pn-junctions

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...that doping can be legal?

The standard silicon (Si) solar cell is based on a semiconductor pn-junction. The contact of n-doped and p-doped layers forms a pn-junction . Doping is a process of introducing impurities into a pure semiconductor.

n-type semiconductor is obtained by doping with impurity atoms having an excess valence electron. In the case of Si (elements of group IV of the periodic table), n-type semiconductor is the result of doping with one of the elements of group V, and these impurity atoms are called electron donors. p-type semiconductors on the other hand are obtained by doping with impurity atoms (from group III for Si) with one valence electron less than the surrounding atoms. This can also be viewed as an atom with an excess hole, which is just the absence of an electron that would be present otherwise. Atoms of elements of group III are called in this case acceptors. Typically, phosphorus is used to dope silicon to create an n-type layer, and boron-doped silicon is used for the p-type layer.

Fig.: n-doped silicon Si(group IV) with phosphorus P(group V).

Each isolated atom has its discrete energy levels. As the interatomic spacing decreases, each degenerate energy level splits to form a band. Further decrease in spacing causes the band originating from different discrete levels to lose their identities and merge together, forming a single band. When the distance between atoms approaches the equilibrium interatomic spacing of the diamond lattice, this band splits again into two bands. These bands are separated by a region which designates energies that the electrons in a solid cannot possess. This region is called the forbidden gap or band gap (Eg). The upper band is called the conduction band ( CB ), while the lower band is called the valence band ( VB ).

Fig.: Energy bands diagram

An insulator has very strong bonds between neighbor atoms, while a metal has very weak bonding. Thermal vibrations can easily break weak bonds. The bonds between neighboring atoms in semiconductor are only moderately strong. Therefore, thermal vibrations will break some of these bonds. When the bond is broken, a free electron along with a free hole is generated. The band gap of a semiconductor is not large, so some free electrons will be able to move from the valence band to the conduction band, leaving holes in valence band. When an electric field is applied, both the free electrons in the conduction band and holes in the valence band will gain kinetic energy and cause an electric current.

In n-type semiconductor, electrons from the doped element are close to the conduction band and will carry electrical current. These new energy levels below the conduction band and called donor levels (D.L.), because an electron there is easily donated to the conduction band.

The existence of extra holes in p-type semiconductor, creates extra energy levels just above the valence band, since it takes relatively little energy to move another electron into a hole. These levels are called acceptor levels (A.L.), because they can easily accept an electron from the valence band.

Fig.: Electrones easy donated from donor level(D.L.) into conduction band(CB). Extra holes accept electron from valence band(VB) into acceptor level(A.L.).

A pn-junction is rectified, which means that it allows a current to flow easily only in one direction. The current increases rapidly as the voltage increases, when we apply a forward bias to the junction. However, by applying a reverse bias, virtually no current flows initially. As the reverse bias is increased, the current remains very small until a critical voltage is reached at which the current suddenly increases.

Fig.: The current-voltage characteristic of pn-junction.

Electrons can occupy only one energy state. The lowest energy band level that can be occupied by valence electrons at absolute zero temperature is called the Fermi level (E F). The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties. The Fermi level is near the valence band edge in the p-type material and near the conduction band edge in n-type.

Fig.: Fermi level in the n-type semiconductor (a) and in the p-type semiconductor (b).