Computers are the jewel in the crown of modern technology. Take one journey on the London Underground and you realise how successfully they have entwined themselves into our daily lives.Tablets, smartphones, laptops, the list goes on and on. Few aspects of life remain untouched by some form of computerized system. Using a computer is now so instinctual that most people cannot comprehend a world without them.
Powering this feat of humanity is a network of semiconductor chips and transistors. Most of these are made out of the chemical element silicon.
To convert a silicon crystal into a semi-conducting material, it must undergo a process known as doping. This is a method used to add impurities to the silicon crystal lattice. There are two ways of doping silicon.
- N-type – Addition of Phosphorous or Arsenic. This adds an ‘extra electron’ that is used to conduct electricity
- P-type – Addition of Boron or Gallium. This results in one less electron, making a ‘hole’ in the crystal structure
If you place an p and n type semiconductor together, interesting behaviour is observed at the junction between the two. At the p-n junction current is only allowed to pass one way – from the p to the n side. It cannot travel in the opposite direction. As a result, the junction acts as a switch, called a diode.
A transistor looks like two diodes placed back to back, making three layers instead of the two layered diode. The layers can either be NPN or PNP. Transistors have switching behavior. When a small current is applied across the central layer, a much larger current can flow through the entire three-layer system. As a result, a small current can turn a large current on and off.
Silicon chips, which are made from thousands of transistors acting as switches, are used to design logic gates, the foundations of a computer. But this reliance on Silicon may soon be a thing of the past.
In an article published in Nature last week, Max Shulaker and colleagues at Stanford University in California revealed that they have made the first computer made entirely out of carbon nanotubes.
Carbon nanotubes are hollow, cylindrical structures made from of a single sheet of carbon atoms. It is believed that transistors made from nanotubes could be faster and more efficient than silicon-based equivalents. The team had to overcome several problems when designing a circuit using carbon nanotubes.
Due to their small size, nanotubes often become mis-positioned in the circuit, resulting in incorrect pathways. To overcome this, the team grew the nanotubes on quartz and then transferred them to a silicon dioxide wafer. This method gave them a 99.5% alignment. To remove “metallic” nanotubes (which cause high leakage currents and incorrect logic functionality) they sent a high voltage through the circuit to vaporise the useless, non semi-conducting nanotubes.
Next, the team designed an operating system for the new circuit to run. The computer can use this operating system to switch between two different tasks: counting and sorting.
Though much smaller and simpler than current silicon-based computers, this new computer provides a significant step in carbon computing. It could be the first of a whole new generation of faster and better electronic systems.
For more on this new carbon nanotube computer, check out the Nature article.