Semiconductor Doping

Doping plays a crucial role in semiconductor technology. It allows engineers to alter the electrical properties and control the conductivity and behavior of semiconductors. Introducing specific impurities into the semiconductor crystal lattice makes it possible to create regions with either excess electrons or electron deficiencies known as “holes”. 

Doping is crucial for fabricating electronic devices like transistors, diodes, and integrated circuits. By carefully controlling doping levels and types in semiconductors, engineers can tailor their electrical properties to meet diverse technological needs, from high-speed integrated computer circuits to efficient solar cells for renewable energy applications.

There are two main types of semiconductor doping: P-type and N-type.

1. P-type Doping

In P-type doping, impurities are introduced to create an excess of positively charged “holes” within the crystal lattice structure, altering its conductivity. This doping process involves incorporating elements from Group III of the periodic table, such as boron, aluminum, or gallium, into the semiconductor crystal lattice. These Group III elements have one less valence electron than the semiconductor material, creating positively charged vacancies in the crystal lattice when they replace the semiconductor atoms. These holes can then attract free electrons, effectively creating positively charged carriers that contribute to the material’s conductivity.

2. N-type Doping

N-type doping increases the number of mobile charge carriers of negative polarity, hence the term “N-type,” where “N” stands for negative. The most commonly used dopants for achieving N-type doping are elements from group V of the periodic table, such as phosphorus, arsenic, and antimony. These atoms have five valence electrons, one more than the four valence electrons of an undoped semiconductor, resulting in an extra electron. When a small amount of these dopant atoms is introduced into the crystal lattice of the semiconductor material during manufacturing, some of the host atoms are replaced by the dopant atoms.

The dopant atoms create excess negative charge carriers, electrons, in the semiconductor material. These extra electrons are relatively free to move within the crystal lattice, contributing to the material’s conductivity. This excess of electrons is what characterizes N-type semiconductor material. N-type doping is essential for creating the majority charge carriers in the region of a semiconductor device known as the “n-region,” which is crucial for operating components like NPN transistors and n-type diodes.

Effect of Doping on Energy Band Gap

In a semiconductor material, the valence band represents the highest energy band fully occupied by electrons at absolute zero temperature. Conversely, the conduction band is the next higher energy band that remains empty at absolute zero.

When a semiconductor is doped with impurities, it introduces additional energy levels within the band gap. For P-type semiconductors, these impurities create holes in the valence band that act as positive charge carriers. Conversely, N-type semiconductors are doped to introduce extra electrons in the conduction band, serving as negative charge carriers.

In general, doping plays a vital role in creating P-N junctions, essential building blocks in semiconductor devices. When a P-doped region is placed close to an N-doped region, a P-N junction forms, creating a barrier that controls the flow of charge carriers. This junction enables the development of diodes, where current can flow in one direction while being significantly hindered in the opposite direction, facilitating applications such as rectification and voltage regulation.