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                                              Electron micrograph of a Silicon

                                              Germanium NPN bipolar transistor

                                               fabricated at IBM Microelectronics

Silicon As Semiconductors

Semiconductors are solids where there is only a small difference in energy, called a band gap, between the filled valency band of electrons and a conduction band. If cooled to absolute zero, the electrons occupy their lowest possible energy levels. The conduction band is empty , and the material is a perfect insulator. At normal temperatures, some electrons are thermally excited from the valency band to the conduction band , and hence they can conduct electricity by the passage of electrons at normal temperatures. The conductivity is in between that of a metal and an insulator and depends on the number of electrons in the conduction band .

Silicon is the most important commercial example of semiconductor. The crystal structures is like diamond. Atoms of Si has four electrons in their outer shell, which form four covalent bonds to other atoms.

Table 3.8 Band gaps of some semiconductors at absolute zero

　

In Si at very low temperature , the valence bad is folled and conduction band is empty . Under these condition , Si and Geis onsulators and cannot carry any electric current.

The band gap is 106 kJ mol-1 for Si , and at room temperature a few valence electrons gain sufficient energy from the thermal vibration of the atoms to be promoted into the conduction band. If the crystal is connected in an electric circuit , these thermally excited electrons carry a small current, and make the Si crystal slightly conduction . This is termed intrinsic semiconductor. Expressed in another way, some bonds are broken, and there valence electrons can migrate, and conduct electricity.

　

As the temperature is increased, the conductivity increases, that is the electrical resistance decreases. (This is the opposite of the situation with metals.) With Si the maximum working temperature is 150 ⁰C. This intrinsic semiconductor is undesirable, and precautions must be taken to limit the working temperature of transistors.

　

Pure Si can be made semiconducting in a controlled way by adding impurities which act as charge carriers. Si is first obtained extremely pure by zone refining. Some atoms with five outer electrons, such as arsenic As, are deliberately added to the silicon crystal. This process is called 'doping' the crystal. A minute proportion of Si atoms are randomly replaced by As atoms with five electrons in their outer shell. Only four of the outer electrons on each As atom are required to form bonds in the lattice, At absolute zero or low temperatures, the fifth electron is localized on the As atom. However, at normal temperatures, some of these fifth electrons on As are excited into the conduction band, where they can carry current quite readily. This is extrinsic conduction, and it increases the amount of semiconduction far above that possible by intrinsic conduction. Since the current is carried by excess electrons, it is n-type semiconduction.

　

Alternatively a crystal of pure Si may be doped with some atoms with only three outer electrons, such as indium In. Each indium atom uses its three outer electrons to form three bonds in the lattice, but they are unable to form four bonds to complete the covalent structure. One bond is incomplete, and the site normally occupied by the missing electron is called a 'positive hole'. At absolute zero or low temperatures, the positive holes are localized around the indium atoms. However, at normal temperatures a valence electron on an adjacent Si atom may gain sufficient energy to move into the hole. This forms a new positive hole on the Si atom. The positive hole seems to have moved in the opposite direction to the electron . By a series of 'dops', the 'positive hole' can migrate across the crystal. This is equivalent to moving an electron in and opposite direction , and thus current is carried. Since current is carried by the migration of positive centres , this is p-type semiconduction.

　

Silicon must be ultra-highly purified before it can be used in semiconductors. First impure silicon ( 98% pure ) is obtained by reducing SiO₂ with carbon in an electria furnaca at about 1900^oC. This may be purified by reacting with HCl, forming trichlorosilane SiHCl₃, which may be distilled to purify it, then decomposed by heating to give silicon .

　

SiO₂ + C à Si + CO₂

Si + 3HCl à H₂ + SiHCl₃à Si

                                 (350^oC)                       (strong heating )

The final purification is by zone refining , where a rod of silicon is melted near one end by an electric furnace . As the furnace is slowly moved along the rod , the narrow molten zone gradually moves to the other end of the rod . The impurities are more soluble in the liquid melt than in the solid, so they concentrate in the molten zone, and eventually move to the end of the rod. The impure end is removed, leaving an ultra-purified rod, with a purity of at least 1 part in 10¹⁰. Purified silicon crystals can be converted to p-type or n-type semiconductors by high temperature diffusion o the appropriate dopant element, up to a concentration of 1 part in 10^8. In principle any of the Group III elements boron, aluminium, gallium or indium can be used to make p-type semiconductors, though indium is the most used because of its low melting point. Similarly Group V elements such as phosphorus or arsenic can be used to make n-type semiconductor , but because of it's low melting point arsenic is most used .

　

If a single crystal is doped with indium at one end , and with arsenic at the other end , then one part is a p-type semiconductor and the other is an n-type semiconductor . In the middle there will be a boundary region where the two sides meet , which is a p-n junction . Such junction are the important part of modern semiconductor devices .

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