Semiconductor Physics – Electrons, Holes, and Energy Levels Explained

Charge of an Electron – The Smallest Unit That Powers Everything

Every electronic device — from your smartphone to satellites — relies on the motion of electrons. But what exactly is an electron, and why does its charge matter?

The electron is a subatomic particle with a negative charge, and it plays a key role in current flow, semiconductor behavior, and every basic electronic component.

Value of Electron Charge:

The negative sign indicates that electrons carry negative electric charge. When electrons move through a conductor or semiconductor, electric current is generated.

Why Don’t Electrons Fall Into the Nucleus? Understanding Electron Orbits

At the heart of every atom lies the nucleus — packed with protons and neutrons. Surrounding it are electrons, zipping around in defined energy levels or "shells." But here’s a big question...

If opposite charges attract, why don’t electrons crash into the nucleus?

Electrons don't orbit like planets around the sun — they exist in probability zones or orbitals, defined by quantum mechanics. If an electron were to spiral into the nucleus, it would have to lose energy. But there's a catch:

• Electrons can only occupy discrete energy levels.

• They cannot exist in between levels, and they cannot lose all energy unless there’s a mechanism (like absorption or emission of photons).

So, the electron stays in its lowest possible energy state (called the ground state) without collapsing into the nucleus.

Innermost vs Outermost Electrons

• Innermost electrons (like those in the K and L shells) are tightly bound to the nucleus.

• They are stable and don’t participate in bonding or conduction.

On the other hand…

• Valence electrons are the outermost electrons, loosely bound and more reactive.

• In conductors and semiconductors, these are the electrons that move, transfer, and form currents.

Why We Focus Only on Valence Electrons in Electronics

Electronics is all about controlling current and manipulating charge. And:

 It’s only the valence electrons that:

• Respond to electric fields

• Break free and become conduction electrons

• Participate in PN junction behavior, band formation, and current flow

Inner electrons? They’re too tightly bound to care about your circuit. 😄.

Why?

As we move away from the nucleus, the energy of the electrons increases. Electrons in outer shells (valence electrons) have higher potential energy compared to those in inner shells because they are less tightly bound to the nucleus. This makes them more likely to participate in chemical reactions and electrical conduction — which is why valence electrons are so important in electronics.

What is a Hole in Semiconductor Physics?

When an electron in the valence band gains energy and jumps into the conduction band, it leaves behind an empty spot in the valence band. This spot is called a hole.

Even though a hole isn’t a physical particle, it behaves like a positive charge carrier. In fact, it acts as if a positive particle is moving in the opposite direction of an electron.

• Electrons move from atom to atom

• The absence of an electron appears to shift — this is the hole movement

• In P-type semiconductors, holes dominate the current flow

Hole vs Electron – Energy Behavior

Let’s look at energy movement now:

• As we move away from the nucleus, the energy of electrons increases.

• Electrons in outer shells (valence band) are less tightly bound and thus have higher potential energy.

Now for the reverse side:

As we move toward the nucleus, the energy of holes increases. This might sound strange at first — but remember, a hole is the absence of an electron in the valence band. So, holes are considered to occupy higher energy states the deeper they are in the valence band (closer to the nucleus).

This mirrored energy behavior is why in band diagrams, the valence band is drawn upside down — so that as you go deeper, hole energy increases.

Electron Transitions – Jumping Between Energy Levels

Electrons in an atom don’t float randomly — they’re confined to specific energy levels or shells. But they’re not stuck there forever.

These energy levels are quantized, meaning an electron can only exist in one of them — never in between.

How Do Electrons Move Between Shells?

Electrons can jump to a higher energy level by absorbing energy, and they can drop to a lower level by releasing energy.

When energy is absorbed:

• The electron moves from a lower shell to a higher shell (excitation).

• This energy could come from heat, light (photons), or electricity.

• The atom enters an excited state.

When energy is released:

• The electron falls from a higher shell to a lower shell.

• It emits that energy as light (a photon) or heat.

• This is the principle behind LEDs and lasers.

Energy Increases With Distance from the Nucleus

As you move to shells farther from the nucleus:

• The energy of the electron increases

• The electron becomes less tightly bound to the atom

• It becomes more available for conduction in metals and semiconductors

This behavior lays the groundwork for how electrons become conduction electrons — free to move and generate current in circuits.

Band Gap Energy and Wavelength Relation:

 Breakdown:

= Band gap energy (in eV).

• $\mathrm{\lambda (lambda)}$ = Wavelength (in angstroms, 1 Å =meters).

• 12400 is the constant derived from Planck’s equation using proper unit conversions for eV and Å.

 Example:

If a material absorbs light of 6200 Å wavelength:

This means the semiconductor’s band gap is 2.0 eV, like Gallium Phosphide (GaP).

This post forms the bedrock of semiconductor physics.

In the next blogs, we’ll dive deeper into energy bands, Fermi levels, intrinsic vs. extrinsic semiconductors, and eventually PN junctions and MOSFET magic.

Stick around — it only gets better from here!

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