Besides the silicon: the search for new semiconductors

Our modern world is based on semiconductors. In addition to the computer, to mobile phones and digital cameras, semiconductors are a key component of a growing number of devices. Think of the LED lights, energy efficient CFL lamps are replacing home, along with everything that has an illuminated display or a control circuit: cars, refrigerators, ovens, coffee makers and more.

While many people have heard of Silicon and Silicon Valley, They do not realize that this is just one example of an entire class of materials.

But silicon used in all types of electronic computers and gadgets has its technical limits, in particular because engineers are trying to use electronic devices to produce or process the light. The search for new semiconductors is active. Where will these innovations of materials?

What is a semiconductor?

As the name suggests, Semiconductors are materials that conduct electricity to some temperatures but not to others – unlike most metals, which are conductive at any temperature, and insulators such as glass, plastic and stone, they usually do not conduct electricity.

However, this is not their most important trait. If built properly, these materials may change the electricity that moves through them, limiting the directions in which flows and amplifying a signal.

The combination of these properties is the basis of diodes and transistors that make up all our modern gadgets. These circuit elements perform a multitude of tasks, including the conversion of electricity from the wall outlet into something usable by the devices and the processing of information in the form of zero and one.

The light can also be absorbed in the semiconductor and transformed into electrical current and voltage. The process works well in reverse, allowing the emission of light. Using this property, We manufacture laser, luci a LED, digital cameras and many other devices.

The rise of silicon

While this all sounds very modern, original discoveries semiconductor date back to the 30s.

Selenium was also used to make some of the first solar cells in 1880.

Ancient Solar cell selenium

Selenium was discovered by chemist Berzelius in 1817 It is in the 1873 Smith discovered that its resistivity was a lighting intensity function falling on the surface of the material. The first selenium cell was manufactured by Siemens in 1876, and it was made by winding two thin platinum wires on the surface of a sheet of mica, and then covering the surface with a thin film of molten selenium. The large surface-to-volume ratio was necessary because the photoconductivity is a surface effect, while the selenium resistivity is quite high.

The selenium cell was used as a detector for an optical communication system developed by Alexander Graham Bell in 1879-1880, the active area is the small central rectangle.

A key limitation was the inability to purify the elements used. Small impurities, small as one of a gazillion, or 0,0000000001 percent, They could radically change the behavior of a semiconductor. As technology evolved to produce more pure materials, They followed better semiconductors.

The first semiconductor transistor was made of germanium in 1948, but silicon rose rapidly to become the dominant semiconductor material. The silicon wafer is mechanically strong, relatively easy to cleanse and has reasonable electrical properties.

It is also incredibly abundant: the 28,2 percent of the earth's crust is silicon. This makes it literally inexpensive. This nearly perfect semiconductor worked well for the creation of diodes and transistors and still forms the basis of almost all computer chips. There was a problem: the silicon is very inefficient in converting the light into an electrical signal or to transform electricity into light.

When the primary use of the semiconductors was in computer processors connected by metal wires, This was not a big deal. Ma, while we were moving towards the use of semiconductors in solar panels, sensors for cameras and other applications related to light, This weakness of the silicon became a real obstacle to progress.

Finding new semiconductors

The search for new semiconductors begins on the periodic table of elements, a part of which is in the right figure.

In the column labeled IV , each element form bonds by sharing electrons with four of its four neighbors. The strongest of these bonds of elements “group IV” It is for carbon (C), forming diamonds. Diamonds are good insulators (and transparent) because carbon retains these electrons closely. Usually, a burn diamond before forcing an electric current through it. Recently structuring the carbon atoms in thin gratings we have arrived at the opposite solution with graphene, equally transparent, strong and highly conductive.

The elements in the lower part of the column, pond (Sn) and lead (Pb), They are much more metal. Like most metals, they keep their bonding electrons so loose that when it is applied a small amount of energy the electrons are free to break their bonds and flow through the material.

Silicon (And) and germanium (Give) They are in the middle and therefore are semiconductors. Because of a quirk in the way both are structured, however, They are inefficient in the exchange of electricity with light.

To find materials that work well with light, we have to go from one part of the Group IV Column. Combining elements of the columns “group III” e “group V” You are obtained by materials with semiconducting properties. These materials “III-V”, such as gallium arsenide (GaAs), They are used to realize laser, luci a LED, photodetectors (as found in digital cameras) and many other devices. They do what that silicon is not good.

But because the silicon is used for solar panels if it is so difficult to convert light into electricity? Cost. The silicon can be refined by a shovel full of dirt collected from anywhere on the Earth's surface; The constituent elements of III-V compounds are much rarer.

A solar panel in standard silicon converts sunlight with efficiency from 10 al 15%. A III-V panel can be three times more efficient, but it often costs more than three times as much. The III-V materials are also more fragile than silicon, making them difficult to work with large panels.

However, the increased speed of the electrons of the III-V materials allows the construction of very faster transistors, at speeds hundreds of times higher than those found in computers. They can open the way for the replacement of cables inside the computer with light beams, significantly improving the speed of data flow.

In addition to the III-V materials, There are also materials II-VI in use. These materials include some of the sulfides and oxides in 800 sought. Zinc Combinations, cadmium and mercury in tellurium were used to create infrared cameras and solar cells. These materials are notoriously fragile and very difficult to fabricate.

The future of semiconductors

As new semiconductor materials could be used?

The electronic semiconductor III-V high-power (gallium nitride) It will be the backbone of our electricity grid system, converting the power for high voltage transmission and vice versa. New materials (antimonidi e bismutoidi) They are leading the way to detection infrared ( http://www.mdpi.com/1424-8220/13/4/5054/htm ) for medical, military, civilians, as well as new possibilities for telecommunications.

And what about the old standby, silicon? His inability to take advantage of the light efficiently does not mean it is destined to basket dust of history. Researchers are giving new life to silicon, creating “silicon photonics” ( http://reap.ece.tufts.edu/ ) to better manage the light, rather than merely exchanging electrons.

One method is the inclusion of small amounts of another element of group IV, pond, in silicon or germanium. This changes their properties, allowing them to absorb and emit light more efficiently.

Future research will tell vali there are directions that have been more successful.

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