Semiconductors: Half Conductor, Half Insulator - Essay Sample

Introduction

Semiconductor is a word derived from joining two words: 'Semi' meaning partly or half and 'Conductor', which is a material that conducts electric current. Therefore, semiconductor is a material that partly conducts electric current while at the same time acting as an insulator (Zhou et l., 2016). Thus, a semiconductor material is neither a good conductor nor a good insulator. It is just in between. Semiconductors can exist in single-element and compound forms. Single-element semiconductors include Silicon (Si), germanium (Ge), astatine (At), antimony (Sb), boron (B), arsenic (As), polonium (Po), and tellurium (Te). Conversely, compound semiconductors comprise silicon carbide, silicon germanium, indium phosphide, and gallium nitride. Semiconductors occur in numerous chemical compositions in a variety of crystal structures.

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Structure of Semiconductors

Semiconductors comprise atoms bonded together to create a uniform structure. An atom comprises a nucleas made up of protons or positively charged particles and neutrons (particles without charge) surrounded by electrons. Normally, electrons and protons are equal, which makes the atom electrically neutral. Each semiconductor has valence electrons that are shared, forming covalent bonds with surrounding atoms. Each covalent bond has two atoms that share a pair of electrons. Therefore, studying the arrangement of these atoms helps one understand the material properties of different types of semiconductors.

Most semiconductors have a diamond, zinc-blende, wurtzite, or rock-salt crystal structure. Elements and binary compounds, which average four valence electrons per atom, form tetrahedral bonds. In particular, a tetrahedral lattice site in a compound AB is one in which each atom A is surrounded symmetrically by four nearest neighboring B atoms.

Properties of Semiconductors

Optical Properties

An optical property of a semiconductor is defined as any property that involves the interaction between electromagnetic radiation or light and the semiconductor such as polarization absorption, reflection, refraction, diffraction, and scattering effects (Zelewski & Kudrawiec, 2017). Most optical properties of semiconductors are related to the specific nature of their electronic band structures, which is in turn related to the form of crystallographic structures, the particular atoms, and their bonding. Nearly all of transitions that contribute to the optical properties of semiconductors are one-electron transitions. Most of them conserve the crystal momentum, and hence, measure the vertical energy differences between the conduction and valence bands.

The properties are subdivided into those that are electronic and those that are lattice in nature. In particular, the electronic properties relate to processes involving the electronic states of the semiconductor. However, lattice properties concern vibrations of the lattice such as creation of phonons and absorption. Notwithstanding, the electrical properties receive the most attention because of the technological importance of their practical applications. Today, common semiconductor optoelectronic technologies encompass lasers, photodetectors, light-emitting-diodes, switches, modulators, and optical amplifiers, which exploit specific elements of the electronic optical properties.

Resistivity

The electrical property of semiconductors majorly referred to as resistivity, changes according to various aspects such as temperature and presence of impurities (Zhou et l., 2016). For example, at low temperature, they have low conductivity while at high temperature they conduct electricity easily. The conductivity increases because some electrons can break away from their covalent bond and move freely within the structure. This is attributed to the band gap between the conduction (Ec) and valence band (Ev), which is small in semiconductors. The valence band is the lower energy level of the semiconductor. On the other hand, the conduction band is the energy level at which electrons are free. In particular, when the temperature is raised, the band gap reduces and electrons move from the valence to the conduction band. Notably, this creates an empty space for an electron from another atom to fill it. This continuous movement is referred to as a 'hole' (Zhou et l., 2016). The minimum energy needed to excite the electron is called band gap (Eg) (Zhou et l., 2016). Consequently, they act as insulators at low temperatures and conductors at high temperatures.

Types of band gaps - There are two kinds of band gap in semiconductors namely direct and indirect. In direct band gap semiconductors, the minimum of the conduction band and maximum of the valence band occurs at the same k-point in the Brillouin zone. Conversely, for the indirect band gap semiconductors, the minimum of conduction band and maximum of valence band occur at different k-values.

Similarly, if semiconductors contain impurities, their conductivity is high unlike when they have none. The process of adding impurities to a pure semiconductor is called doping and the impurities are referred to as dopants (Tietze et al., 2018). Mainly, dopants are classified into two groups. The first set give negative charge carriers to make n-type semiconductors while the other group gives positive charge carriers to make p-type semiconductors. Regarding percentage, it takes only from 0.000001 to 0.1% of a dopant to bring a semiconductor material to useful resistivity range, a property that permits the creation of regions of very precise values in the material ((Tietze et al., 2018). The ability to vary semiconductors' resistivity makes them suitable for manufacturing semiconductor electronics. For example, according to Tietze et al. (2018), doping silicon with boron varies its resistivity between 10-3 cm to around 103 cm, which makes it ideal for integrated circuits. Moreover, it is quite easy to create areas of differential electrical properties. During manufacturing, scaling of semiconductor devices such as complimentary metal-oxide semiconductor (CMOS) makes the electrical performance of the device more strongly dependent on changes in dopant distribution (Agarwal et al., 2019).

