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These are driven by integrated circuits, flat panel displays, Light Emitting Diodes and solar cells. These are all based on semiconductors.

A semiconductor is a material whose conductivity can be manipulated. Materials consist of atoms  with a very small nucleus surrounded by electrons in a series of discrete energy levels. Semiconductors have a unique electronic structure, in which the atoms have  electrons that are tightly bound to each nucleus and have a low conductivity. The electrons can only have select energies and those in the highest energy level are in the "ground state".  In semiconductors, above the ground state there is  an energy "band gap". If the electrons can jump over the band gap to the conduction band, where they are not tightly bound in atoms,  they will have a high conductivity. Semiconductors have a band gap that is larger than than the thermal energy of the environment, but small enough to be manipulated by voltages and photons from the sun. 

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Semiconductors have a unique place in the periodic table; pure Group 4 elements (Silicon), or compounds of Group 3 and 5 (such as GaN). The periodic table is organized based on the electronic structure of the elements, so it is not surprising that electrical properties follow  the periodic table. 

The electrical properties of semiconductors can be modified by "doping". If a Group 5 element replaces some of the  Group 4 atoms in a Group4 crystal lattice, the material has a extra unbonded electrons and conducts electricity. Likewise, if a Group 3 element is used, there is a shortage of electrons and their is a spare location for the electrons to move into and conductivity increases. The spot where the electron is missing is called a "hole".



Device Types 


Diodes - LED, Camera, Solar cells

A diode is a device that passes current in one direction and blocks in the other. It consists of a piece of semiconductor with one half doped so that it has excess electrons, the other so that holes. A voltage can force electrons in the electron rich side to move into the side with holes, however a voltage in the opposite direction cannot pass current. 

A photo diode is an example of the diode using the internal photo-electric effect. Understanding the photo-electric effect won Einstien his Nobel Prize, as he showed that when light is shined on a material electrons are emitted (external photoelectron effect) with energy that are dependent on wavelength not intensity - proof of the quantum properties of light. 


A solar cell uses the internal photoelectric effect uses a photon of energy equal or greater than the band gap that is absorbed and lifts a electron into the conduction band and creates a flow of current. A camera uses the same effect, except that the electrons are stored in a memory cell and then read out. 

An LED is the opposite of a solar cell, it  uses a voltage across the material to pass current, lifting the electrons into the conduction band, and then letting them  relax back across the band gap emitting a light photon with a wavelength corresponding to the band gap.  Quantum wells are used to prevent the exited electrons from loosing energy by collisions. Quantum wells are very thin layers around 10 atoms (3 nm) think  with different band energy. 

Transistors IC, FPD

Transistors use an external voltage in a capacitor to reduce the band gap and induce conductivity in a switch or amplifier.

The transistors are the universal building block of modern  electronics; arranged into storage cells in memories, as linked switching logic to execute mathematical operations, or as analog devices in amplifiers or radios. These can consist of billions of transistors. Flat panel displays use a structure that looks just like a memory cell in each display pixel, and can  consist of several million transistors.  

How they are made

The commonalities between these 3 devices means that the technical developments feed off each other. 

The remarkable developments in electronics is primarily a economic story. IC's, FPD, Solar cells and LED's are all manufactured on ever larger substrates at high speed, and that is what has driven costs down and/or capability up. 

All these devices are built up like a patterned layer cake. Each layer is patterned by a sequence of creating the layer, patterning a photographic resist material, transfer of the pattern into the layer, and removing the resist. An LED or Solar cell is simple 2-3 layers, a FPD transistor is 5 layers, a IC can be 30 or more layers!

The most expensive process in the photography step, creating original features 100 atoms wide over huge areas. Molecular Imprints, the start up that drew me to Austin was an alternative patterning process. 

The formation of the semiconductor is the device unique step. In many ways the simplest is the IC, it is a perfect single crystal of silicon. It is grown as a cylinder of the correct diameter, and then  sliced and polished into wafers that are polished flat. By perfect, I mean NOT a single atom out place ! 

Solar cells are similar except they can tolerate some defects and do not need to be flat so they can be much cheaper. 

LED's requires unique materials with specific  band gaps. These are all III/V materials, which are very expensive to grow. As a result they start with a cheaper material that has a crystal lattice that is close to the target material, sapphire is the most popular. The III/V material is grown as a crystal on this "hetero" lattice. 

Displays have the unique challenge that they must be meters on a side, and transparent. The solution is to start with a huge piece of glass and deposit silicon on the glass. Because the glass is amorphous,  there is no reference lattice  and the silicon is formed as lots of small crystals "polysilicon", or with no crystals "amorphous". The device design must be tolerant of the much poorer electrical properties compared to single crystal silicon. 

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