Metal–semiconductor junction

Key takeaways

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

Reference excerpt

In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest type of practical semiconductor device. M–S junctions can either be rectifying or non-rectifying. The rectifying metal–semiconductor junction forms a Schottky barrier, making a device known as a Schottky diode, while the non-rectifying junction is called an ohmic contact. (In contrast, a rectifying semiconductor–semiconductor junction, the most common semiconductor device today, is known as a p–n junction.) Metal–semiconductor junctions are crucial to the operation of all semiconductor devices. Usually, an ohmic contact is desired so that electrical charge can be conducted easily between the active region of a transistor and the external circuitry. Occasionally, however, a Schottky barrier is useful, as in Schottky diodes, Schottky transistors, and metal–semiconductor field effect transistors.

The critical parameter: Schottky barrier height

Whether a given metal-semiconductor junction is an ohmic contact or a Schottky barrier depends on the Schottky barrier height, ΦB, of the junction. For a sufficiently large Schottky barrier height, that is, ΦB is significantly higher than the thermal energy kT, the semiconductor is depleted near the metal and behaves as a Schottky barrier. This is typically between 0.4 eV and 0.7 eV for a material like silicon. For lower Schottky barrier heights, the semiconductor is not depleted and instead forms an ohmic contact to the metal. The Schottky barrier height is defined differently for n-type and p-type semiconductors (being measured from the conduction band edge and valence band edge, respectively). The alignment of the semiconductor's bands near the junction is typically independent of the semiconductor's doping level, so the n-type and p-type Schottky barrier heights are ideally related to each other by:

Φ B ( n ) + Φ B ( p ) = E g {\displaystyle \Phi _{\rm {B}}^{(n)}+\Phi _{\rm {B}}^{(p)}=E_{\rm {g}}}

where Eg is the semiconductor's band gap. In practice, the Schottky barrier height is not precisely constant across the interface, and varies over the interfacial surface.

Schottky–Mott rule and Fermi level pinning

The Schottky–Mott rule of Schottky barrier formation, named for Walter H. Schottky and Nevill Mott, predicts the Schottky barrier height based on the vacuum work function of the metal relative to the vacuum electron affinity (or vacuum ionization energy) of the semiconductor:

Φ B ( n ) ≈ Φ m e t a l − χ s e m i {\displaystyle \Phi _{\rm {B}}^{(n)}\approx \Phi _{\rm {metal}}-\chi _{\rm {semi}}}

This model is derived based on the thought experiment of bringing together the two materials in vacuum, and is closely related in logic to Anderson's rule for semiconductor-semiconductor junctions. Different semiconductors respect the Schottky–Mott rule to varying degrees. Although the Schottky–Mott model correctly predicted the existence of band bending in the semiconductor, it was found experimentally that it would give grossly incorrect predictions for the height of the Schottky barrier. A phenomenon referred to as "Fermi level pinning" caused some point of the band gap, at which finite DOS exists, to be locked (pinned) to the Fermi level. This made the Schottky barrier height almost completely insensitive to the metal's work function:

Φ B ≈ 1 2 E b a n d g a p {\displaystyle \Phi _{\rm {B}}\approx {\frac {1}{2}}E_{\rm {bandgap}}}

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Metal–semiconductor junction

Start with the simplest possible case. Write down what Metal–semiconductor junction 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 Metal–semiconductor junction 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 Metal–semiconductor junction 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 Metal–semiconductor junction

In research: Metal–semiconductor junction 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 Metal–semiconductor junction 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: Metal–semiconductor junction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Semiconductor structures, so understanding it makes those chapters shorter.
In everyday life: Look for Metal–semiconductor junction 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.

How to study Metal–semiconductor junction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Metal–semiconductor junction 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.
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Frequently asked questions

What is Metal–semiconductor junction in simple terms?

In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest type of practical semiconductor device.

Why does Metal–semiconductor junction 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 Metal–semiconductor junction?

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 Metal–semiconductor junction.