Semiconductors – Familiarity with semiconductors

Atoms of different elements have electrons that orbit the nucleus in different orbits. Electrons circulating in orbits close to the nucleus have less energy, but the more gravitational force is exerted on them by the nucleus, so these electrons cannot be easily separated from the atom.

In each atom, the last circuit is called the valence layer, and the electrons in this layer are called valence electrons.

Valence electrons have more energy than other electrons but are less dependent on the nucleus than other electrons.

Figure (1) shows the atomic structure of the copper atom. As you can see in this figure, the copper atom has only one electron valence.

Objects in nature are divided into three categories based on the number of valence electrons of their constituent atoms: conductive bodies, insulating bodies, and semiconductor bodies, which we will examine in the following.

Conductor bodies:

The atoms that make up these bodies have less than four valence electrons, and these electrons are easily released from the nucleus.

Conductors have many free electrons, and these free electrons conduct electricity. These objects are also called conductive objects.

Monovalent metals are good conductors and the best conductors are silver, copper, and gold.

Insulating Bodies:

The atoms that make up these bodies usually have five to eight electron valences.

Because the energy given to the atoms of insulating objects is divided among a large number of valence electrons, the energy received by each electron is very small and therefore these electrons are difficult to separate from the atom, which causes the insulating objects to be in a state of Normally they have very few free electrons and do not conduct electricity.

Semiconductor Bodies:

The atoms that make up a semiconductor body usually have four valence electrons. Semiconductor bodies are almost insulated at absolute zero (-273 ° C).

At room temperature (25 ° C), ambient thermal energy releases several valence electrons and increases electrical conductivity in the body.

At room temperature, the electrical conductivity of semiconductors is better than that of insulators and worse than that of conductors. Commonly used semiconductors include carbon, silicon (silicon), germanium, thorium, zirconium, and hafnium, of which silicon and germanium are widely used in electricity and electronics.

In the following, we will examine silicon and germanium semiconductors.

Silicon and germanium semiconductors

Silicon has an atomic number of 14. That is, it has 14 protons and 14 electrons. Germanium has an atomic number of 32.

That is, it has 32 protons and 32 electrons. Figure (2) shows the silicon atomic structure and Figure (3) shows the germanium atomic structure.

As you can see in these figures, each silicon and germanium atom has four valence electrons.

Because silicon and germanium have four electrons in their last circuit, they tend to complete their last circuit and reach a steady state.

For this purpose, each atom shares an electron with each of its four adjacent atoms. This type of bond between atoms is called a covalent bond.

Figure (4) shows the covalent bonds between silicon atoms.

In a silicon or germanium crystal, at absolute zero temperature, the silicon or germanium crystal is a complete insulator because all the covalent bonds are between the atoms and there are no free electrons.

But as the temperature increases, the motion of the valence electrons increases, and some of the covalent bonds between the atoms are broken and electrons are released, thus increasing the electrical conductivity of silicon and germanium crystals.

The higher the temperature, the more covalent bonds are broken and the more free electrons there are, increasing the electrical conductivity of the crystal.

As each electron separates from an atom, an electron void is created in that atom called a hole. Figure (5) shows how to create a hole.

The electrons released in the crystal move irregularly. If an electron accidentally approaches a hole, it is absorbed into the hole.

Therefore, until an external force is applied to the crystal, the release of electrons and their absorption by the holes continues irregularly.

But when voltage is applied to both ends of the crystal, the free electrons move to the positive pole of the battery, and a current is applied to the crystal that is caused by the movement of the electrons, called the electron current.

There is another current in the crystal that is caused by the movement of cavities.

If there is a hole in an atom, it absorbs electrons from the adjacent atom. But instead of the absorbed electron, a new hole is created, and although the holes do not move, they appear to be moving.

In this way, when an electron moves from right to left, for example, the hole appears to be moving from left to right.

Note that the hypothetical direction of the holes is always opposite to the direction of the electrons.

Because the number of free electrons and holes created in silicon and germanium crystals due to heat energy is not large enough, these crystals do not have good electrical conductivity. To increase the electrical conductivity of these semiconductors, impurities are added to them.

