Semiconductors: How They Emit Light or Heat in Modern Devices

When we talk about semiconductors powering LEDs, lasers, and processors, one key concept governs their behavior—band gap transitions. Whether a semiconductor emits light or simply dissipates heat depends on its band structure—specifically whether it's a direct or indirect band gap semiconductor.

In this blog, we’ll dive deep into:

  • What makes a band gap direct or indirect?

  • How electrons behave in each?

  • Why some semiconductors shine (literally) while others don’t?

  • And what real-world devices use them.

The Foundation: Band Structure in Semiconductors

A semiconductor's energy band structure determines how electrons move and how energy is released. Two major bands are involved:

  • Valence Band – Where electrons are bound to atoms.

  • Conduction Band – Where electrons are free to move and conduct electricity.

The gap between them is called the band gap. When electrons jump from the valence band to the conduction band (absorbing energy) and fall back down (releasing energy), we observe the output—either as light or heat.

What is a Direct Band Gap Semiconductor?

In a direct band gap semiconductor:

  • The lowest point of the conduction band and the highest point of the valence band are aligned at the same momentum (k-value).

  • This means an electron can directly fall back from the conduction band to the valence band without needing external help (like a phonon).

  • The energy released in this process is emitted as a photon, i.e., light.

Key Insight:

This is why direct band gap semiconductors are used in LEDs, laser diodes, and optoelectronic devices. They emit light efficiently.

Examples:

  • Gallium Arsenide (GaAs)

  • Indium Phosphide (InP)

  • Gallium Nitride (GaN)

What is an Indirect Band Gap Semiconductor?

In an indirect band gap semiconductor:

  • The lowest point of the conduction band and the highest point of the valence band occur at different momentum values.

  • So, when an electron falls back, it cannot do so directly—it needs to interact with a phonon (a quantum of lattice vibration) to conserve momentum.

  • The process is slower, and instead of emitting light, the electron loses energy as heat.

Real-World Effect:

These semiconductors do not emit light efficiently. Instead, they dissipate energy as heat.

Examples:

  • Silicon (Si)

  • Germanium (Ge)

Trapped Energy Levels and Carrier Behavior in Indirect Band Gaps

Here’s something deeper: In indirect band gap materials, there are often trap levels or localized energy states between the conduction and valence bands. These act like intermediate steps, where electrons may:

  • Get temporarily stuck

  • Lose energy slowly

  • Take time before recombination

This trapping results in non-radiative recombination, where energy is released as lattice vibrations (phonons)—which we perceive as heat.

Meanwhile, direct band gap semiconductors don’t rely on these trap states. The transition is quick and clean, leading to radiative recombination—i.e., light emission.

Summary: Light vs Heat – What's the Output?

Band Gap Type: Direct

  • Electron Transition: Straight from conduction to valence band at same momentum
  • Energy Output: Light(Photon)

Band Gap Type: Indirect 

  • Electron Transition: Needs a phonon to assist due to different momentum
  • Energy Output: Heat(Phonon)

Applications Based on Band Gap Type

Devices Using Direct Band Gap Semiconductors:

  • Light Emitting Diodes (LEDs)

  • Laser Diodes

  • Optical Fiber Communication

  • Photodetectors

 Devices Using Indirect Band Gap Semiconductors:

  • Silicon-based Microprocessors

  • Power Transistors

  • Heat Sensors and Thermistors

  • Solar Cells (with engineered trapping)

Conclusion: Why Understanding Band Gaps Matters

Whether you want to generate light or handle heat, the type of band gap in a semiconductor defines your possibilities. Direct band gap semiconductors shine—literally—in optoelectronics, while indirect ones serve silently in powerful processors and sensors.

So next time you look at an LED glowing or feel your laptop heating up, remember—it’s all about how electrons fall.

To understand the Classification of Semiconductors do visit our previous blog - Classification of Semiconductors: Intrinsic, Extrinsic, Band Gaps & Their Role in Modern Electronics

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