Semiconductor materials

A semiconductor is a substance that has conductivity (~103 to 10−8 Scm-1) in between an insulator and conductor, This can be due either to the dopant or to temperature effects. Generally semiconductors are crystalline solids, but amorphous and liquid semiconductors are also identified. Semiconductor materials are commonly used in modern electronics, such as radios, telephones, computers, and many other electronic devices. Semiconductors fall into two main categories; intrinsic and extrinsic.

Semiconductor band gaps are small enough, such that a small increase in temperature promotes a sufficient numbers of electrons from the valence band to the conduction band. This creates electron holes, or unoccupied levels in the valence band, and very loosely held electrons in the conduction band. The conductivity of intrinsic semiconductors is reasonably low at room temperature. Conductivity can be considerably improved by the doping processes making it an extrinsic semiconductor. The negative charge of the electrons is balanced by an equivalent positive charge in the centre of the dopant atoms. Hence, the net electrical charge of the semiconductor material is not changed.

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1.6.1 Intrinsic semiconductors

Intrinsic semiconductors are also called pure semiconductors and they do not contain any dopant species. Therefore the number of charge carriers is determined by the properties of the material itself instead of the amount of impurities. In an intrinsic semiconductor, the number of electrons in the conduction band is equal to the number of holes in the valence band. The electrical conductivity of intrinsic semiconductors can be due to crystal defects or to thermal excitation.

1.6.2 Extrinsic semiconductors

An extrinsic semiconductor is a semiconductor that has been doped with other substances in order to change their conductive properties. This means that their electronic properties and conductivity may change in a controlled manner by adding specific dopants to the intrinsic material. Dopant atoms come from different elements than the atoms of the intrinsic semiconductor. The electrical conductivity may be changed, not only by the number of dopant atoms, but also by the type of dopant which may produce 103-106 increase in conductivity. Therefore, specific properties of extrinsic semiconductors depend on the specific dopantadded to them.

Dopant atoms act as either donors or acceptors to the intrinsic semiconductor atoms and change its electron and hole carrier concentrations at thermal equilibrium. Depending on the carrier, concentrations of an extrinsic semiconductor, they are classified as either n-type or p- type. Electrical properties of these types of semiconductors make them essential components of many electronic devices, solar cells, fuel cells, etc.

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1.6.3 N-type and P-type semiconductors

Electrons are the charge carriers for n-type semiconductors, while p-type semiconductors carry current mainly as electron deficiencies called holes. A hole has a positive electric charge, equal and opposite to the charge on an electron. Applying an external voltage to the sample, both the electron and the hole can move across the material. Flows of holes occur in a direction opposite to the flow of electrons in a semiconductor. In an n-type semiconductor, the dopant donates extra electrons which considerably increases the conductivity. However, in a p-type semiconductor, the dopant creates extra holes which similarly increase the conductivity. Therefore, both n- and p-type semiconductors are important for many applications.

1.6.4 Fermi level and band theory for extrinsic semiconductors

The Fermi level is important to describe the behaviour of extrinsic semiconductors and also helps to explain the different behaviours between insulators, metals and intrinsic and extrinsic semiconductors. The Fermi level of a substance is defined as the highest occupied energy level found in that substance at absolute zero temperature (0 K).

In band theory for semiconductors, there are mainly two bands to consider. The fully occupied lowermost band, called the valence band and the (almost unoccupied) uppermost band is called the conduction band. Only when electrons are excited to the conduction band can current flow in these materials. One of the main mechanisms for electrons to be excited to this conduction band is due to thermal activation. Therefore, conductivity of semiconductors is strongly dependent on the temperature of the material.

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Donor impurity atoms contain more valence electrons than the atoms they replace in the intrinsic semiconductor lattice and provides extra valence electrons to a semiconductor's conduction band. These excess electrons increase the electron carrier concentration of the semiconductor, making it n-type. Acceptor impurity atoms contain less valence electrons than the atoms they replace in the intrinsic semiconductor, it accepts electrons from the semiconductor's valence band and provides extra holes to the intrinsic semiconductor. These extra holes increase the hole carrier concentration of the semiconductor, making it p-type.

Band theory of n-type semiconductors shows that they have a Fermi level higher than the intrinsic Fermi energy level and it lies closer to the conduction band, while p-type semiconductors have Fermi energy levels below the intrinsic semiconductors and lie closer to the valence band. N-type and p-type semiconductors show that extra energy levels have been added by the dopant (Figure 1.11). In n-type materials, extra electron energy levels lie near to the top of the band gap, hence they can be more easily excited into the conduction band, while in p-type materials, creation of an acceptor energy level, close to the valence band, allows excitation of valence band electrons, leaving mobile holes in the valence band. The n- and p-type labels specify which charge carrier is its majority carrier. The opposite carrier is called the minority carrier, which is present due to thermal excitation at a much lower concentration compared to the majority carrier.

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Figure 1.11: Schematic diagrams for band structures of extrinsic semiconductors