The energy gap between the valance and conduction band is very small. A semiconductor is a material that is between conductors and insulators in its ability to conduct electrical current. A semiconductor in its pure intrinsic state is neither a good conductor nor a good insulator. The most common single-element semiconductors are silicon,germanium, and carbon. Compound semiconductors such as gallium arsenide are also commonly used.
The most common examples of intrinsic semiconducting materials are silicon. Each atom of silicon has four valance electrons. Moreover each atom of silicon is surrounded by four atoms. A silicon Si atom with its four valence electrons shares an electron with each of its four neighbors to form covalent bond. This effectively creates eight shared valence electrons for each atom and produces a state of chemical stability.
The semiconducting materials have negative temperature coefficient of resistivity. At low temperatures, the valence band is completely filled and conduction band is completely empty. Thus the semiconducting materials behave like insulator at low temperatures. At comparatively higher temperature, the electrons in valance band acquire sufficient energy to jump in conduction band.
As the temperature increases, the probability of the electrons to jump from valance to conduction band increases. Therefore, the conductivity of semiconductors increases with increase in temperature. At absolute zero, the intrinsic semiconducting materials behaves like insulators because they have no free electrons. But as the temperature of increases, the thermal agitation in the atoms breaks some covelent bonds which result in formation of electron hole pairs.
The electrons jump from valance band to conduction band by absorbing the thermal energy. As the result, the conductivity of semiconductor increases with increase in temperature. The conductivity of silicon and germanium can be drastically increased by the controlled addition of impurities to the intrinsic pure semiconductive material. This process, called doping, increases the number of current carriers electrons or holes.
The two categories of impurities are n-type and p-type. N-Type Semiconductor To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms e. Each pentavalent atom antimony, in this case forms covalent bonds with four adjacent silicon atoms. Four of the antimony atom's valence electrons are used to form the covalent bonds with silicon atoms, leaving one extra electron.
This extra electron becomes a conduction electron because it is not attached to any atom. Because the pentavalent atom gives up an electron, it is often called a donor atom. The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon. Majority and Minority Carriers in N-Type Semiconductor In an n-type semiconducting material, most of the current carriers are electrons. Of course, due to the mass amounts of different molecular orbital mixings, bands of varying energy will form.
The difference between these band energies is known as the band gap , as indicated in Figure 2. Figure 2. The blue boxes represent the conduction bands while the yellow boxes represent valence bands. The shading of the boxes is indicative of electron density within the band. The band theory looks at the jump of electrons across the band gap. In particular, the jump of electrons from their valence band to their conduction band across their Fermi energy level. This 'jump' dictates optical and magnetic properties of the solid.
The band of energy where all of the valence electrons reside and are involved in the highest energy molecular orbital. The band energy where positive or negative mobile charge carriers exist. Negative mobile charge carriers are simply electrons that had enough energy to escape the valence band and jump to the conduction band.
Here, they move freely throughout the crystal lattice and are directly involved in the conductivity of semiconductors. Positive mobile charge carriers are also referred to as holes.
Holes refer to the lack of an electron in the conduction band. In other words, a hole refers to the fact that within the band there is a place where an electron can exist ie. Because the electron has the potential to be there and yet isn't there, it is referred to as positive mobile charge carrier. This level refers to the highest occupied molecular orbital at absolute zero. It is usually found at the center between the valence and conduction bands.
The particles in this state each have their own quantum states and generally do not interact with each other. When the temperature begins to rise above absolute zero, these particles will begin to occupy states above the Fermi level and states below the Fermi level become unoccupied. Semiconductors are defined to have conductivity in between an insulator and a conductor. Due to this property, semiconductors are very common in every day electronics since they likely will not short circuit like a conductor.
They get their characteristic conductivity from their small band gap. Note: this article is in German. Heitler, Walter, and Fritz London.
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