N-Type Semiconductor - It is created when pentavalent impurity atoms are added to conduction-band electrons. Pentavalent atoms have five valence electrons. They include arsenic (As), phosphorus (P), antimony (Sb), and bismuth (Bi). For instance, if added to silicon, each pentavalent atom forms covalent bonds with four adjacent silicon atoms. The remaining electron becomes a conduction electron since it is not involved in bonding. Consequently, it does not form a hole in the valence band. Besides, it is in excess of the number needed to fill the valence band.

P-Type Semiconductor - A p-type semiconductor is created when trivalent impurity atoms are added. Such atoms include boron (B), gallium (Ga) and indium (In). Each trivalent atom forms covalent bonds with four adjacent atoms of the intrinsic semiconductor. Since all three valence electrons are used; a hole is created. However, this hole is not accompanied by a conduction or free electron.

Organic semiconductors - There are two sets of organic semiconductors namely low molecular weight materials and polymers. Both of these have a conjugated p-electron system created by the pzorbitals of sp2-hybridized C atoms in the molecules. The nature of bonding in organic semiconductors is different from inorganic ones. In particular, organic molecular crystals are van-der-Waals-bonded solids, which suggest a relatively weaker intermolecular bonding as compared to covalently bonded semiconductors such as silicon (Zelewski & Kudrawiec, 2017). The outcomes are visible in mechanical and thermodynamic properties, for instance, lower melting point and reduced hardness but even more significantly in a much weaker delocalization of electronic wave functions amongst neighboring molecules. Notably, this has direct effects for optical properties and charge carrier transport.

Hall Effect in Semiconductors

The Hall Effect occurs due to the nature of the current in a conductor. It describes the behavior of the free carriers in a semiconductor when applying electric and magnetic fields (Thirumavalavan et al., 2015). When a current-carrying semiconductor is placed in a magnetic field, its charge carriers experience a force in a direction perpendicular to both the magnetic field and the current.

Figure 1: Hall setup and carrier motion for holes and electrons

As shown in figure 1 above, the holes move in the positive x-direction. The magnetic field results in a force that acts on the mobile particles in a direction dictated by the right-hand rule (Thirumavalavan et al., 2015). Consequently, there is a force, Fy, along the positive y-direction that moves the holes to the right. In a steady-state, this force is balanced by an electric field, Ey, such that no net force exists on the holes. Consequently, a Hall voltage (VH) across the sample occurs that one can measure with high-impedance voltmeters.

Figure 2: Hall setup and carrier motion for electrons

From figure 2 above, the hall voltage is positive for holes. Conversely, electrons travel in the negative x-direction. Hence, the force, Fy, is in the positive y-direction because of the negative charge. Just like holes, the electrons move to the right. The balancing electric field, Ey, now results in a negative hall voltage.

A measurement of the Hall voltage is utilized to determine the free carrier density, carrier mobility, and the type of semiconductor, which is whether n-type or p-type. Repeating this measurement at various temperatures enables the measurement of the free carrier density, mobility, and the function of temperature (Thirumavalavan et al., 2015). A measurement of the carrier density versus temperature avails information concerning the ionization energies of the donor and acceptors present in the semiconductor.

Conclusion

To conclude, understanding the chemistry of semiconductors is crucial especially when dealing with electronic devices like transistors, diodes, and integrated circuits. Moreover, one comprehends the structure of semiconductors and the interaction of atomic particles especially when different types of semiconductive materials are joined. One also understands the nature of current in both normal and magnetic fields.

References

Agarwal, S., Dixit, A., & Johnson, J. B. (2019). U.S. Patent No. 10,482,200. Washington, DC: U.S. Patent and Trademark Office. https://patentimages.storage.googleapis.com/9a/e5/5c/ef48042c46c02f/US10482200.pdf

Thirumavalavan, S., Mani, Y., & Suresh, S. S. (2015). Studies on Hall effect and DC conductivity measurements of semiconductor thin films prepared by chemical bath deposition (CBD) method. Zurnal nano-ta elektronnoyi fiziki, (7, 4), 04024-1. https://jnep.sumdu.edu.ua/download/numbers/2015/4/articles/en/jnep_2015_V7_04024.pdf

Tietze, M. L., Benduhn, J., Pahner, P., Nell, B., Schwarze, M., Kleemann, H., ... & Leo, K. (2018). Elementary steps in electrical doping of organic semiconductors. Nature communications, 9(1), 1-9. https://www.nature.com/articles/s41467-018-03302-z?report=reader

Zelewski, S. J., & Kudrawiec, R. (2017). Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals. Scientific reports, 7(1), 1-11. https://www.nature.com/articles/s41598-017-15763-1

Zhou, J., Liao, B., & Chen, G. (2016). First-principles calculations of thermal, electrical, and thermoelectric transport properties of semiconductors. Semiconductor Science and Technology, 31(4), 043001. https://dspace.mit.edu/bitstream/handle/1721.1/109361/Topical%20Review%20-%20manuscript.pdf?sequence=1