Band Gap and Semiconductor Current Carriers | Intermediate Electronics

Summary

This video explains the concept of band gap and current carriers in semiconductors. It covers the structure of atoms, the energy levels of electrons, and the difference between the valence and conduction bands. The video explores how the band gap affects the conductivity of materials like insulators, semiconductors, and conductors. It also introduces free electrons and holes as the two types of current carriers in semiconductors and explains how they contribute to electron and hole currents. Viewers gain a fundamental understanding of how current is produced in semiconductor materials.

Takeaways

• 🔬 Band gap is the energy difference between the valence band and conduction band, determining how easily electrons can move.
• ⚛️ In the Bohr model, atoms consist of neutrons, protons, and electrons, with valence electrons in the outermost shell, called the valence band.
• ⚡ Electrons in the valence band can gain enough energy to jump to the conduction band, enabling current flow in semiconductors.
• 🚫 Insulators have a large band gap, preventing easy electron movement, which results in poor conductivity.
• 🟢 Semiconductors have a smaller band gap, allowing electrons to move into the conduction band when external energy is applied.
• ⚡ Conductors, like copper, have no band gap, allowing electrons to move freely and conduct electricity easily.
• 🛠️ In semiconductors, current is carried by both free electrons in the conduction band and holes in the valence band.
• 🌡️ At room temperature, some electrons in intrinsic silicon gain enough energy to jump into the conduction band, creating free electrons and holes.
• 🔋 Electron current is produced when free electrons move toward the positive end of a voltage source in the conduction band.
• 🔄 Hole current occurs in the valence band as electrons move into nearby holes, creating current without overlapping with electron current.

Q & A

What are the fundamental components of an atom according to the Bohr model?

According to the Bohr model, an atom consists of a central nucleus containing protons and neutrons, with electrons orbiting around the nucleus in specific energy levels or shells.

What is the valence band in an atom?

The valence band refers to the outermost shell of an atom, where the valence electrons are located. It represents a band of energy levels that the electrons are confined to.

How does an electron move from the valence band to the conduction band?

An electron moves from the valence band to the conduction band when it gains sufficient energy from an external source to overcome the band gap, allowing it to escape the valence band.

What is the 'band gap' in a semiconductor?

The band gap is the difference in energy between the valence band and the conduction band. It is the amount of energy an electron needs to jump from the valence band to the conduction band.

Why do materials with a large band gap have poor conductivity?

Materials with a large band gap have poor conductivity because the electrons in the valence band require a large amount of energy to jump to the conduction band, making it difficult for current to flow.

How does the band gap affect the conductivity of insulators, semiconductors, and conductors?

In insulators, the band gap is very large, preventing electron movement and resulting in poor conductivity. In semiconductors, the band gap is smaller, allowing electrons to move to the conduction band with external energy. In conductors, the conduction and valence bands overlap, allowing free movement of electrons and excellent conductivity.

What are the two types of current carriers in a semiconductor?

The two types of current carriers in a semiconductor are free electrons (which move in the conduction band) and holes (which represent the absence of an electron in the valence band).

How is electron current produced in a semiconductor?

Electron current is produced when free electrons in the conduction band are attracted to the positive terminal of a voltage source, moving through the material and generating a flow of current.

What is hole current and how is it different from electron current?

Hole current occurs in the valence band when electrons move into nearby holes, creating a flow of current. Unlike electron current, which happens in the conduction band, hole current is the result of electron movement within the valence band.

How do external factors, such as temperature or voltage, affect the generation of current in semiconductors?

External factors like temperature or voltage can provide the energy needed for electrons to jump from the valence band to the conduction band, increasing the number of free electrons and holes, which in turn increases the current flow in semiconductors.

Outlines

00:00

🔍 Introduction to Band Gap and Semiconductor Current Carriers

This paragraph introduces the topics of band gap and current carriers in semiconductors. It begins by recalling basic chemistry concepts about atoms, including protons, neutrons, and electrons. The Bohr model is used to explain the structure of an atom with energy levels, or shells, surrounding the nucleus. The valence shell, also referred to as the valence band, contains electrons that can gain enough energy to escape to the conduction band, which is crucial for understanding how current is produced in semiconductors.

💡 Understanding Band Gap

Here, the concept of band gap is defined as the energy difference between the valence band and the conduction band. This energy must be supplied to a valence electron for it to jump to the conduction band, where it can move freely. The size of the band gap determines a material's conductivity. If the gap is large, the material has poor conductivity because electrons cannot easily reach the conduction band.

📊 Examining Energy Diagrams and Conductivity of Materials

The paragraph dives into energy diagrams of different materials—insulators, semiconductors, and conductors—and explains their conductivity based on the size of the band gap. Insulators have a large band gap, making it difficult for electrons to move. Semiconductors have a smaller band gap, allowing electrons to jump into the conduction band when provided with external energy. Conductors, like metals, have overlapping valence and conduction bands, which allows electrons to move freely and conduct electricity.

