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Photo–Dember effect

Photo–Dember effect is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photo–Dember effect rather than just read about it. In short: In semiconductor physics, the photo–Dember effect (named after its discoverer Harry Dember) is the formation of a charge dipole in the vicinity of a semiconductor surface after ultra-fast photo-generation of charge carriers. The dipole forms owing to the difference of mobilities (or diffusion constants) for holes and electrons which combined with the break of symmetry provided by the surface lead to an effective cha…

Photo–Dember effect — main illustration
Photo–Dember effect — illustration

Key takeaways

  • Photo–Dember effect belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Photo–Dember effect to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Photo–Dember effect from memory before moving on to harder problems.

Reference excerpt

In semiconductor physics, the photo–Dember effect (named after its discoverer Harry Dember) is the formation of a charge dipole in the vicinity of a semiconductor surface after ultra-fast photo-generation of charge carriers.

The dipole forms owing to the difference of mobilities (or diffusion constants) for holes and electrons which combined with the break of symmetry provided by the surface lead to an effective charge separation in the direction perpendicular to the surface. In an isolated sample, where the macroscopic flow of an electric current is prohibited, the fast carriers (often the electrons) are slowed and the slow carriers (often the holes) are accelerated by an electric field, called the Dember field. One of the main applications of the photo–Dember effect is the generation of terahertz (THz) radiation pulses for terahertz time-domain spectroscopy. This effect is present in most semiconductors but it is particularly strong in narrow-gap semiconductors (mainly arsenides and antimonides) such as InAs and InSb owing to their high electron mobility. The photo–Dember terahertz emission should not be confused with the surface field emission, which occurs if the surface energy bands of a semiconductor fall between its valence and conduction bands, which produces a phenomenon known as Fermi level pinning, causing, at its time, band bending and consequently the formation of a depletion or accumulation layer close to the surface which contributes to the acceleration of charge carriers. These two effects can contribute constructively or destructively for the dipole formation depending on the direction of the band-bending.

See also Photoelectrochemical process

References

Illustrations

Photo–Dember effect: Simplified Monte-Carlo simulation of the photo–Dember effect in semiconductors. Electrons are assumed to have a mobility 3 times larger than holes (for visualisation purposes). It can be observed how electrons diffuse away from the surface faster than holes shifting the "centre of negative charge" deeper into the semiconductor while the holes ("centre of positive charge") remain closer to the surface, thus forming a dipole.
Simplified Monte-Carlo simulation of the photo–Dember effect in semiconductors. Electrons are assumed to have a mobility 3 times larger than holes (for visualisation purposes). It can be observed how electrons diffuse away from the surface faster than holes shifting the "centre of negative charge" deeper into the semiconductor while the holes ("centre of positive charge") remain closer to the surface, thus forming a dipole.

Worked examples

Example 1 — a first encounter with Photo–Dember effect

Start with the simplest possible case. Write down what Photo–Dember effect claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photo–Dember effect before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photo–Dember effect ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photo–Dember effect

In research
Photo–Dember effect appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Photo–Dember effect in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Photo–Dember effect is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optoelectronics, Semiconductors, Terahertz technology, so understanding it makes those chapters shorter.
In everyday life
Look for Photo–Dember effect outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Photo–Dember effect in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Photo–Dember effect means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Photo–Dember effect out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photo–Dember effect in simple terms?

In semiconductor physics, the photo–Dember effect (named after its discoverer Harry Dember) is the formation of a charge dipole in the vicinity of a semiconductor surface after ultra-fast photo-generation of charge carriers. The dipole forms owing to the difference of mobilities (or diffusion const…

Why does Photo–Dember effect matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photo–Dember effect?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photo–Dember effect.

Tags

  • Optoelectronics
  • Semiconductors
  • Terahertz technology

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