**2. Technical background**

It is important to assimilate the band theory that explains the nature of semiconductors in terms of energy levels between the valence and the conduction bands in order to gain knowledge about semiconductor materials. The main difference between metallic materials and semiconductors is that the current is provided by electron flow in metallic conductors, whereas in semiconductors this flow occurs not only by the electron flow but also by the flow of positively charged holes.

Electrical conductivity is directly related to the band structure of a material. If we look at the basis of the theory, atomic energy levels of each atom are equal when two different atoms are sufficiently distant from each other. However, as these atoms approach each other, differences in the original energy levels of the atoms are observed, and as a result of these differences, an interaction occurs that creates molecular bands between the atoms. Therefore, it is for this reason that materials with different band structures show different conductivity properties. In conductors, the energy difference between partially filled energy levels and empty levels is very low. Therefore, when a potential is applied to metals, electron mobility between the filled and empty levels, which takes place by using very low energy, is easily realized, and the flow of electrons is provided. Therefore, it would not be the right approach to talk about an obstacle between the levels defined as valence band and conduction band in metals. As known, an electron must have an empty energy level to move, otherwise electrons cannot move in solid material. Based on this approach, it can be clearly understood why electrical current is not observed in insulating structures. In insulators, the valence band is the highest band fully filled by electrons, and the conduction band is the lowest empty band, with a forbidden band gap of about 5–10 eV between these two bands [2]. This broadband between the valence band and the conduction band prevents the transmission of electrons to the conduction band, and no electrical current is generated in the insulating materials. Similar to the band structure of the insulators, semiconductors have a valence band occupied by electrons and a conduction band ready to be filled with electrons. In semiconductors, just like insulating materials, there is a band gap between these two bands. The main difference between the band gap in these two groups excluding conductors is that the band gap value is much smaller in semiconductors (1.1 eV for silicon) than in the insulator [3]. Since the thermal energy in semiconductors creates the driving force for the movement of electrons, the conductivity of these materials is directly related to the temperature. The conductivity of a semiconductor material as a result of a decrease in resistivity can be associated with increased kinetic energy with temperature. **Figure 1** schematically shows the energy band gaps in conductors, semiconductors, and insulators.

In semiconductors, the valence band that is below the forbidden band gap is almost completely full. On the other hand, there is a nearly empty conduction band over the prohibited band gap. When a semiconductor material is excited, if the

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holes [4].

**Figure 1.**

*Energy band gaps in materials.*

**3. Types of semiconductors**

recombination steps [5].

*Technological Background and Properties of Thin Film Semiconductors*

energy of the photon is greater than the band gap, the electrons will easily pass to the conduction band, and electrical current will occur. According to the theory, as a result of the transition of an electron to the conduction band, the excited electron leaves a hole in place that can flow through the material and act as a positively charged particle. Here, carrier production and recombination can be defined as two basic factors in the creation of charge-carrying electrons and positively charged

In general, depending on the level of doping, semiconductors can be classified into two main groups such as *intrinsic semiconductors* and *extrinsic semiconductors*. The intrinsic semiconductors are pure semiconductors and no addition is made. In this type of semiconductor, conductivity is provided by the thermal stimulation of electrons. At the same time, the number of excited electrons and positively charged holes is equal. The behavior here appears as a result of the carrier production and

On the other hand, extrinsic semiconductors have low conductivity values, and an important process called *doping* is applied to overcome the problems encountered in applications and to increase the conductivity [6]. This process can be explained simply by adding small amounts of impurities in the concentrations of charge-carrying electrons and positively charged holes, thereby increasing the conductivity level. The aim is to change the electronic structure by impurity addition into the structure without changing the crystal structure. For example, arsenic with five valence electrons to an atom and germanium with four valence electrons will cause the arsenic atom to covalently bond with the germanium atom. The extra fifth electron of the arsenic atom will have the electrical conductivity as it will have the freedom to move from one atom to another [4]. Such semiconductors, which the dopant element donates an electron, are called n-type semiconductors. In addition to producing free electrons in n-type doping, an equal number of positive charges are also produced in pairs with free electrons. As a result, the doped semiconductor material remains electrically neutral. However, these positive charges should not be understood as positively charged holes. These charges occur in the absence of free electrons, but do not contribute to a current flow. Another contribution of free electrons to pure semiconductors is that the donor electron is much closer to the conduction band than an electron in the valence band of the original atom.

