Semiconductor Devices
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 Semiconductor physics, one of the largest branches of solid state physics, studies both the fundamental principles of nature and the complexity of its systems. Semiconductors reflect the enormous diversity in phenomena and complexity in nature more vividly than most other physical systems. Clearly, semiconductors form the heart of modern technology, as every chip and every laser in any high tech gadget is made out of them. Without semiconductors we would have no compact discs, no computers, no walkmen, no gameboys, no internet, no space travel, and no mobile phones. What renders semiconductors really unique and fascinating, however, is their incredible richness in phenomena, and the enormous range of their physical parameters. 

In addition to their commercial significance, advanced semiconductor devices are also a wonderful playground for testing quantum physics. In these systems we can directly observe quantum mechanical tunnelling, quantum interference of electrons, quantisation of angular momentum - just by measuring the resistance of a solid state device. Where else is it possible to study atoms in the extreme conditions that only occur naturally in neutron stars? Where can we study the break up of the electron (a fundamental particle) into free quarks ?

In a MOSFET the electrons that carry current between the source and drain are confined to move only in a thin, two-dimensional, sheet. Shortly after the invention of the MOSFET it was realised that it might be possible to detect effects of the two-dimensional nature of the electrons just by measuring the sample resistance. In 1980, while investigating the low temperature properties of a two-dimensional electron system formed in a high quality silicon transistor, von Klitzing, Dorda and Pepper made a completely unexpected discovery -- they found that instead of varying smoothly with magnetic field, the low temperature Hall resistance of a two-dimensional system increases in sharp steps. Most importantly they showed that the Hall resistance is quantised in terms of two fundamental constants, the electron charge e and Plank's constant h. This quantisation is accurate to one part in 100 million, independent both of the sample and even of the material system in which the electrons are situated, and is now used as the international standard definition of resistance. In 1985 the Nobel prize for physics was awarded to von Klitzing for the discovery of the quantum Hall effect.

Just as the theorists were beginning to understand why this small-scale quantum effect should produce such accurate quantisation effects in macroscopic samples new experiments using a very high quality to the electron systems formed in a gallium arsenide quantum well showed a fractional quantisation. It appeared as if current in these new samples was being carried not by ordinary electrons, but by new quasiparticles that carry a fraction of an electron charge. The discovery of this fractional quantum Hall effect, and the theoretical understanding of fractionally charged quasiparticles that form this new quantum fluid, was rewarded with the 1998 Nobel prize for physics.

Both of these discoveries were made possible by advances in semiconductor device fabrication techniques. Using ultra high vacuum molecular beam epitaxy it is possible to grow extremely pure single crystal semiconductor structures one atomic layer at a time. By growing difference semiconductors on top of each other it is possible to create the atomic scale sandwiches, such as used in the quantum well laser. This ability to artificially engineer the band structure of a device has made MBE an invaluable tool for explorations of quantum electronic devices.

Advanced semiconductor nanofabrication techniques make it possible to further constrain the electrons to only travel in a one-dimensional quantum wire, or to trap them in zero dimensional quantum boxes. This allows us to form a whole new world in the laboratory - artificial atoms with the shape of a square box, artificial molecules a thousand times larger than natural ones so we can inspect their properties easily, long wires with just a single atom's diameter, and nanosized machines that may one day travel in our blood vessels.


First Integrated Circuit
The first integrated circuit, made by Texas Instruments, contained a grand total of six components. Today's Pentium-III processor contains over 13 million transistors.


The MOSFET
The Metal-Oxide-Silicon Field Effect Transistor forms the basis of the modern semiconductor industry. However we still don't fully understand the fundamental properties of semiconductor devices such as the MOSFET.



Integer Quantum Hall Effect
When electrons (or holes) are confined to a 2D sheet, the Hall resistance no longer increases smoothly with magnetic field B, but exhibits a series of plateaus at low temperatures.


Fractional Quantum Hall Effect
In very high quality 2D systems, the Hall resistance becomes fractionally quantised electrons (or holes) are confined to a 2D sheet, the Hall resistance no longer increases smoothly with magnetic field B, but exhibits a series of plateaus at low temperatures.


MBE crystal growth system
Using Molecular Beam Epitaxy it is possible to grow extremely pure single crystal semiconductor structures with atomic precision. moreover different semiconductors can be grown on top of each other, allowing band structure engineering.