⚡ Current Carriers in Semiconductors

This paragraph introduces two main types of current carriers in semiconductors: free electrons and holes. It explains how atoms form solid crystalline materials through covalent bonding, and the example of silicon is used to illustrate how an intrinsic silicon crystal is formed. The text describes how valence electrons can jump into the conduction band, leaving behind holes in the valence band, both of which contribute to current generation in a semiconductor.

🌀 Electron Current in Semiconductors

Here, electron current is explained. At room temperature, intrinsic silicon crystals gain enough energy for some valence electrons to jump into the conduction band, creating free electrons. When a voltage is applied, these free electrons move toward the positive terminal of the voltage source, producing current in the material. This type of current is known as electron current.

🔄 Hole Current in Semiconductors

This section focuses on hole current, which occurs in the valence band. As valence electrons jump into the conduction band, vacancies, or holes, are left behind in the valence band. Electrons in the valence band can move into nearby holes, creating movement that generates a current known as hole current. Despite being caused by electron movement, it’s referred to as hole current to distinguish it from electron current in the conduction band.

📚 Conclusion and Recap of Band Gap and Semiconductor Current Carriers

The video concludes by summarizing the key topics covered: band gap, current carriers in semiconductors, and the energy diagrams of different materials (insulators, semiconductors, and conductors). The role of free electrons and holes in producing current in semiconductors is briefly revisited, and viewers are encouraged to ask questions or subscribe for more content.

Keywords

💡Band Gap

Band Gap refers to the energy difference between the valence band and the conduction band in a material. It is a fundamental concept in semiconductor physics. In the video, band gap is used to explain why certain materials conduct electricity while others do not. For instance, insulators have a large band gap, making it difficult for electrons to move freely, whereas conductors have no band gap, allowing free movement of electrons. Semiconductors have a smaller band gap, which can be overcome with external energy, making them useful for electronic devices.

💡Semiconductor Current Carriers

Semiconductor current carriers are particles that facilitate the flow of electric current in semiconductor materials. The video discusses two types: free electrons and holes. These carriers are crucial for understanding how semiconductors function in electronic devices. Free electrons move in the conduction band, while holes represent the absence of an electron in the valence band. The movement of these carriers in response to an electric field is what generates current in semiconductors.

💡Valence Band

The valence band is the highest energy band in a solid where electrons are normally present at absolute zero temperature. It is named after the valence electrons that occupy this band. In the context of the video, when a valence electron gains enough energy, it can jump from the valence band to the conduction band, which is essential for creating current in semiconductors.

💡Conduction Band

The conduction band is a range of energy levels in a solid where electrons can move freely and thus conduct electricity. The video explains that when electrons reach the conduction band, they can move throughout the material, contributing to electrical conductivity. This is in contrast to the valence band, where electrons are bound to their atoms.

💡Energy Levels

Energy levels are the specific amounts of energy that electrons in an atom or solid can have. The video uses the concept of energy levels to describe the organization of electrons around the nucleus and how they are grouped into shells or bands. Understanding energy levels is key to grasping how electrons move between the valence and conduction bands in semiconductors.

💡Electron

Electrons are negatively charged subatomic particles that orbit the nucleus of an atom. In the video, electrons play a critical role in the operation of semiconductors. When they gain enough energy, they can move from the valence band to the conduction band, becoming free electrons that contribute to the flow of current.

💡Hole

A hole is the absence of an electron in the valence band of a semiconductor. The video explains that when an electron is excited to the conduction band, it leaves behind a hole. The movement of these holes, as nearby electrons fill the vacancy, is described as hole current, which is a type of current carrier in semiconductors.

💡Covalent Bonding

Covalent bonding is a type of chemical bond formed by the sharing of electron pairs between atoms. The video uses the example of silicon atoms covalently bonding to form a crystalline structure. This bonding is important for creating the regular arrangement of atoms that allows for the movement of electrons and holes in semiconductors.

💡Intrinsic Silicon Crystal

An intrinsic silicon crystal is a pure form of silicon used in semiconductors, containing no impurities. The video mentions that at room temperature, some valence electrons in an intrinsic silicon crystal gain enough energy to become free electrons, creating holes in the process. This is a fundamental process in the operation of semiconductor devices.

💡Energy Diagram

An energy diagram is a graphical representation that shows the distribution of energy levels in a material. The video uses energy diagrams to illustrate the difference in band gaps between insulators, semiconductors, and conductors. This visual tool helps viewers understand how the band gap affects the conductivity of different materials.

💡External Energy

External energy refers to energy sources outside the material that can provide the necessary energy for electrons to move from the valence band to the conduction band. In the video, external energy is mentioned as a factor that can excite valence electrons, enabling them to jump the band gap and contribute to the current in semiconductors.

