Wondering what is a semiconductor, exactly? It’s a material whose conductivity sits between a conductor and an insulator. You can tune it with temperature, light and impurities.
Read this page as part of the wider set of guides on semiconductor basics.
| Item | Value |
|---|---|
| Silicon band gap | About 1.12 eV at room temperature |
| Gallium arsenide band gap | About 1.42 eV |
| Typical semiconductor gap range | Between 0 and 3.5 eV |
| Insulator gap | Greater than 3.5 eV |
| Diamond band gap | 5.5 eV |
| Intrinsic silicon electron concentration | 1.0 × 10¹⁰ cm⁻³ at 298 K |
These figures come from a Wevolver explainer on semiconductors and an MIT OpenCourseWare lecture summary. Values can change with source and conditions, so check the sources before you rely on them.
Conductors, insulators and semiconductors compared
A conductor such as copper, aluminum or silver passes current easily. An insulator such as glass, ceramics or polymers passes almost none, and a semiconductor sits between them, as the Wevolver comparison table lays out.
The three differ in a few clear ways:
- Conductors: very high conductivity, carried mostly by free electrons. Conductivity generally decreases as temperature rises.
- Semiconductors: conductivity you can control over a wide range, carried by electrons and holes. Conductivity generally increases as temperature rises.
- Insulators: very few mobile carriers under normal conditions, so conductivity stays very low.
Sitting in the middle is not what makes the material valuable. The real value is control: you can set how many charge carriers are present and how they move.
Why the band gap sets the rules
When atoms pack into a crystal, their energy levels crowd together into continuous energy bands, as a CircuitBread explainer describes. The valence band holds electrons bound to atoms, and the conduction band holds electrons that can move through the crystal.
The energy difference between those bands is the band gap. In metals the adjacent bands overlap, so there is no band gap at all, which is why they conduct so readily.
An MIT OpenCourseWare lecture summary puts semiconductors between 0 and 3.5 eV, and insulators above 3.5 eV. Diamond is the lecture’s example of an insulator, with a gap of 5.5 eV.
Treat 3.5 eV as a rough dividing line rather than a hard law. If you want the full picture of the gap itself, the guide on what is band gap in semiconductors goes further.
Real materials sit at different points in that range. Silicon has a band gap of about 1.12 eV at room temperature, while gallium arsenide is about 1.42 eV, according to the Wevolver article.
The band gap is a range of energies in which allowed electronic states are absent in the ideal crystal. Its size determines how readily electrons can be excited from the valence band into the conduction band, whether by thermal energy, photons or other forms of excitation, according to Wevolver. The band gap, in other words, determines how readily a semiconductor generates mobile charge carriers.
The same gap explains why the three material classes behave so differently:
- Conductor: occupied and available states overlap, or a relevant band is partially filled, so electrons respond readily to an applied electric field.
- Insulator: the gap is large enough that very few electrons can be thermally excited into conducting states under ordinary conditions.
- Semiconductor: the smaller gap lets temperature, light, doping and external fields control the carrier population and the conductivity.
Atomic arrangement matters as well as the element itself. The MIT lecture summary stresses that the band layout depends on how the atoms are bonded, not just on their atomic orbitals. Pure carbon makes the point: diamond and graphite are both carbon, yet their properties couldn’t be more different.
Keep that in mind when you compare gap values. Each figure describes a particular material in a particular form, such as silicon or gallium arsenide.
How electrons and holes carry current
In crystalline silicon, each atom forms covalent bonds with four neighbours by sharing its four valence electrons. At absolute zero, an intrinsic semiconductor has a filled valence band and an essentially empty conduction band, so nothing moves.
Add energy and the picture changes. Here is the sequence:
- Heat or light gives an electron enough energy to cross the band gap.
- The electron lands in the conduction band, where it can move through the crystal.
- It leaves a vacancy in the valence band, and that vacancy behaves as a positively charged carrier called a hole.
- Electrons and holes both contribute to the current.
At 298 K, intrinsic silicon has an electron concentration of about 1.0 × 10¹⁰ cm⁻³, according to Wevolver. That is many orders of magnitude lower than the free-electron concentration in a typical metal.
This also explains the temperature trend. More thermal energy frees more carriers, so a semiconductor’s conductivity generally rises with temperature, the opposite of a metal.
How doping tunes a semiconductor
Doping is the deliberate addition of small concentrations of impurities to control carrier concentration. As the MIT lecture puts it, this is not the doping you read about in sport, but the introduction of a small amount of an impurity.
The type of impurity decides which carrier dominates:
- Donor dopants make n-type material, where electrons are the majority carriers. Phosphorus in silicon is the standard example, because it has more valence electrons than the host.
- Acceptor dopants make p-type material, where holes are the majority carriers. Gallium in silicon is the example, because it has fewer valence electrons than the host.
Both examples come from the MIT OpenCourseWare lecture summary. The guide on n-type vs p-type semiconductor compares the two types side by side.
Because of doping, silicon’s carrier concentration can be changed dramatically, which is the foundation of device engineering. That is the idea behind diodes and transistors.
Which materials are semiconductors, and what they become
The Wevolver table lists silicon, germanium, gallium arsenide, SiC and GaN as examples. Silicon remains the dominant material for microprocessors, memory, integrated circuits and many photovoltaic applications.
- Silicon: the everyday workhorse for chips.
- Gallium arsenide: another well-known compound semiconductor, with a wider gap than silicon.
- Silicon carbide and gallium nitride: wider-band-gap materials that support higher-temperature and higher-power applications.
These materials are fabricated into diodes, transistors and integrated circuits through processes such as doping, deposition, etching and lithography. A chip is a die containing a functional circuit, and the semiconductor material itself is not synonymous with the finished device.
Frequently asked questions
Is silicon the only semiconductor?
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No. Germanium, gallium arsenide, silicon carbide and gallium nitride are also listed as semiconductors in the Wevolver table. Silicon is simply the dominant one for microprocessors, memory and integrated circuits.
Does a semiconductor conduct better when it is hot?
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Generally, yes. Conductivity in a semiconductor generally increases as temperature rises, while in a conductor it generally decreases.
What is a hole?
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A hole is the vacancy left in the valence band when an electron crosses the band gap. It behaves as a positively charged carrier.
Is a chip the same thing as a semiconductor?
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No. The semiconductor is the material, while a chip is a die containing a functional electronic circuit.
