Semiconductor Basics

What is Semiconductors?

Semiconductors are special materials, like silicon, that can act as both a conductor (allowing electricity to flow) and an insulator (blocking electricity) depending on how they are treated. By adding tiny amounts of other materials (a process called doping), we can control how electricity flows through them. This makes semiconductors the building blocks of electronic devices like computers, phones, and solar panels. They are used in components like transistors and diodes to control and process electrical signals, making modern technology possible.

Conductor

A conductor is a material that allows electricity to flow through it easily because it has many free electrons, which are loosely bound to their atoms. These free electrons can move freely within the material when an electric field is applied, creating an electric current. Copper, for example, is one of the best conductors due to its atomic structure. A single atom of copper has 29 electrons, with one electron in its outermost shell (also called the valence shell). This outermost electron is only weakly attracted to the nucleus, making it “free” to move around the material.

Conductors are electrically neutral overall, meaning the total number of positive charges (protons) in their nuclei equals the total number of negative charges (electrons). However, the presence of these free-moving electrons is what enables conductors to carry electricity efficiently. In metals like copper or silver, the dense arrangement of atoms also allows free electrons to pass through easily without much resistance, which is why these materials are widely used in electrical wiring, circuits, and power transmission systems.

Insulators

An insulator is a material that does not allow electricity to flow through it easily because it has very few or no free electrons. In these materials, the electrons are tightly bound to their atoms and cannot move freely, even when an electric field is applied. This lack of free-moving electrons prevents the material from conducting electricity. Examples of insulators include rubber, glass, plastic, and wood.

In insulators, the outermost shell of electrons (valence shell) is either completely full or tightly held by the atomic nucleus, making it very difficult for the electrons to gain energy and move. For instance, in glass, the electrons are so strongly bound that they cannot participate in the flow of electric current. Insulators are neutral overall, with the number of positive charges (protons) equal to the number of negative charges (electrons), but since there are no free electrons, the material resists the flow of electricity.

Because of their high resistance to electric current, insulators are used to protect and isolate conductors in electrical systems. For example, rubber or plastic coatings on wires prevent electricity from escaping and protect users from electric shocks. Insulators are also essential in high-voltage applications, ensuring that electrical energy flows only where it is intended.

Semiconductors

A semiconductor is a material that falls between a conductor (like copper) and an insulator (like rubber) in its ability to conduct electricity. Semiconductors have a moderate number of free electrons, and their ability to conduct electricity can be controlled or altered by introducing impurities (a process called doping) or by changing conditions like temperature or light exposure. Silicon, the most widely used semiconductor, is the backbone of modern electronics due to its abundance and excellent properties.

In its pure form, silicon has four electrons in its outermost shell (valence shell), which form strong bonds with neighboring silicon atoms in a crystal structure. This means there are very few free electrons, so pure silicon behaves more like an insulator. However, when small amounts of other elements are added (such as phosphorus or boron), the number of free charge carriers increases. Phosphorus adds extra electrons, creating an n-type semiconductor (with negative charge carriers), while boron creates “holes” by accepting electrons, forming a p-type semiconductor (with positive charge carriers).

Semiconductors are neutral overall, with the same number of positive charges (protons) and negative charges (electrons), but the doping process creates conditions for controlled electrical flow. Silicon is used to make diodes, transistors, solar cells, and integrated circuits (chips) because its properties can be precisely manipulated, making it essential for modern technology like smartphones, computers, and renewable energy devices.

Silicon Crystals

Silicon crystals are the structured arrangement of silicon atoms in a repeating, three-dimensional pattern, forming the foundation of modern semiconductor technology. Each silicon atom in the crystal has four electrons in its outermost shell, which it shares with four neighboring atoms to create strong covalent bonds. This results in a stable and rigid crystal lattice. In its pure state, silicon crystals act as poor conductors of electricity because all the electrons are tightly bound in these bonds. However, when doped with small amounts of other elements, like phosphorus or boron, the crystal gains free electrons or holes, making it conductive and suitable for electronic applications. Silicon crystals are used in devices like solar panels, transistors, and microchips due to their excellent ability to regulate electrical flow and their abundance in nature.

Covalent bonds

Covalent bonds are strong chemical bonds formed when two atoms share one or more pairs of electrons to achieve stability. These bonds are common in nonmetals, such as in a silicon crystal, where each silicon atom shares electrons with four neighboring atoms, creating a stable and rigid lattice structure. Covalent bonds are essential for the structural integrity of many compounds and materials.

The Holes

In semiconductors, holes refer to the absence of an electron in the valence band, which behaves like a positive charge carrier. When an electron moves from one atom to fill a neighboring hole, the hole appears to move in the opposite direction. Holes are created in p-type semiconductors, where doping with elements like boron introduces fewer electrons, leaving “gaps” for electrons to occupy. These holes, though not physical particles, play a critical role in the flow of electric current within semiconductors.

Flow of Free Electrons and Holes

The flow of free electrons and holes in a semiconductor is crucial for electrical conduction. Free electrons, which are negatively charged, move toward the positive terminal under an applied electric field, contributing to current in materials like n-type semiconductors. Conversely, holes, representing the absence of electrons in the valence band, move in the opposite direction, toward the negative terminal. This hole movement in p-type semiconductors contributes to current, even though it is the movement of electrons that fills the holes. Together, these processes allow current to flow in semiconductors, with free electrons and holes playing complementary roles in conduction.

Doping a Semiconductor

Doping a semiconductor in silicon means adding small amounts of other elements to change how it conducts electricity. Pure silicon is not a good conductor, but by introducing impurities like boron (from group III elements) or phosphorus (from group V elements), we can make it either more positive or negative. Adding boron creates “holes” or missing electrons, making the material a p-type semiconductor, while adding phosphorus adds extra electrons, making it an n-type semiconductor. These changes allow us to control the flow of electricity, which is essential for making electronic devices like transistors work.

n-Type Semiconductor

An n-type semiconductor is a type of semiconductor that has been doped with elements from group V of the periodic table, such as phosphorus or arsenic. These elements have more valence electrons than silicon, so when they are introduced into the silicon crystal, they contribute extra electrons that are free to move through the material. This results in an abundance of negative charge carriers (electrons), making the material negatively charged. The majority charge carriers in an n-type semiconductor are electrons, which enhance its ability to conduct electricity.

p-Type Semiconductor

On the other hand, a p-type semiconductor is doped with elements from group III of the periodic table, like boron or gallium. These elements have fewer valence electrons than silicon, which creates “holes” in the crystal structure where electrons are missing. These holes act as positive charge carriers, and when an external voltage is applied, electrons from neighboring atoms move to fill these holes, allowing the flow of current. The majority charge carriers in a p-type semiconductor are holes, which makes the material positively charged.

p-n junctions

Together, n-type and p-type semiconductors are used to form p-n junctions, which are the basic building blocks of many electronic devices, such as diodes, transistors, and solar cells. The interaction between the free electrons in the n-type material and the holes in the p-type material enables the control of current flow, crucial for electronic switching and amplification.