Highlights

Introduction to band gap and semiconductor current carriers to understand how current is produced in a semiconductor.

Atoms consist of neutrons, protons, and electrons, with the exception of normal hydrogen atoms which lack neutrons.

The Bohr model describes atoms as having a nucleus of protons and neutrons, surrounded by orbiting electrons.

Energy levels surrounding the nucleus are grouped into shells, and the outermost shell is called the valence shell.

Valence electrons are confined to the valence band, which represents a band of energy levels.

When a valence electron gains enough energy, it can move from the valence band to the conduction band.

The energy difference between the valence and conduction bands is called the band gap.

In materials with a large band gap, electrons have difficulty moving to the conduction band, leading to poor conductivity.

Insulators have a large band gap, preventing electron movement, while semiconductors have a smaller band gap.

In conductors like copper, there is no band gap; the conduction band and valence band overlap, allowing free electron movement.

Current carriers in semiconductors include free electrons and holes, and they are responsible for producing current.

Intrinsic silicon crystals are formed through covalent bonding, where silicon atoms bond with adjacent atoms.

At room temperature, silicon gains heat energy that allows valence electrons to move into the conduction band.

When electrons jump to the conduction band, vacancies known as holes are left in the valence band.

Electron current is produced when thermally generated free electrons move towards the positive end of a voltage source.

Hole current is created when electrons in the valence band move into nearby holes, contributing to overall current flow.

Transcripts

In this video, we are going to discuss about band gap and semiconductor current carriers,

which will help us understand how current is produced in a semiconductor.

If we recall some basic topics in chemistry, we’ll remember that all atoms consist of

neutrons, protons, and electrons, except for a normal hydrogen atom which doesn’t have

a neutron.

Using the Bohr model, we can visualize that an atom has a central nucleus consisting of

protons and neutrons that is surrounded by orbiting electrons.

The orbits surrounding the nucleus are grouped into energy levels known as shells and the

outermost shell is called the valence shell.

The valence shell of an atom represents a band of energy levels, which is why it’s

also called a “valence band” and valence electrons are confined to that band.

When a valence electron gains enough energy from an external source, it can escape from

the valence band and goes to the conduction band.

Band Gap The difference in energy between the valence

band and the conduction band is called “band gap”.

It is the amount of energy a valence electron must possess so that it can jump from the

valence band to the conduction band, wherein the electron is free to move throughout the

material.

If the band gap is really big, electrons will have a hard time jumping to the conduction

band, which is the reason of material’s poor conductivity.

Energy Diagrams Let’s try to examine the energy diagram

of the three types of materials used in electronics and discuss the conductivity of each material

based on their band gap.

As we can see, the band gap between the valence band and conduction band in an insulator is

really big.

That is why it doesn’t conduct current.

The band gap in a semiconductor is smaller compared to an insulator and allows valence

electrons in the valence band to jump into the conduction band if it receives external

energy.

In a conductor, like copper, there’s no band gap.

Actually, the conduction band and valence band overlaps, which means that electrons

can freely move into the conduction band.

This is why you may hear the electrons in metal referred to as a “sea of electrons”

- they’re just floating around.

Current Carriers Now that we know more about band gap, let’s

discuss the two types of current carriers in a semiconductor, free electrons and holes,

and see how they produce current in a semiconductor.

Atoms may combine to form a solid crystalline material through covalent bonding.

For example, a silicon atom covalently bonds with four adjacent silicon atoms to form an

intrinsic silicon crystal.

Intrinsic because it doesn’t contain impurities and a crystal because there is a pattern in

how the atoms are connected.

Electron Current At room temperature, intrinsic silicon crystal

gains enough heat energy that enables some of the valence electrons to jump into the

conduction band, becoming free electrons.

When this happens, vacancies are left in the valence band within the crystal.

These vacancies are known as holes.

Now, if we put a voltage source across an intrinsic silicon material, the thermally-generated

free electrons in the conduction band will be attracted to the positive end of the voltage

source.

They will move toward the positive end and this movement produces current in the material.

This type of current is called electron current.

Hole Current While electron current happens in the conduction

band, the other type of current, hole current, happens in the valence band.

Remember that as valence electrons jumped into the conduction band, vacancies or holes

are left in the valence band.

Electrons that remain in the valence band can move into a nearby hole when it receives

a small amount of energy.

This movement produces a current in the valence band called hole current.

Though the current is produced by valence electrons moving into a nearby hole, it is

called hole current so that it won’t be confused with the electron current produced

in the conduction band.

In this video, we learned about band gap and the current carriers in a semiconductor.

We examined the energy diagram of the three types of materials used in electronics, insulators,

semiconductors, and conductors and briefly discussed their conductivities.

We also talked about the two types of semiconductor current carriers, free electrons and holes,

and mentioned how they produce current in a semiconductor material.

If you have any questions, leave it in the comments below and if you’ve found this

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