*DOI: http://dx.doi.org/10.5772/intechopen.91751*

*Technological Background and Properties of Thin Film Semiconductors DOI: http://dx.doi.org/10.5772/intechopen.91751*

**Figure 1.** *Energy band gaps in materials.*

*21st Century Surface Science - a Handbook*

semiconductors.

**2. Technical background**

materials is one of the most important sources of energy and new-generation energy. In short, semiconductor devices for nanotechnology and polymer science have taken the advancement of research in semiconductors to a new step, aiming to improve the chemical and physical properties of these materials. To understand the nature of these crucial engineering materials, the difference and theory between conductors, insulators, and semiconductors must be fully understood. In addition, basic concepts such as band theory, doping processes, and p-n connection theory of solids are theoretical bases that will give a general idea of understanding

It is important to assimilate the band theory that explains the nature of semiconductors in terms of energy levels between the valence and the conduction bands in order to gain knowledge about semiconductor materials. The main difference between metallic materials and semiconductors is that the current is provided by electron flow in metallic conductors, whereas in semiconductors this flow occurs not only by the electron flow but also by the flow of positively charged holes. Electrical conductivity is directly related to the band structure of a material. If we look at the basis of the theory, atomic energy levels of each atom are equal when two different atoms are sufficiently distant from each other. However, as these atoms approach each other, differences in the original energy levels of the atoms are observed, and as a result of these differences, an interaction occurs that creates molecular bands between the atoms. Therefore, it is for this reason that materials with different band structures show different conductivity properties. In conductors, the energy difference between partially filled energy levels and empty levels is very low. Therefore, when a potential is applied to metals, electron mobility between the filled and empty levels, which takes place by using very low energy, is easily realized, and the flow of electrons is provided. Therefore, it would not be the right approach to talk about an obstacle between the levels defined as valence band and conduction band in metals. As known, an electron must have an empty energy level to move, otherwise electrons cannot move in solid material. Based on this approach, it can be clearly understood why electrical current is not observed in insulating structures. In insulators, the valence band is the highest band fully filled by electrons, and the conduction band is the lowest empty band, with a forbidden band gap of about 5–10 eV between these two bands [2]. This broadband between the valence band and the conduction band prevents the transmission of electrons to the conduction band, and no electrical current is generated in the insulating materials. Similar to the band structure of the insulators, semiconductors have a valence band occupied by electrons and a conduction band ready to be filled with electrons. In semiconductors, just like insulating materials, there is a band gap between these two bands. The main difference between the band gap in these two groups excluding conductors is that the band gap value is much smaller in semiconductors (1.1 eV for silicon) than in the insulator [3]. Since the thermal energy in semiconductors creates the driving force for the movement of electrons, the conductivity of these materials is directly related to the temperature. The conductivity of a semiconductor material as a result of a decrease in resistivity can be associated with increased kinetic energy with temperature. **Figure 1** schematically shows the energy band

**50**

gaps in conductors, semiconductors, and insulators.

In semiconductors, the valence band that is below the forbidden band gap is almost completely full. On the other hand, there is a nearly empty conduction band over the prohibited band gap. When a semiconductor material is excited, if the

energy of the photon is greater than the band gap, the electrons will easily pass to the conduction band, and electrical current will occur. According to the theory, as a result of the transition of an electron to the conduction band, the excited electron leaves a hole in place that can flow through the material and act as a positively charged particle. Here, carrier production and recombination can be defined as two basic factors in the creation of charge-carrying electrons and positively charged holes [4].
