physics

Impact ionization

Impact ionization is a physics 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 Impact ionization rather than just read about it. In short: Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or hole) with enough kinetic energy can knock a bound electron out of its bound state (in the valence band) and promote it to a state in the conduction band, creating an electron-hole pair.

Key takeaways

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

Reference excerpt

Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or hole) with enough kinetic energy can knock a bound electron out of its bound state (in the valence band) and promote it to a state in the conduction band, creating an electron-hole pair. For carriers to have sufficient kinetic energy a sufficiently large electric field must be applied, in essence requiring a sufficiently large voltage but not necessarily a large current. If this occurs in a region of high electrical field then it can result in avalanche breakdown. This process is exploited in avalanche diodes, by which a small optical signal is amplified before entering an external electronic circuit. In an avalanche photodiode the original charge carrier is created by the absorption of a photon. The impact ionization process is used in modern cosmic dust detectors like the Galileo Dust Detector and dust analyzers Cassini CDA, Stardust CIDA and the Surface Dust Analyser for the identification of dust impacts and the compositional analysis of cosmic dust particles. In some sense, impact ionization is the reverse process to Auger recombination. Avalanche photodiodes (APD) are used in optical receivers. Before the signal is given to the receiver circuitry the photon is multiplied with the photocurrent and this increases the sensitivity of the receiver since photocurrent is multiplied before encountering of the thermal noise associated with the receiver circuit.

See also Multiphoton ionization Avalanche breakdown Avalanche diode Avalanche photodiode

References

External links Animation showing impact ionization in a semiconductor

Illustrations

Worked examples

Example 1 — a first encounter with Impact ionization

Start with the simplest possible case. Write down what Impact ionization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 Impact ionization 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 Impact ionization 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 Impact ionization

In research: Impact ionization appears in physics 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 Impact ionization 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: Impact ionization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ionization, Nuclear and atomic physics stubs, Semiconductors, so understanding it makes those chapters shorter.
In everyday life: Look for Impact ionization 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Impact ionization in 20 minutes

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

Frequently asked questions

What is Impact ionization in simple terms?

Why does Impact ionization matter?

Because it connects several physics 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 Impact ionization?

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 Impact ionization.