Adding impurities to semiconductors is done in two ways.
1- Impurity of semiconductor crystal with pentavalent atom
2- Impurity of semiconductor crystal with trivalent atom

Impurity of a semiconductor crystal with a pentavalent atom: In this method, pentavalent elements such as arsenic (As), antimony (Sb), or phosphorus (P), which have five electrons in their capacitance layer, are added to the silicon or germanium crystal.

For example, in Figure (6), the pentavalent element arsenic is added to the silicon crystal.

As you can see in Figure (6), the impurity atom of arsenic forms a common bond with the four adjacent silicon atoms, and since there are only 8 electrons in the capacitance layer of the arsenic atom, one electron of the impurity atom is easily released from the nucleus. It is released as an electron.

So with the addition of each impurity atom, a free electron is created. By adjusting the number of impurity atoms, they control the number of free electrons in the crystal.

In addition to the free electrons that result from the addition of an impurity atom in the crystal, a small number of electrons are released from the nucleus due to the heat energy of the environment, and avoid is created in their void.

An impurity atom that gives a crystal a free electron and turns into a positive ion is called a donor atom.

Because the number of free electrons in these crystals that conduct electrical conduction is far greater than the number of holes, free electrons are called majority carriers and holes are called minority carriers.

These types of crystals, most of which are carried by electrons, are called N-type crystals, where N is derived from the word negative.

Because in this type of crystal, the charge of the majority carriers, ie electrons, is negative. Of course, the whole crystal is electrically neutral because its positive and negative charges are equal.

Figure (7) shows a symbolic image of majority and minority carriers in N-type semiconductors. In this figure, the white circles represent the hole and the red comet circles represent the free electrons in motion.

Impurity of a semiconductor crystal with a trivalent atom: Whenever we add a trivalent element such as aluminum (Al), gallium (Ga), or indium (In) which has three electrons in its capacitance to the crystal of pure silicon or germanium, the electrons of the circuit Finally, an impure element such as aluminum forms a covalent bond with the valence electrons of adjacent atoms.

In this way, seven electrons are circulating in the last circuit of the impurity atom, resulting in a void or hole.

Figure (7) shows the addition of an aluminum impurity atom to a silicon crystal.

As you can see in Figure (8), a hole is created by adding a trivalent atom to a semiconductor crystal. The electron may break its bond with the other electron due to sufficient kinetic energy and fill the hole, in which case a new hole is created in the crystal.

So for every trivial atom added to a semiconductor, a hole is sure to form in that semiconductor. The trivalent atom that can absorb a free electron is called the acceptor atom.

The acceptor atom becomes a negative ion when it receives an electron.

Due to the heat of the environment, a small number of electrons also acquire the necessary energy and separate from their nuclei and become free electrons.

Thus, in the crystal, in addition to the large number of cavities that carry the majority, there are also a small number of free electrons, the minority carriers.

Because in this type of crystal, the holes that have a positive charge are the carriers of the majority, these types of crystals are called P-type crystals, where P is derived from the word Positive meaning positive.

Figure (9) shows a symbolic image of the majority and minority carriers in the P-type semiconductor.

Figure (9)

Superconductor happy at room temperature

A team of physicists in New York has discovered a material that conducts electricity with perfect efficiency at room temperature — a long-sought scientific milestone. The hydrogen, carbon, and sulfur compound operates as a superconductor at up to 59 degrees Fahrenheit, the team reported today in Nature. That’s more than 50 degrees hotter than the previous high-temperature superconductivity record set last year.

“This is the first time we can claim that room-temperature superconductivity has been found,” said Ion Errea, a condensed matter theorist at the University of the Basque Country in Spain who was not involved in the work.

“It’s a landmark,” said Chris Pickard, a materials scientist at the University of Cambridge. “That’s a chilly room, maybe a British Victorian cottage,” he said of the 59-degree temperature.

Yet while researchers celebrate the achievement, they stress that the newfound compound — created by a team led by Ranga Dias of the University of Rochester — will never find its way into lossless power lines, frictionless high-speed trains, or any of the revolutionary technologies that could become ubiquitous if the fragile quantum effect underlying superconductivity could be maintained in truly ambient conditions. That’s because the substance superconducts at room temperature only while being crushed between a pair of diamonds to pressures roughly 75% as extreme as those found in the Earth’s core.

