Insulators: The band gap energy or the forbidden energy gap is very high in between conduction band and valence band. This is the reason why there’s no conductivity present in such substances.
In insulating material case, there is a big forbidden gap between valence band and conduction band. It is practically impossible for the electron to jump to conduction band from valence band. As a result, such materials are not able to conduct, and they are called insulators.
The forbidden energy gap between the conduction and valence band is wide enough, nearly 7 eV in insulators. An insulator example, diamond, whose forbidden energy gap is approximately 6 eV. These specific materials might conduct at really high temperatures only or if they are at high voltage.
This conduction type is rare and is known as “insulation breakdown” or the “breakdown of an insulator.” More insulating materials are wood, glass, paper, mica, etc.
The band which divides the two bands (V & C) is known as the Forbidden Band (F).
Insulators do not have any free charge carriers and therefore, they are non-conductive in nature.
The atomic bond
Atomic bond is fully based on sharing of non-metals’ electron pairs. Those elements that act like non-metals are having the ambition to seize electrons, hence, no free electrons are there that can do the work as charge carriers.
The ionic bond
The ions in a solid state are arranged in the form of grid system. With the help of electrical forces, the particles are holded together. No free charge carriers are present which enables the current flow. Therefore, substances comprised of ions can be insulator and conductor, both.
Because of the large gap, no electrons are excited from V.B to vacant C.B. The remaining valence band is filled completely.
Mostly, the solid substances are insulators, and when it comes to solids’ band theory, it means that there’s a huge forbidden energy gap between valence electrons’ energies.
Let’s say, Glass is an insulating material which might be transparent to light which is visible for approximately co-related reasons along with its own kind as the electrical insulator. The light photons that are visible don’t have that much quantum energy to fill the band gap or energy gap and achieve the electrons to a applicable energy level present in conduction band.
Glass’ visibility properties can even offer some understanding regarding the doping effects on solids’ properties.
The small percentage of impure atoms or impurity atoms present in glass can provide it the color by giving particular energy levels’ availability that absorbs specific visible lights’ colors. Remember that, doping in insulators can completely change the optical properties of it.