The Complete Guide to Doping in Semiconductors: What Is it and Why Is it Necessary?

Semiconductor doping is a key process in semiconductor manufacturing that helps wafers conduct electricity better. Doping is essential for building electronic devices like transistors and diodes, so before you buy a silicon wafer, it's key to understanding this process properly.

Silicon doping makes N-Type and P-Type wafers, which are crucial for electronic devices like transistors and diodes to work properly. In this article, we’ll explain its benefits and what you need to know about them.

Doping in Semiconductors

In electronics, a process known as semiconductor doping involves adding minute quantities of impurities to pure semiconductor materials, like silicon, to modify their electrical characteristics.

In fact, silicon is not a good electrical conductor when pure. Doping is the process of adding minute amounts of impurities to make it suitable for electronics.

Elements from groups III and V of the periodic table are used for doping. Phosphorus, which has five valence electrons, and boron, which has three, are the two most significant substances that can be used to dope silicon.

P-type Silicon

In P-type doping, impurities create an excess of positively charged holes in the crystal lattice, changing its conductivity. Elements from Group III of the periodic table, like boron, aluminum, or gallium, are used.

These elements have one fewer valence electrons than semiconductors, so when they replace semiconductor atoms, they create positively charged vacancies. In fact, the “P” stands for “positive.”  

These elements cause silicon to become abundant in "holes," which attract free electrons and contribute to the material’s conductivity. There are a few common ways to dope silicon with group III acceptor atoms:

• Adding boron or gallium impurities to molten silicon while it is crystallizing into an ingot is known as "doping" during crystal growth.  

• Thermal diffusion is the net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. Diffusion causes materials to gradually mix together. The wafer surface is exposed to boron tribromide or nitride at temperatures above 1000°C in order to infuse atoms.  

• A beam of boron ions is directed straight into the silicon lattice during ion implantation. The voltage required to accelerate the ions can be changed to precisely control the penetration depth.

The majority of cutting-edge semiconductor factories chose the latter approach because it provides exact control over the doping concentration across incredibly thin layers inside the silicon crystal.

To maximize the positive charge carrier mobility, calibration must be done correctly.

N-type Silicon

N-type doping increases the quantity of mobile negative charge carriers. Common N-type dopants include group V elements such as antimony, phosphorus, and arsenic.  

These elements have an extra electron because they have five valence electrons, one more than an undoped semiconductor's four. When these dopants are added to the semiconductor, certain host atoms are swapped out for dopant atoms.  

As a result, conductivity is improved, and excess electrons are produced that are free to move. Electrons are negative charge carriers—hence the “N” for “negative.”

Much like producing a P-type wafer, creating an N-type silicon wafer starts with refining raw silicon into an ultra-pure monocrystalline form. The difference lies in how impurity is embedded to enable negative charge carriers.  

Common doping techniques for N-type silicon wafers include:  

• Diffusion of phosphine gas. A carrier gas that has been enriched with phosphine gas (PH3), the desired dopant, is introduced to silicon wafers at temperatures higher than 900°C so that an even concentration balance can occur.

• By dripping POCl3 liquid onto wafers and heating them to about 1000°C, liquid phosphorus diffusion is achieved. Next, the liquids are passed through a carrier gas, which carries the dopant in a gaseous form. Since not every wafer needs to be doped, silicon dioxide can be used to mask off specific regions. Doping does not occur at these sites because the dopants are unable to pass through the oxide.  

• Phosphorus ions are injected directly into silicon through ion implantation, which allows for precise doping control.  

Ion implantation frequently yields the greatest outcomes when it comes to N-type wafers designed for cutting-edge electronics. For specific applications, it is possible to precisely create N-type regions in silicon by calibrating beam currents and acceleration voltages.  

Reasons for the Need for N-Type and P-Type Wafers

The fascinating thing about semiconductor devices is that they require both P and N types of silicon to operate. One cannot just use the other to create integrated circuits, solar cells, transistors, and other devices.  

This is due to the necessity of the P-N junction interaction.  

When a P-type semiconductor comes into contact with an N-type semiconductor, the line separating the silicon's positive and negative halves acts like a gate, controlling the flow of electrical current.

It allows current to flow easily in one direction but not in the opposite direction, which is essential for devices like diodes used in rectifiers and voltage regulators.

The unique qualities on either side of the intersection enable a wide range of advantageous applications.

For example, diodes and LEDs work by applying a voltage across the P-N junction, which causes current to flow. Transistors change the width of the P-N junction depletion region to amplify signals and turn on and off currents.  

Effect of Doping on Energy Band Gap

Band theory is a fundamental concept in semiconductor physics that helps us understand how electrons are positioned in different materials.

In a semiconductor material, the valence band is the highest energy band that is completely filled with electrons at absolute zero temperature. The conduction band, on the other hand, is the next higher energy band that remains empty at absolute zero.  

When a semiconductor is doped with impurities, new energy levels are introduced within the band gap between the valence band and the conduction band. They are known as the donor level in N-type and the acceptor level in P-type semiconductors.  

For P-type semiconductors, the impurities create holes in the valence band. Conversely, for N-type semiconductors, the impurities introduce extra electrons in the conduction band.  

P-type Silicon Wafers vs. N-type Silicon Wafers

While the doping process is what distinguishes P and N-type wafers, the substrate specs also impact quality and performance.

Comparable P-type wafers typically have lower resistivity than N-type wafers. Longer diffusion lengths can be achieved, and the lifetime of N-type silicon carriers is in the order of milliseconds, despite the fact that the bipolar diffusion coefficient in N-type silicon is three times smaller than in P-type silicon.

When compared to boron-doped P-type-based technologies, N-type-based crystalline silicon technologies exhibit lower temperature coefficients and higher efficiencies (23–25%). Additionally, metallic impurities and somewhat lower oxygen levels are frequently used in their production.

Nonetheless, their market share is still quite small (5% in 2015, 5% in 2019, and 5% in 2020), most likely as a result of the increased fabrication costs.

Learn From Wafer World and Buy a Silicon Wafer You Need

Nowadays, a wide variety of substrate and semiconductor materials can be used to enhance our technology, and common substrates like silicon can be enhanced to optimize this feature.  

Moreover, combining P-type and N-type silicon wafers opens the door to a wider range of applications. These days, nearly all semiconductor electronics demand both. If you’re interested in learning more, reach out to Wafer World and learn about what we have to offer!