Recently, I took the opportunity during my free time to go through some course materials on semiconductors, and found some knowledge points quite interesting, so I organized my understanding of semiconductor knowledge.

Although it's not quite appropriate to write essays and professional knowledge at the same time on the public account, fortunately, there are not many followers now, and many of them are classmates and friends, without the burden of thought, so I will write some extracurricular knowledge!

Semiconductors are the industries that the country has been focusing on developing in the past two years. What exactly are semiconductors?

All objects in life can be roughly divided into three categories according to their electrical conductivity: conductors, semiconductors, and insulators.

This is easy to understand. Objects are either conductive or not conductive, or they have a little bit of conductivity. It is this kind of half-hearted, unclear material that provides physicists with different opportunities for play.

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Too absolute conductive and non-conductive materials are not very interesting, but things that change their conductivity under different conditions are interesting.

Here is a picture to intuitively see the conductivity of objects:

According to electrical conductivity, they can be divided into:

Insulators: very low electrical conductivity, about between 20-18 S/cm to 10-8 S/cm, such as fused quartz and glass;Conductors: Have a high electrical conductivity, ranging from 10^4 S/cm to 10^6 S/cm, such as metals like aluminum and silver.

Semiconductors: Their electrical conductivity lies between that of insulators and conductors.

Common elemental semiconductors found in nature include silicon and germanium. It is said that germanium-based semiconductors were discovered and applied earlier than silicon-based ones. However, the natural advantage of silicon is its low cost! Common sand and stone in nature contain a large amount of silicon elements, imagine how much there is!

Even though there is a lot of silicon sand in nature, it contains too many impurities and defects, and cannot be used directly. It needs to be refined.

How to refine it? In one word - burn!

As learned in junior high school chemistry, carry out redox reactions.

①SiC + SiO2 → Si(solid) + SiO2(gas) + CO(gas)

②Si(solid) + 3HC → SiHCl3(gas) + H2(gas)

③SiHCl3(gas) + H2(gas) → Si(solid) + 3HCl(gas)

After three high-temperature chemical reactions, we obtained solid silicon, but at this time, the silicon is polycrystalline silicon.What is polycrystalline silicon?

Just as when we peel an orange, there are many segments inside the orange (polycrystalline orange), and the taste of different segments is different (crystal orientation), we need to choose the best-tasting segment and let this segment grow alone!

How to make a small single crystal grow alone?

Physicists are very smart and have invented a method to grow single crystals, called the Czochralski method, which may be named after this scientist.

The industry also has an intuitive name for it, called the pulling method!

Because when growing single crystals, it is to pull the small crystal up! The speed is a bit slow when pulling, let's take a look at this device:

The blue round rod in the picture is the single-crystal silicon. During the pulling process, it rotates while being pulled up, and the crystal ingot grown by the pulling method is a cylinder.The ingot that has been grown is then cut into slices using a mechanical blade, cut into round disc shapes, and thus wafers are made.

Does it look familiar?

Wafers are produced in this way.

Although we have obtained the wafers, the electrochemical properties of the single-crystal silicon at this point are not up to par and cannot be directly used to make chips. Engineers then find ways to improve the electrochemical properties of the single-crystal silicon.

How to improve the single-crystal silicon?

Let's delve into the element silicon first. In the periodic table, silicon is ranked 14th, and the silicon atom has 4 electrons in its outermost shell, which are shared with 4 surrounding atoms to form 4 pairs of shared electrons. This structure of shared electron pairs is called covalent bonding. Each pair of electrons forms a covalent bond.

The picture on the left is the crystal structure of single-crystal silicon, which is the diamond crystal structure. The picture on the right shows the situation of silicon atoms sharing electrons, with a central silicon atom sharing electrons with four silicon siblings.

One day, a physicist suddenly thought of a question: what if the silicon family did not share electrons with silicon siblings and brought other siblings into the group?

