Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
The valence band is the range of energies occupied by the outermost electrons of atoms; these electrons are bound and normally do not carry current. The conduction band is the band of higher energy just above it. An electron in the conduction band is free to move through the solid, so it can carry electric current. In short: valence band electrons are bound, conduction band electrons are free.
The energy gap Eg is the empty energy region between the top of the valence band and the bottom of the conduction band. No electron of the solid can have an energy that lies inside this region, so it is called the forbidden gap. Eg is measured in electron volt (eV). To move an electron from the valence band into the conduction band, you must give it at least Eg of energy.
An electron cannot stay or rest at an energy inside the gap, so no allowed energy states exist there. That is why it is 'forbidden'. But an electron can jump across it in one step if it gains energy equal to or greater than Eg. The word forbidden describes the region of energies, not the jump itself.
Not always. At absolute zero (0 K), a semiconductor or insulator has a full valence band and an empty conduction band. As temperature rises, some electrons gain enough thermal energy to cross Eg, so the conduction band gets a few electrons. In metals the conduction band is partly filled even at 0 K, which is why metals conduct at all temperatures.
The size of Eg decides how easily current flows. A large Eg (about 5 eV or more, like diamond) means almost no electrons cross, so the solid is an insulator. A small Eg (about 1 eV, like silicon Eg = 1.1 eV, germanium Eg = 0.7 eV) means some electrons cross at room temperature, so it is a semiconductor. Metals have no gap or overlapping bands.
They need extra energy at least equal to Eg. This energy can come from heat (higher temperature), from light (a photon of energy greater than Eg), or from an applied electric field. When an electron jumps up, it leaves behind an empty space called a hole in the valence band, and both the free electron and the hole can then carry current.
An LED is constructed from a p-n junction diode using GaAsP. The energy gap is 1.9 eV. The wavelength of the light emitted will be equal to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Eg is measured in electron volt (eV). One eV is the energy gained by one electron moving through a potential difference of one volt. For example, silicon has Eg = 1.1 eV and germanium has Eg = 0.7 eV.
The conduction band is higher in energy. The valence band sits below it, and the empty forbidden gap Eg separates the top of the valence band from the bottom of the conduction band.
Yes, slightly. For most semiconductors Eg decreases a little as temperature rises. This is a minor effect for NEET; the key idea to remember is that higher temperature gives electrons more energy to cross Eg, so more electrons reach the conduction band.
It leaves behind an empty space called a hole. The hole behaves like a positive charge carrier. So one jump creates two carriers: a free electron in the conduction band and a hole in the valence band.
A bigger Eg means electrons need more energy to reach the conduction band, so far fewer electrons make the jump at a given temperature. Fewer free carriers means lower conductivity, which is why large-gap solids like diamond are insulators.
