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Conductor vs Semiconductor vs Insulator: Band Gap, Resistivity and Temperature Compared

What separates conductors, semiconductors and insulators is how many electrons are available to carry current, and band structure explains this. In a metal the conduction band is partly filled with electrons, so there are many free electrons from the start. An insulator has a large band gap, so at room temperature almost no electrons can rise into the conduction band. A semiconductor sits in between: its band gap is moderate, around 1 eV, so temperature, light and added impurities can change how easily it conducts by a large amount.

AspectConductor (metal)SemiconductorInsulator
Band structureThe conduction band is partly filled with electrons (or the valence and conduction bands overlap)The valence band is full, with a moderate band gap to the conduction bandThe valence band is full, with a large band gap to the conduction band
Typical band gapNoneOften around 1 eV (Si about 1.12 eV, Ge about 0.66 eV, GaAs about 1.42 eV). Wide-bandgap semiconductors such as SiC and GaN exceed 3 eVLarge (about 9 eV for SiO2)
Typical resistivity at room temperatureVery low (about 1.7×10⁻⁶ Ω·cm for copper)In between, and varies by orders of magnitude with impurity content (on the order of 10⁵ Ω·cm for pure silicon)Very high (10¹⁴ Ω·cm or more for SiO2 and similar materials)
Charge carriersFree electrons: 10²² or more per cm³ (about 8.5×10²² for copper)Two kinds: electrons in the conduction band and holes in the valence band. Their density can be changed greatly by adding impuritiesAlmost none
As temperature risesResistance increases (more scattering from lattice vibrations)In a pure semiconductor, resistance drops sharply (thermally generated carriers increase rapidly)Resistance stays very high, but falls slightly at high temperature
Adding impuritiesConductivity barely changes (resistance rises slightly)Even tiny amounts change conductivity by orders of magnitude, making it possible to form n-type and p-type regionsInsulation must be preserved; impurities and defects cause leakage current
Under lightAlmost no changeLight with energy above the band gap creates carriers and increases current (photoconductivity)Almost no change under visible light (SiO2 is transparent to visible light)
Typical materialsCopper, aluminum, gold, tungstenSilicon, germanium, GaAs, SiC, GaNSiO2, silicon nitride (Si3N4), alumina (Al2O3), resins
Role in semiconductor devicesInterconnects, electrodes, contacts, bonding wiresThe parts that control current, such as transistors and diodesGate dielectrics, interlayer dielectrics between wiring layers, device isolation, passivation

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Conductor (metal)
When you want to carry electricity with little loss: on-chip wiring and electrodes, and the wires and leads of a package.
Semiconductor
When you want to switch current on and off, amplify it, or convert between light and electricity. Semiconductors form the core of transistors, diodes, image sensors and solar cells.
Insulator
When you need to separate places where current must not flow: MOSFET gate dielectrics, interlayer dielectrics filling the space between wires, and isolation between neighboring transistors.

FAQ

Why is it called a "semi"-conductor?

Because its resistivity lies between that of conductors (metals) and insulators. What really matters, though, is not the value itself but the fact that doping, temperature, light and electric fields can change how easily it conducts by a large amount. That property is what makes transistors and diodes work.

Why does resistance change in opposite directions with temperature in metals and semiconductors?

In a metal the number of free electrons is nearly fixed, so as temperature rises, more scattering from lattice vibrations raises the resistance. In a pure semiconductor, the number of carriers excited across the band gap grows exponentially with temperature, outweighing the extra scattering, so the resistance falls.

Above what band gap is a material an insulator?

There is no sharp boundary. Materials with a large band gap and almost no carriers at room temperature are usually called insulators, but materials that can be made to conduct by adding impurities, such as diamond at about 5.5 eV or Ga2O3 at about 4.5–4.9 eV, are treated as semiconductors (ultra-wide-bandgap semiconductors).

Is silicon a conductor or an insulator?

It is a semiconductor. Pure silicon has a fairly high resistivity at room temperature, on the order of 10⁵ Ω·cm, but adding a tiny amount of phosphorus or boron lowers it by orders of magnitude. For example, n-type silicon with an electron density of 1×10¹⁶ cm⁻³ is about 0.5 Ω·cm.

Check your understanding

Which value is closest to the band gap of silicon (Si) at room temperature?

  1. AAbout 0.66 eV
  2. BAbout 1.12 eV
  3. CAbout 3.4 eV
Show answer and explanation

Answer: B. About 1.12 eV

Silicon's band gap is about 1.12 eV at room temperature, so it absorbs light up to about 1.1 µm (1240 ÷ 1.12). About 0.66 eV is germanium, whose small gap makes leakage rise quickly at high temperature; about 3.4 eV is GaN, a wide-bandgap semiconductor suited to high voltage, high temperature and blue LEDs. Silicon sits in between, and together with its high-quality thermal oxide (SiO2) this made it the material of choice for integrated circuits.

Which of the following materials has the largest band gap?

  1. ASilicon (Si)
  2. BGermanium (Ge)
  3. CSilicon dioxide (SiO2)
Show answer and explanation

Answer: C. Silicon dioxide (SiO2)

SiO2 has a very large band gap of about 9 eV and is an excellent insulator, far larger than silicon (about 1.12 eV) and germanium (about 0.66 eV). Besides the wide gap, its conduction-band offset to silicon (about 3.1 eV) is large, so electrons have difficulty crossing a gate oxide. High-k materials (HfO2 is about 5.8 eV) trade a smaller gap for a higher dielectric constant and are chosen by balancing this against leakage.

Which correctly describes how the resistivity of an intrinsic semiconductor depends on temperature?

  1. AResistivity decreases as temperature rises
  2. BResistivity increases as temperature rises
  3. CResistivity is constant regardless of temperature
Show answer and explanation

Answer: A. Resistivity decreases as temperature rises

In an intrinsic semiconductor, the number of carriers excited across the band gap rises exponentially with temperature, so resistivity falls (a negative temperature coefficient). In metals the carrier count is nearly fixed and lattice-vibration scattering increases, so resistivity rises. NTC thermistors exploit this behavior. Note that in doped silicon near room temperature the carrier count is roughly constant, so there is a range where resistance actually rises as mobility drops.

  1. What happens to the intrinsic carrier density when the temperature of an intrinsic semiconductor is raised?
  2. How many valence (outermost-shell) electrons does a silicon atom have?
  3. Which value is closest to the band gap of germanium (Ge) at room temperature?

Practice Semiconductor physics questions

Key terms explained

Band gap
The energy difference between the top of the valence band and the bottom of the conduction band. About 1.12 eV for silicon at room temperature. A larger band gap means better tolerance to high temperatures and voltages.
Intrinsic carrier density
The density of thermally generated electrons and holes in an undoped semiconductor, written ni. For silicon it is about 1×10¹⁰ cm⁻³ at room temperature and rises steeply with temperature. In equilibrium, n × p = ni².
Mobility
How easily carriers move when an electric field is applied. In silicon, electron mobility is higher than hole mobility.
Fermi level
The energy at which the probability of an electron occupying a state is exactly one half. It lies closer to the conduction band in n-type material and closer to the valence band in p-type.

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