engineering

Ridley–Watkins–Hilsum theory

Ridley–Watkins–Hilsum theory is a engineering 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 Ridley–Watkins–Hilsum theory rather than just read about it. In short: In solid state physics the Ridley–Watkins–Hilsum theory (RWH) explains the mechanism by which differential negative resistance is developed in a bulk solid state semiconductor material when a voltage is applied to the terminals of the sample. It is the theory behind the operation of the Gunn diode as well as several other microwave semiconductor devices, which are used practically in electronic oscillators to produc…

Key takeaways

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

Reference excerpt

In solid state physics the Ridley–Watkins–Hilsum theory (RWH) explains the mechanism by which differential negative resistance is developed in a bulk solid state semiconductor material when a voltage is applied to the terminals of the sample. It is the theory behind the operation of the Gunn diode as well as several other microwave semiconductor devices, which are used practically in electronic oscillators to produce microwave power. It is named for British physicists Brian Ridley, Tom Watkins and Cyril Hilsum who wrote theoretical papers on the effect in 1961. Negative resistance oscillations in bulk semiconductors had been observed in the laboratory by J. B. Gunn in 1962, and were thus named the "Gunn effect", but physicist Herbert Kroemer pointed out in 1964 that Gunn's observations could be explained by the RWH theory. In essence, RWH mechanism is the transfer of conduction electrons in a semiconductor from a high mobility valley to lower-mobility, higher-energy satellite valleys. This phenomenon can only be observed in materials that have such energy band structures. Normally, in a conductor, increasing electric field causes higher charge carrier (usually electron) speeds and results in higher current consistent with Ohm's law. In a multi-valley semiconductor, though, higher energy may push the carriers into a higher energy state where they actually have higher effective mass and thus slow down. In effect, carrier velocities and current drop as the voltage is increased. While this transfer occurs, the material exhibits a decrease in current – that is, a negative differential resistance. At higher voltages, the normal increase of current with voltage relation resumes once the bulk of the carriers are kicked into the higher energy-mass valley. Therefore the negative resistance only occurs over a limited range of voltages. Of the type of semiconducting materials satisfying these conditions, gallium arsenide (GaAs) is the most widely understood and used. However RWH mechanisms can also be observed in indium phosphide (InP), cadmium telluride (CdTe), zinc selenide (ZnSe) and indium arsenide (InAs) under hydrostatic or uniaxial pressure.

Worked examples

Example 1 — a first encounter with Ridley–Watkins–Hilsum theory

Start with the simplest possible case. Write down what Ridley–Watkins–Hilsum theory claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Ridley–Watkins–Hilsum theory 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 Ridley–Watkins–Hilsum theory 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 Ridley–Watkins–Hilsum theory

In research: Ridley–Watkins–Hilsum theory appears in engineering 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 Ridley–Watkins–Hilsum theory 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: Ridley–Watkins–Hilsum theory is common in secondary-school and first-year university syllabi. It links to neighbouring topics Condensed matter stubs, Electronic engineering, so understanding it makes those chapters shorter.
In everyday life: Look for Ridley–Watkins–Hilsum theory 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 Ridley–Watkins–Hilsum theory in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ridley–Watkins–Hilsum theory 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 Ridley–Watkins–Hilsum theory out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ridley–Watkins–Hilsum theory in simple terms?

Why does Ridley–Watkins–Hilsum theory matter?

Because it connects several engineering 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 Ridley–Watkins–Hilsum theory?

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 Ridley–Watkins–Hilsum theory.