Semiconductors are found everywhere and are ubiquitous. All our gadgetry and scientific equipment, chips and electronic devices like LED, transistors etc wouldn't have been as efficient without semiconductors. So let's learn a bit of introductory theory related to semiconductors.
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Introduction
- Electrical Resistance(R) and Conductance(G) : The electrical resistance of an object is a measure of its opposition to the flow of electric current. The inverse quantity is electrical conductance,
and is the ease with which an electric current passes. Electrical
resistance shares some conceptual parallels with the notion of
mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).
- Current Density ( J ) : is a measure of the amount of current flowing per unit cross-sectional area of the material.
- Semiconductor : Semiconductors are those materials with resistivities between that of the conductors and insulators. Examples : Ge, Si, GaAs, PbS. Of these Germanium(Ge) and Silicon(Si) are of course the most popular. Germanium is a better semiconductor as compared to Si, still Si is widely used in electronics since Si is inexpensive and abundant as sand(Quartz).
- Doping : Semiconductors(in their pure/intrinsic state) are not very conductive. To make them sufficiently conductive so they can be used in electronics, some impurities are added to them. This process is called Doping! The doped semiconductors are often referred to as extrinsic semiconductors.
- Types of Extrinsic Semiconductors : Semiconductors can be doped with electron donor or electron acceptor impurities. Those doped with electron-donor impurities become N-type semiconductors, and the latter are called P-type semiconductors.
- N-type semiconductors : N-type semiconductors are created by doping an intrinsic semiconductor with an electron donor element during manufacture. The term n-type comes from the negative charge of the electron. In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. A common dopant for n-type silicon is phosphorus or arsenic. In an n-type semiconductor, the Fermi level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band.
- P-type semiconductors : P-type semiconductors are created by doping an intrinsic semiconductor with an electron acceptor element during manufacture. The term p-type refers to the positive charge of a hole. As opposed to n-type semiconductors, p-type semiconductors have a larger hole concentration than electron concentration. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. A common p-type dopant for silicon is boron or gallium. For p-type semiconductors the Fermi level is below the intrinsic Fermi level and lies closer to the valence band than the conduction band.
Basic Terminologies
- Bands : The different energy levels that electrons in a semiconductor can occupy are often categorised into groups called bands.
- Band Theory of solids : Band Theory of Solids:
A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically.
- Band Gap / Energy Gap / Stop Band : In solid state physics and related applied fields, the band gap, also called an energy gap or stop band, is a region where a particle or quasi particle is forbidden from propagating. For insulators and semiconductors, the band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band.
- Fermi Level : An important parameter in the band theory is the Fermi level, the top of the available electron energy levels at low temperatures. The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties.
- Energy Band Gaps in Solids
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References
- https://en.wikipedia.org/wiki/Extrinsic_semiconductor#N-type_semiconductors
- https://en.wikipedia.org/wiki/Semiconductor
Thanks for reading!
Martin Medro
