1.1 Characteristics of PN junction
Everyone knows that although there are a lot of free electrons in a conductor, the conductor itself is not charged. Similarly, whether it is a P-type semiconductor or an N-type semiconductor, although they have a large number of carriers, they themselves are still uncharged neutral substances without external conditions. However, if the P-type semiconductor and the N-type semiconductor are closely “combined”, then a PN junction will be formed at the junction of the two. The PN junction is the basis of a variety of semiconductor devices such as solar cells, diodes, triodes, and thyristors. Therefore, it is necessary to discuss the formation of PN junction and its conductivity.
1.1.1 Diffusion motion and drift motion
People know that substances always diffuse from places with high concentrations to places with low concentrations, just like dropping a few drops of blue ink into a glass of clear water, and the blue ink diffuses around the clear water to make a glass of water blue. When the P-type semiconductor and the N-type semiconductor are “combined” together, the same situation occurs: at the junction, because the hole concentration in the P-type region is greater than that of the N-type region, the electron concentration in the N-type region is greater than that of the P-type region, so Produce the “diffusion motion” of the interceptor. As a result, the electrons in the N-type region diffuse to the P-type region, as shown in Figure 1-1(a). The thin layer A near the N area is positively charged due to the loss of electrons; the holes in the P type area go to the N type area
Domain diffusion, as shown in Figure 1-1(b), the thin layer B near the P-type region is negatively charged due to the loss of holes. As a result, a positively charged thin layer A and a negatively charged thin layer B are formed at the junction of the PN zone. Due to the accumulation of positive and negative charges, an internal electric field is formed between A and B, as shown in Figure 1-1 (c ) Shown. Its direction is from A to B (the direction of the electric field is from positive charge to negative charge, from high potential to low potential).
Figure 1-1: Diffusion of electrons and holes in PN junction
The generation of the electric field in this PN junction is caused by the diffusion movement, and the internal electric field is only when there is diffusion. So what effect does this internal electric field have on diffusion? Obviously, on the one hand, this internal electric field prevents electrons in the N-type region from continuing to diffuse to the P-type region, and the holes in the P-type region diffuse to the N-type region, which is to the majority of current carriers. The diffusion movement of the son plays an obstructive role. But on the other hand, it promotes the movement of electrons with very little content in the P-type region (the minority electron carriers in the P-type semiconductor) to the N-type region, and the few holes in the N-type region (the few in the N-type semiconductor) Hole carriers) move into the P-type region. The regular movement of such minority carriers under the action of an electric field is called “drift movement”. The internal electric field here assists the drifting movement of minority carriers. This drift motion is exactly the opposite of the direction of the diffusion motion caused by the concentration difference, so it plays a role of mutual hindrance and restriction, so the A and B layers are called barrier layers or PN junctions.
Since there are two opposite directions of diffusion motion and drift motion inside the PN junction, at the beginning, the diffusion motion is dominant, and the thin layers A and B become thicker and thicker, but with the continuous diffusion of electrons and holes, the inner The electric field is getting stronger and stronger, so the drifting motion becomes stronger and stronger under the action of the internal electric field, until the drifting motion and the diffusion motion reach a dynamic balance, the electrons in the N-type region and the holes in the P-type region no longer increase, and the barrier layer The thickness also no longer increases. At this time, the thickness of the barrier layer is 10-5 to 10-4 cm. Of course, the drift motion and the diffusion motion are still going on at this time, but the two are in a dynamic equilibrium state, and the macroscopically shows that the total current of the one-pole tube is zero. This is the condition of the semiconductor diode when there is no applied voltage.
1.2 Turn-on and cut-off of PN junction
If the PN junction is connected to a forward voltage (the positive pole of the external voltage is connected to the P area, and the negative pole is connected to the N area), the direction of the external electric field is opposite to the direction of the internal electric field. The external electric field moves the electrons in the N-type region to the left and the holes in the P-type region to move to the right, so that the positive and negative charges in the original space charge region are neutralized, and the amount of charge in the charge region is reduced, and the space charge The area becomes narrower, that is, the barrier layer becomes narrower. Therefore, the external electric field acts to weaken the internal electric field. This greatly facilitates the diffusion movement. As a result, most carriers pass through the barrier smoothly under the action of an external electric field, and at the same time, the external power supply continuously supplies holes and electrons to the semiconductor. Therefore, a larger current appears in the circuit, which is called a forward current. Therefore, the resistance of the PN junction during forward conduction is very small.
On the contrary, if the PN junction is connected to the reverse voltage (the negative electrode of the external voltage is connected to the P area, and the positive electrode is connected to the N area) , the direction of the external electric field is consistent with the direction of the internal electric field, which strengthens the internal electric field. , The space charge area is widened, that is, the barrier layer is widened. In this way, the diffusion motion of majority carriers becomes impossible, but the drift motion will be strengthened by the increase of the internal electric field. However, the drift current is formed by minority carriers in the semiconductor, and its number is very small.Therefore, when a reverse voltage is applied to the PN junction, the reverse current is extremely small and exhibits a very reverse resistance. Basically, it can be considered that no current flows. People call this phenomenon “cut-off”. Because the PN junction has the above-mentioned unidirectional conductivity characteristics, semiconductor diodes are widely used in circuits such as rectification and detection.