Due to their role in the fabrication of electronic devices, semiconductors are an important part of our lives. Imagine life without electronic devices. Semiconductors are all around us. They control the computers we use to conduct business, the phones and mobile devices we use to communicate, the cars and planes that get us from place to place, the machines that diagnose and treat illnesses, the military systems that protect us, and the electronic gadgets we use to listen to music, watch movies, and play games, just to name a few.
So what are semiconductors? Semiconductors are materials that have conductivity between conductors (generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. Silicon and gallium arsenide are some of the most commonly used semiconductors. Silicon is a critical element used in electronic circuit fabrication and gallium arsenide is used in solar cells, laser diodes, etc. A solar energy plant produces megawatts of electricity generated by solar cells made from gallium arsenide.
Semiconductor materials have conducting properties that may be altered in useful ways by introducing impurities ("doping") into the crystal structure. For example, a Silicon atom has 4 valence electrons, it forms a covalent bond with 4 different silicon atoms to form a silicon crystal. As there are very few free electrons available to move around the silicon crystal, crystals of pure silicon (or germanium) are therefore good insulators, or at the very least very high-value resistors. But simply connecting a silicon crystal to a battery supply is not enough to extract an electric current from it. To do that we need to create a “positive” and a “negative” pole within the silicon allowing electrons and therefore electric current to flow out of the silicon.
These poles are created by doping the silicon with certain impurities. If silicon is doped with an atom with 5 valence electrons (antimony, phosphorus, or arsenic) or 3 valence electrons (boron, gallium, indium), the conductivity of silicon is increased by the movement of holes or electrons depending on the dopant. For example, doping with a pentavalent atom allows four out of the five orbital electrons to bond with its neighboring silicon atoms leaving one “free electron” to become mobile when an electrical voltage is applied (electron flow). As each impurity atom “donates” one electron, pentavalent atoms are generally known as “donors”. Similarly, doping with trivalent atoms, which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as holes in the structure of the crystal where electrons are effectively missing.
As there is now a hole in the silicon crystal, a neighboring electron is attracted to it and will try to move into the hole to fill it. However, the electron filling the hole leaves another hole behind it as it moves. This in turn attracts another electron which in turn creates another hole behind it, and so forth gives the appearance that the holes are moving as a positive charge through the crystal structure (conventional current flow). As each impurity atom generates a hole, trivalent impurities are generally known as “Acceptors” as they are continually “accepting” extra or free electrons.
Doping greatly increases the number of charge carriers within the crystal. When a doped semiconductor contains free holes, it is called "p-type", and when it contains free electrons, it is known as "n-type". Also increasing the temperature increases the conductivity of a semiconductor, unlike metals that decrease conductivity with temperature. Quantum physics is used to explain the movement of charge carriers within the crystal lattice. When two differently doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers, which include electrons, ions, and electron holes, at these junctions is the basis of diodes, transistors, and most modern electronics.
Currently, there is a semiconductor supply chain shortage due to increased demand and we are facing a shortage in car manufacturing, toys for Christmas, etc…. Yes, semiconductors are very important to us.
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By: Zubin Sidhu
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References:
Dcadmin. “About Semiconductors | SIA.” Semiconductor Industry Association, 14 July 2021, www.semiconductors.org/semiconductors-101/what-is-a-semiconductor.
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