Using different doping processes, P-type semiconductors and N-type semiconductors are made on the same semiconductor (usually silicon or germanium) substrate through diffusion, and a space charge region called a PN junction is formed at their interface. The PN junction has unidirectional conductivity and is a characteristic utilized by many devices in electronic technology, such as the material basis of semiconductor diodes and bipolar transistors. In this issue, Millennial Solar brings you the basic knowledge of PN junctions.

Basic principles

Photovoltaic effect: "Photovoltaic effect" refers to the phenomenon that light causes a potential difference between different parts of a non-uniform semiconductor or a combination of a semiconductor and a metal.

N-type semiconductor

N is the prefix of Negative. This name is derived from the negative charge of electrons. In silicon crystals (or germanium crystals) doped with a small amount of impurity phosphorus (or antimony), semiconductor atoms (such as silicon atoms) are replaced by impurity atoms. , four of the five outer electrons in the outer layer of the phosphorus atom form covalent bonds with the surrounding semiconductor atoms, and the extra electron is almost unrestrained and can easily become a free electron. As a result, N-type semiconductors become semiconductors with a higher electron concentration, and their conductivity is mainly due to free electron conductivity.

P-type semiconductor

P is the prefix of Positive. It is named because the holes are positively charged. In silicon crystals (or germanium crystals) doped with a small amount of impurity boron element (or indium element), because semiconductor atoms (such as silicon atoms) are impurity atoms Substitution, when the three outer electrons in the outer layer of the boron atom form a covalent bond with the surrounding semiconductor atoms, a "hole" will be generated. This hole may attract bound electrons to "fill" it, making the boron atom become negatively charged of ions. In this way, this type of semiconductor becomes a material that can conduct electricity because it contains a higher concentration of "holes" ("equivalent to" positive charges).

Formation of PN junction

The PN junction is composed of an N-type doped region and a P-type doped region in close contact, and the contact interface is called a metallurgical junction interface. On a complete silicon wafer, different doping processes are used to form an N-type semiconductor on one side and a P-type semiconductor on the other side. We call the area near the interface of the two semiconductors a PN junction. After the P-type semiconductor and the N-type semiconductor are combined, since the free electrons in the N-type region are majority carriers, they are called minority carriers, while the holes in the P-type region are majority carriers and the free electrons are minority carriers. There is a concentration difference between electrons and holes at the junction. Due to the difference in concentration of free electrons and holes, some electrons diffuse from the N-type region to the P-type region, and some holes diffuse from the P-type region to the N-type region. As a result of their diffusion, the P region loses holes, leaving negatively charged impurity ions, and the N region loses electrons, leaving positively charged impurity ions. The ions in a semiconductor in an open circuit cannot move freely and therefore do not participate in conduction. These immovable charged particles form a space charge region near the interface between the P and N regions. The thickness of the space charge region is related to the dopant concentration. After the space charge region is formed, due to the interaction between positive and negative charges, an internal electric field is formed in the space charge region, and its direction is from the positively charged N region to the negatively charged P region. Obviously, the direction of this electric field is opposite to the direction of carrier diffusion movement, preventing diffusion. On the other hand, this electric field will cause the minority carrier holes in the N region to drift toward the P region, and the minority carrier electrons in the P region to drift toward the N region. The direction of the drift movement is exactly opposite to the direction of the diffusion movement. The holes drifting from the N region to the P region supplement the holes lost in the P region on the original interface, and the electrons drifting from the P region to the N region supplement the electrons lost in the N region on the original interface, which makes the space The charge decreases and the internal electric field weakens. Therefore, the result of the drift motion is to narrow the space charge region and strengthen the diffusion motion. Finally, the diffusion of majority carriers and the drift of minority carriers reach dynamic equilibrium. A thin layer of ions is left on both sides of the junction between the P-type semiconductor and the N-type semiconductor. The space charge region formed by this thin ion layer is called a PN junction. The direction of the internal electric field of the PN junction is from the N region to the P region. In the space charge region, due to the lack of multiple carriers, it is also called a depletion layer.