Impurity doping is a very important step in chip manufacturing. Almost all integrated circuits, LEDs, power devices, etc. require doping. So why doping? What are the doping methods? What is the role of doping?
Why doping?
Intrinsic silicon has poor conductivity. It is necessary to introduce a small amount of impurities into intrinsic silicon to increase the number of movable electrons or holes to improve its electrical properties so that silicon can meet the standards for semiconductor manufacturing.
What is intrinsic silicon?
Intrinsic silicon refers to pure silicon, silicon that is not doped with any impurities, and its number of free electrons and holes is equal. Intrinsic silicon is a semiconductor material with poor conductivity at room temperature.
What is N-type silicon?
N-type silicon is made by doping pure silicon with pentavalent elements (such as P, As, etc.). Atoms of pentavalent elements such as phosphorus and arsenic replace the position of silicon atoms. Since silicon is 4-valent, there will be an extra electron. The extra electrons can move freely and carry negative charge. N means negative.
N+: heavily doped N-type semiconductor. N-: lightly doped N-type semiconductor.
What is P-type silicon?
P-type silicon is made by doping pure silicon with trivalent elements (such as B, Ga, etc.). Atoms of trivalent elements such as boron and gallium replace the position of silicon atoms, but compared with silicon atoms, it lacks an electron. A hole appears in the position where the electron is missing. The hole itself has a positive charge and can accept electrons, so it is called P-type silicon. P stands for positive.
P+, which means a highly doped P-type semiconductor with a high concentration. P-, which means a P-type semiconductor with a low doping concentration.
Common pentavalent elements
Pentavalent elements are group VA elements in the periodic table, represented by P and As. These two elements have 5 electrons in the outermost layer, 4 of which can form covalent bonds with silicon atoms, and the remaining one is a free electron.
Phosphorus (P) is a non-metallic element with a variety of allotropes, the most common of which are white phosphorus, red phosphorus and black phosphorus. Arsenic (As) is a metalloid element with a metallic luster and chemical properties similar to phosphorus, but arsenic compounds are generally more stable. Arsenic trioxide is an arsenic oxide, As2O3.
Common trivalent elements
Trivalent elements are group IIIA elements in the periodic table, represented by boron (B) and gallium (Ga). These two elements have 3 electrons in the outermost layer.
- Boron (B) is a hard and brittle non-metallic element with a black or brown color. In nature, it mostly exists in the form of its oxides or borates, and common substances include boric acid, borax, etc.
- Gallium (Ga) is a soft metal with a low melting point. Its melting point is about 29.76°C, and GaAs is widely used as a semiconductor material. In addition, gallium is also used in the production of solar cells, LEDs, etc.
Common doping methods
There are currently two main methods, namely diffusion and ion implantation:
- Diffusion
First, the semiconductor wafer is cleaned to ensure that there is no contamination on its surface. Subsequently, the silicon wafer is heated at high temperature (diffusion furnace). Dopant atoms can enter the semiconductor material, and after diffusion, post-processing such as annealing is performed to stabilize the distribution of dopants.
- Ion implantation
Ion implantation uses high voltage to accelerate ionized dopants to very high speeds, and the accelerated ions are precisely shot at the surface of the silicon wafer. Because the ions have high kinetic energy, they will penetrate the surface of the silicon wafer and enter its interior.
