# Forbidden gap in semiconductors.

• savi
In summary, the conversation discusses the concept of the forbidden gap between the conduction and valence bands in a solid and the role of the Fermi level in determining the density of electrons and holes in a semiconductor. It raises the question of how the Fermi level can exist in the forbidden gap, despite the fact that electrons cannot normally exist in this region. The thread referenced further explores this topic in the context of doping and thermodynamic equilibrium.
savi
in books we have read that there lies a forbidden gap between conduction band and valence band. and electrons can not exist in this gap i.e. probability of electron being found in this region should be zero.
BUT the fermi level,which has 50% prabality to contain an electron is found in forbidden gap for a semi conductor generally. how can this be possible if no electron can exist in forbidden gap?

savi said:
in books we have read that there lies a forbidden gap between conduction band and valence band. and electrons can not exist in this gap i.e. probability of electron being found in this region should be zero.
BUT the fermi level,which has 50% prabality to contain an electron is found in forbidden gap for a semi conductor generally. how can this be possible if no electron can exist in forbidden gap?
When atoms are brought close together, as in a solid, the electrons come under the influence of forces from other atoms, where the energy level merges into bands of energy levels. There are two distinct energy bands in which electrons could exist: the valence band and the conduction band . Separating these two bands is an energy gap , termed the forbidden gap, in which electrons cannot normally exist.

The central task of basic semiconductor physics is to establish formulas for the position of the Fermi level EF relative to the energy levels EC and EV (the level of the bottom of the conduction band and the top of the valence band), taking into account the effects of "doping". Doping introduces additional electron energy levels into the band gap, that may or may not be populated by electrons, dependent on circumstances and temperature, and causes the Fermi level EF to shift from the energy level (relative to the band structure) that it would have had in the absence of doping. This energy level that the Fermi level has in the absence of doping is called the intrinsic Fermi level (or "intrinsic level") and is usually denoted by the symbol Ei.

The theory of semiconductor physics is constructed in such a fashion that – in a situation of complete thermodynamic equilibrium – the position of the Fermi level, relative to the band structure, determines both the density of electrons and the density of holes.
Avinash Singh
Jr. YSR (ISCA)
Mech. Eng.
KIIT Univesity
BBSR

## What is the forbidden gap in semiconductors?

The forbidden gap, also known as the band gap, is the energy gap between the valence band and the conduction band in a semiconductor material. It is the range of energies that electrons cannot occupy, and it determines the electrical conductivity of the material.

## Why is the forbidden gap important in semiconductors?

The forbidden gap plays a crucial role in the functioning of semiconductor devices. It determines the type of material (p-type or n-type) and its ability to conduct electricity. The size of the forbidden gap also affects the absorption of light and the emission of photons in optoelectronic devices.

## How does the forbidden gap affect the behavior of electrons in semiconductors?

The forbidden gap essentially acts as a barrier for electrons. In a material with a smaller gap, electrons can easily move from the valence band to the conduction band, making it a good conductor. In a material with a larger gap, electrons require more energy to cross the gap, making it a poor conductor.

## Can the forbidden gap in semiconductors be manipulated?

Yes, the forbidden gap can be manipulated through a process called doping. By introducing impurities into the semiconductor material, the size of the forbidden gap can be altered, allowing for control of the material's electrical properties.

## How does the temperature affect the forbidden gap in semiconductors?

As temperature increases, the forbidden gap in semiconductors decreases. This is because at higher temperatures, more electrons are able to gain the energy needed to cross the gap, making the material more conductive. This phenomenon is known as thermal excitation.

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