagram of PN junction
Near the interface of P-type and N-type semiconductors, due to the high concentration of free electrons in the N region, negatively charged free electrons will diffuse from the N region to the P region with low electron concentration; as a result of the diffusion, the P region side of the PN junction is negatively charged, and the N region side is positively charged, forming an electric field from the N region to the P region, that is, the internal electric field of the PN junction. The internal electric field will hinder the continued diffusion of the majority carriers, also known as the barrier layer.
The unidirectional conductivity of diodes is widely used. What is the reason that makes electrons so obedient? What is its microscopic mechanism? Here is a brief and vivid introduction.
Suppose there is a P-type semiconductor (yellow represents more holes) and an N-type semiconductor (green represents more electrons). They are both electrically neutral in their natural state, that is, they are not charged. See Figure 2.
Figure 2 P-type and N-type semiconductors
When they are combined together, a PN junction is formed. The electrons of the N-type semiconductor at the boundary will naturally run to the P-type region to fill the holes, leaving behind atoms that have lost electrons and are positively charged. The atoms at the boundary of the corresponding P-type region are negatively charged due to the gain of electrons, thus forming a space charge region at the boundary. Why is it called a "space charge region"? Because these charges are composed of atoms that cannot move in the microscopic space.
The space charge region forms a built-in electric field, and the direction of the electric field is from N to P. This electric field prevents the electrons behind from continuing to fill the holes, because the negative space charge in the P-type region repels electrons. The combination of electrons and holes will become slower and slower, and finally reach equilibrium, which is equivalent to the exhaustion of carriers, so the space charge region is also called the depletion layer. At this time, the PN junction is still electrically neutral as a whole, because the positive and negative space charges cancel each other out. As shown in Figure 3.
Figure 3 PN junction forms a built-in electric field
When a forward voltage is applied, the direction of the electric field changes from positive to negative, which is opposite to the built-in electric field, weakening the built-in electric field, so the diode is easy to conduct. The green arrow indicates the direction of electron flow, which is opposite to the direction defined by the current. As shown in Figure 4.
Figure 4 Forward conduction state
When the reverse voltage is applied, the direction of the electric field is the same as the built-in electric field, which enhances the built-in electric field, so the diode is not easy to conduct. As shown in Figure 5. Of course, non-conduction is not absolute, and there will generally be a small leakage current. As the reverse voltage continues to increase, it may cause the diode to break down and leak sharply.
Figure 5 Reverse non-conduction state
Figure 6 is the current-voltage curve of the diode for reference.
Figure 6 Diode current-voltage curve
Figure 7 shows in a graphic way why diodes can and cannot conduct in different directions, making it easier to understand.
Figure 7: Different conduction effects in different directions
There are many examples of one-way conduction in life, such as the one-way gate at the subway entrance, which is equivalent to the effect of a diode: it conducts in the forward direction and does not conduct in the reverse direction. If you insist on passing through in the reverse direction, it may damage the gate due to "reverse breakdown" due to excessive force.
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