The New Silicon

Antimony is one of the earliest discovered chemical elements. For a while confused with lead for centuries, it was first isolated by Biringuccio and extensively characterised in the mid 1500s.

Antimony is an element of different forms, much like carbon and phosphorus. Of the four allotropes of antimony known, only one is stable, with the rest (yellow,black and explosive) without much of a use. Metallic antimony is a bright, silvery-white metal which is quite soft. Interestingly, due to the metal’s structure, it is quite dense and yet very easily crumbles.  

Not surprisingly with today’s health and safety standards, 60% of metallic antimony is oxidised to Antimony trioxide and used in flame retardants. When formed into a resin with other ingredients, the resulting material will burn if a flame is held to it but will immediately self-extinguish when the source of ignition is removed. Other places where Sb is used include Pb-Sb alloys, where the antimony content increases the strength of the alloy. In other alloys, such as the Lead-Antimony alloys used in some lead-acid batteries, the addition of this metal reduces the charging time and amount of hydrogen released by the battery.

One of the most exciting emerging uses for high purity Antimony is in the semiconductor industry as a dopant. Usually, silicon is quite a poor conductor of electricity. However by ‘doping’ (adding small amounts distributed throughout the structure) with elements such as Germanium, Arsenic and others, its conductivity can be drastically changed. Antimony is one of these dopants – with a few novel properties included.

Most of the high purity antimony in today’s world is used in the semiconductor industry. Within this sector, the main home of Antimony is with Indium as an InSb compound. This semiconductor material has been routinely crystalised since the mid 50s and has some very noteworthy properties. For example, Indium Antimonide has a ballistic length which is rivalled only by carbon nanotubes, has a very high electron mobility and even a quantum efficiency that has been measured at 100%.

Ballistic length, roughly speaking, is a measure of the average distance an electron travels before it encounters an obstacle within a substance, while electron mobility describes the greatest velocity at which an electron can travel through a material. Finally, quantum efficiency (in the context of CCDs in cameras/scanners and other detectors operating in the IR-Visual-UV spectrum) refers to the ratio of the the number of photons (discrete packets of light energy) incident on a solar cell, for example, to the number of electrons released into the semiconductor. The quantum efficiency is (in simplistic terms) the efficiency at which light is converted to electricity.

What does all of this mean? These characteristics are golden, and are regularly sought by manufacturers of various components. For example, if one is making an infrared detector, it would be very much better if lots of the incoming infrared light was converted to electrons because one doesn’t have to amplify the signal as much.

A quotation from Intel’s Director of Technology Strategy, Paolo Gargini:

“Increased mobility in the transistor channel leads to higher performance and less energy consumption…compound semiconductors have higher electron mobility than silicon; indium antimonide is highest of all…Where gallium arsenide has 8 times higher mobility than silicon, indium arsenide is 33 times higher and indium antimonide is 50 times higher.”

This alloy has most recently been developed for use in Bipolar Junction Transistors (BJTs) operating at over 200GHz (where a standard computer runs at around 2GHz) and some have even postulated theoretical switching speeds of 1THz (1000000000000 times per second!) These transistors can also be run on very low voltages – as low as 0.5V – which means that the computers of tomorrow may be just as power efficient as the ones of today. This material really is the next big thing, according to some analysts. The “Indium Antimonide Valley” – an update to the slowing performance possible with the silicon counterpart – could soon be a real thing.