One day, the physicist brought the arsenic brother into the group, and a miracle happened:

The arsenic brother has 5 electrons in the outermost shell, 4 of which found a partner from the silicon family, and the other electron was left single. This electron became a vagrant, wandering around, and because the electron carries a charge, it changed the conductivity of the silicon family.At this point, the arsenic atom donates an additional electron to the silicon family, hence the arsenic atom is referred to as a donor.

With an increase in the number of free electrons in the silicon family, the negative charge carriers increase, turning the silicon into an n-type semiconductor.

Why is it called n-type? In English, "Negative" represents a negative charge, taking the first letter of this word, it becomes N.

Similarly, physicists thought, since they could bring in arsenic, which has more electrons, could they also bring in boron, which has fewer electrons? So, physicists tried to incorporate boron atoms.

Because the boron atom has only three electrons in its outermost shell, one less than silicon, what was originally a covalent bond with two pairs of electrons now has only one pair, leaving an extra space, which becomes a positively charged hole.

At this time, the silicon-based semiconductor is called a p-type semiconductor, with P coming from the first letter of the English word "Positive" (positive pole), and the boron atom is referred to as an acceptor.

It is precisely the different atoms added to the silicon single crystal that form n-type and p-type semiconductors.When we have monocrystalline silicon and can find ways to oxidize the surface of the monocrystalline silicon into silicon dioxide, silicon dioxide can serve as an insulator for many device structures, or as a barrier layer for diffusion or ion implantation during the device manufacturing process.

For example, in the manufacturing process of a p-n junction, a silicon dioxide thin film can be used to define the area of the junction.

Let's take a look at a schematic diagram, where (a) shows a silicon wafer without a cover layer, ready for the oxidation step, and (b) only shows the upper surface of the oxidized wafer.

With the theoretical knowledge of P-type and N-type semiconductors, we can also play with more complex things, such as modifying the surface of silicon dioxide into the shapes we want, such as drawing a cat, a flower, etc...

The method to modify the surface of the wafer is photolithography!

Isn't photolithography going to require high-end photolithography equipment? I heard that this kind of equipment is very powerful... Let's first take a look at the principle of photolithography:

Using a high-speed spin coating device (spinner), a layer of material sensitive to ultraviolet (UV) light, called photoresist, is spin-coated on the surface of the wafer. After taking the wafer off the spin coater, it is baked at a temperature between 80°C and 100°C to remove the solvent in the photoresist and harden the photoresist, enhancing the adhesion between the photoresist and the wafer. Next, UV light is used to expose the wafer through a patterned mask. Then, buffered hydrofluoric acid is used as an acid etching solution to remove the surface of silicon dioxide that is not protected by the photoresist. Finally, the photoresist is stripped using chemical solvents or a plasma oxidation system.

The text may sound a bit complicated, but the intuitive understanding is somewhat similar to carving a seal. First, paint a model on the stone with paint, and then carve according to the size of the model. Basically, that's the principle.

There is a difference between positive and negative carvings in seals, and wafers are the same. Depending on the choice of photoresist, positive and negative carvings can also be achieved. The photoresist chosen by people is called positive photoresist and negative photoresist.After lithography, the silicon surface is exposed to the external environment. At this point, physicists introduce other elements to this silicon surface using different methods, which is called ion implantation.

Because after the implantation of B or As ions, these ions change the tradition of the silicon family after being added to the silicon family, and the electrochemical properties of silicon have changed. The semiconductor at this time is called an extrinsic semiconductor.

And the junction formed by the contact of p-type semiconductor and n-type semiconductor is called a p-n junction!

After doping is completed, we need to find a way to lead out the performance of this semiconductor, so the surface of this semiconductor is metallized, ohmic contact and interconnection are completed in the following metallization steps, and the metal film can be formed by PVD or CVD.

With the completion of metallization, the p-n junction can work!

That's all for the introduction of simple semiconductor knowledge!