“People have talked about room-temperature superconductivity forever,” Pickard said. “They may not have quite appreciated that when we did it, we were going to do it at such high pressures.”

Materials scientists now face the challenge of discovering a superconductor that operates not only at normal temperatures but under everyday pressures, too. Certain features of the new compound raise hope that the right blend of atoms could someday be found.

Electrical resistance occurs in normal wires when freely flowing electrons bump into the atoms that make up the metal. But researchers discovered in 1911 that at low temperatures, electrons can induce vibrations in a metal’s atomic lattice, and those vibrations, in turn, draw electrons together into couples known as Cooper pairs. Different quantum rules govern these couples, which stream together in a coherent swarm that passes through the metal’s lattice unimpeded, experiencing no resistance whatsoever. The superconducting fluid also expels magnetic fields — an effect that could allow magnetically levitating vehicles to float frictionlessly above superconducting rails.

As the temperature of a superconductor rises, however, particles jiggle around randomly, breaking up the electrons’ delicate dance.

Researchers have spent decades searching for a superconductor whose Cooper pairs tango tightly enough to withstand the heat of everyday environments. In 1968, Neil Ashcroft, a solid-state physicist at Cornell University, proposed that a lattice of hydrogen atoms would do the trick. Hydrogen’s diminutive size lets electrons get closer to the nodes of the lattice, augmenting their interactions with the vibrations. Hydrogen’s lightness also allows those guiding ripples to vibrate faster, further strengthening the glue that binds the Cooper pairs.

Progress took off in the 2000s when supercomputer simulations let theorists predict the properties of various hydrides, and the widespread use of compact diamond anvils let experimentalists squeeze the most promising candidates to test their mettle.

Suddenly, hydrides started setting records. A team in Germany showed in 2015 that a metallic form of hydrogen sulfide — a pungent compound found in rotten eggs — superconducts at −94 degrees Fahrenheit under 1.5 million times the pressure of the atmosphere. Four years later, the same lab used lanthanum hydride to hit −10 degrees under 1.8 million atmospheres, even as another group found evidence for superconductivity in the same compound at 8 degrees.

Dias’ lab in Rochester has now shattered those records. Guided by intuition and rough calculations, the team tested a range of hydrogen compounds searching for the goldilocks ratio of hydrogen. Add too little hydrogen, and a compound won’t superconduct as robustly as metallic hydrogen does. Add too much, and the sample will act too much like metallic hydrogen, metalizing only at pressures that will crack your diamond anvil. Throughout their research, the team busted many dozens of $3,000 diamond pairs. “That’s the biggest problem with our research, the diamond budget,” Dias said.

The winning recipe proved to be a riff on the 2015 formula. The researchers started with hydrogen sulfide, added methane (a compound of carbon and hydrogen), and baked the concoction with a laser.

“We were able to enrich the system and introduce just the right critical amount of hydrogen necessary to maintain these Cooper pairs at very high temperatures,” said Ashkan Salamat, Dias’ collaborator and a condensed matter physicist at the University of Nevada, Las Vegas.

But the fine details of the hydrogen-carbon-sulfur potion they’ve cooked up elude them. Hydrogen is too small to show up in traditional probes of the lattice structure, so the group doesn’t know how the atoms are arranged, or even the substance’s exact chemical formula.

Eva Zurek, a computational chemist at the University at Buffalo, belongs to a group of theorists loosely affiliated with Dias’ lab. Earlier this year they predicted the conditions under which one metal that might have formed between the diamond anvils should superconduct, and they found different behavior. She suspects that high pressures instead transformed Dias’ substance into an unknown form whose superconductivity is especially robust.

Once Dias’ group can figure out exactly what they’ve got on their hands (details he and Salamat say are coming soon), theorists will build models exploring the features that give this hydrogen-carbon-sulfur mixture its superconducting power, in hopes of further modifying the recipe.

Physicists have proved most two-element hydrogen hybrids to be dead ends, but the new three-element blend marks a potentially significant advance into the world of complex chimera materials. One of the elements involved seems particularly promising to some.

“What I like about this work: They bring carbon into the system,” said Mikhail Eremets, an experimentalist at the Max Planck Institute for Chemistry in Germany whose lab set the hydride records of 2015 and 2019.

Pushing Towards Room-Temperature Superconductivity