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Diamond's structure is a large network covalent crystal, where there are strong covalent bonds between neighboring tetrahedral carbon atoms. The explanation I was given was:

The electrons are held tightly in the covalent bonds of the structure, so they are unable to move around. Thus, diamond cannot conduct electricity.

Why is it then, that in silicon (also a network covalent solid), which has an identical crystal structure to diamond, is known to conduct electricity relatively well and even dubbed a semiconductor? Aren't the electrons held tightly in the covalent bonds between neighboring silicon atoms?

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    $\begingroup$ Actually, diamond is a high band gap semiconductor. For most purposes, it gets treated as an electrical insulator. Look up “Electronic band gap” in wiki and scroll down to filling the bands. Intrinsic Si has a thermally-generated population of electrons in its conduction band and holes in its valence band. With diamond, the band gap is over 5 eV, so there are almost no electrons and holes in the relevant bands. Doped diamonds, like the famous Hope blue diamond, are extrinsic high band gap semiconductors. $\endgroup$
    – Ed V
    Mar 22, 2020 at 22:55
  • $\begingroup$ I just searched for “semiconductor” here and there are 158 questions or answers. Lots to learn reading through these! $\endgroup$
    – Ed V
    Mar 22, 2020 at 23:22
  • $\begingroup$ Yes, a high band gap semiconductor and when doped with Boron, a diamond electrode for wastewater treatment, see infoscience.epfl.ch/record/79837 . $\endgroup$
    – AJKOER
    Mar 22, 2020 at 23:34
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    $\begingroup$ 40 years ago we considered diamond an insulator. Now it is a wide band gap semiconductor. Either way, it doesn’t conduct that well compared to silicon, much less a metal. But it doesn’t have to in order to be useful. $\endgroup$
    – Jon Custer
    Mar 22, 2020 at 23:40
  • $\begingroup$ @JonCuster As per your comment to the answer here! Nice to know that AlN can be usefully doped. Thanks! $\endgroup$
    – Ed V
    Mar 22, 2020 at 23:44

2 Answers 2

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I'm not a solid-state physicist. Yet, this is how I understand this problem, so bare with me.

According to this article:

Every solid has its own characteristic energy-band structure. This variation in band structure is responsible for the wide range of electrical characteristics observed in various materials. In semiconductors and insulators, electrons are confined to a number of bands of energy, and forbidden from other regions. The term "band gap" refers to the energy difference between the top of the valence band and the bottom of the conduction band. Electrons are able to jump from one band to another. However, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials. Electrons can gain enough energy to jump to the conduction band by absorbing either a phonon (heat) or a photon (light).

This forbidden gap plays a major role in determining the electrical conductivity of material. The materials can be classified into three types based on the forbidden gap. they are:

  • Insulators: The materials, which does not allow the flow of electric current through them are called insulators. The band gap between the valence band and conduction band is very large in insulators, which is approximately equal to $\pu{15 eV}$. Normally, in insulators the valence band is fully occupied with electrons due to sharing of outer most orbit electrons with the neighboring atoms whereas its conduction band is empty, i.e., no electrons are present in the conduction band.
  • Semiconductors: The material, which has shown electrical conductivity between that of a conductor and an insulator is called a semiconductor. The band gap of semiconductors is is very small, and it falls between that of valence band and conduction band. The value of this forbidden gap is about $\pu{1-3 eV}$. As a rule of thumb, if the material has $\Delta E_\mathrm{gap} \le \pu{3.0 eV}$, it is considered as a semiconductor (Semiconductors- Band Gaps, Colors, Conductivity and Doping).
  • Conductors: In a conductor, the valence and conduction bands are overlapped each other. Therefore, there is no forbidden gap in a conductor (the value is generally negative). Thus, the valence band electrons to move into conduction band needs only a little amount of applied external energy.

Experimental values of $\Delta E_\mathrm{gap}$ for diamond and silicon are $\pu{5.48 eV}$ and $\pu{1.17 eV}$, respectively (Ref.1). Keep in mind that band gap get narrowed when going down on a group in periodic table. For example, $\Delta E_\mathrm{gap}$ of germanium is $\pu{0.74 eV}$ (Ref.1).

Therefore, silicon and germanium behave like semiconductors, while diamonds fall into insulator category in general (yet, please refer to Ed V's comment above for better definition).

References:

  1. Fabien Tran, Peter Blaha, "Accurate Band Gaps of Semiconductors and Insulators with a Semilocal Exchange-Correlation Potential," Phys. Rev. Lett. 2009, 102, 226401, 4 pages (DOI: 10.1103/PhysRevLett.102.226401).
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    $\begingroup$ +1 Excellent answer: at the right level of detail, without getting bogged down in the little band diagrams, Fermi levels, and on and on. $\endgroup$
    – Ed V
    Mar 23, 2020 at 13:30
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    $\begingroup$ Thanks for the great answer, but I am curious as to why the band gap decreases going down a group? $\endgroup$
    – timeinbaku
    Mar 23, 2020 at 18:00
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    $\begingroup$ Below germanium, you have tin and lead. Lead is a metal (no band gap), but tin has two allotropes: white tin is ordinary metallic tin, but gray tin has the diamond, Si, and Ge crystal structure and is not a metal. See the wiki article on ‘tin’. Metals have no band gap, but gray tin does. $\endgroup$
    – Ed V
    Mar 23, 2020 at 19:07
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    $\begingroup$ @Ed V: Great comment to answer OP's question. I appreciate it. :-) $\endgroup$ Mar 23, 2020 at 19:10
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Diamond is made of Carbon. The valence Shell of the carbon atoms in a dielectric Crystal like diamond are filled. Conductivity can only oceur for electrons in a electronic shell that is partially filled.So Diamond crystal with no defects can not Conduct electricit. On the other hand Silicon reacted as a semiconductor.So Silicon have the ability to conduct electricit. But diamond has a much higher band gap than Silicon.So diamond doesn’t conduct electricity but Silicon does.

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