Forbidden Zone
Definition and meaning of Forbidden Zone in chemistry.
The forbidden zone is a specific range of energy levels inside a solid material. Electrons in the material are physically not allowed to have these specific energies. They must jump completely across this energy gap to conduct flowing electricity.
In more detail
Atoms in a solid crystal pack very closely together to share their electrons. Their shared electron orbits merge into large continuous bands of allowed energy. The bottom band is called the valence band, and it is totally full.
The upper band is called the conduction band, and it is usually empty. The forbidden zone is the totally empty space resting between these two bands. Electrons are completely blocked from resting anywhere inside this empty gap region.
To conduct electricity, an electron must absorb enough energy to jump the gap. Metals do not have a forbidden zone because their energy bands simply overlap. Insulators have a massive forbidden zone that normal electrons can almost never cross.
Semiconductors have a narrow forbidden zone that a little heat or light can bridge. A common misconception is that the forbidden zone is an actual physical empty space. The zone is strictly a measure of energy, not a physical location inside the crystal.
Scientists can purposefully add chemical impurities to a semiconductor to change its behavior. This doping process adds new stepping stones inside the previously empty forbidden zone. These new stones make it much easier for eager electrons to jump the gap.
Key facts
| Alternative names | Band gap, forbidden band |
|---|---|
| Metals | No gap (bands overlap) |
| Semiconductors | Narrow gap (electrons can jump) |
| Insulators | Wide gap (electrons blocked) |
| Silicon gap size | 1.1 electron-volts |
| Doping effect | Adds energy steps inside the gap |
Silicon is the most famous semiconductor used in modern computer chips today. It has a forbidden zone measuring about 1.1 electron-volts at normal room temperature. An electron in the valence band needs exactly that much energy to jump. The electron can easily steal this required energy from nearby heat or light. Once it jumps the gap into the conduction band, the silicon conducts electricity. A piece of diamond has a massive gap of 5.5 electron-volts instead. Normal heat cannot push electrons across that giant gap, making diamond a perfect insulator.
Frequently asked questions
Is the forbidden zone the exact same thing as the band gap?
Yes, chemists and physicists use both terms to describe the exact same energy barrier. "Band gap" is simply the more modern and common term used today.
Why can't electrons just hang out inside the forbidden zone?
The rigid math rules of quantum mechanics simply do not allow it. An electron must have an exact amount of energy that matches an allowed band.
How does heat change the forbidden zone in a material?
Heat gives resting electrons the extra energy boost they need to jump the gap. The gap itself stays the same size, but more electrons can successfully cross it.