Earth Science: Chemistry – Inorganic Chemistry – Semiconductors
Temperature and the ability of certain materials to conduct electricity are related. This was discovered in the 19th century and roughly a century later became the basis of information technology.
Computers, cell phones, digital cameras, and medical technologies, such as computed tomography and artificial pacemakers, depend on semiconductors; these may be made of elemental materials and vary in their band structure. At 68°F (20°C) semiconductors conduct electricity better than insulators but not as well as metals.
Temperature is an important factor. For most semiconductors, the ability to conduct goes up with rising heat. When we hear the term “semiconductor,” we usually think of silicon, one of the most important semiconductors.
However, it is just one of 600 known inorganic semiconductors. Other important ones are the lll-V semiconductors, such as gallium arsenide.
The perfect crystal
Most semiconductors are close to perfect crystals. A bar of manufactured semiconductor material is melted piece by piece, then, during slow cooling, the atoms arrange themselves in a crystalline structure.
To produce a very thin layer of crystal, the gaseous semiconductor is condensed on a cold surface. Today, great importance is placed on the methods used to crystallize semiconductor materials.
Desired donor atoms
The conductivity of a semiconductor can be changed by adding impurities in a process called doping. Donor atoms are introduced into the crystalline structure by firing an ion beam into the material.
The higher the energy with which the ions collide with the semiconductor, the deeper they penetrate.
Semiconductor applications
Semiconductors are critical in microtechnology and information technology. They are used to convert light to electrical energy and electrical energy to light. Many calculators are powered by solar cells that convert light into electricity. Light-emitting diodes are used in many electronic displays.
While electrons in free atoms have energy levels, electrons in solids form energy bands. This band structure explains the properties of semiconductors. Between the bands of different energy there are gaps. In a semiconductor the highest energy level occupied with electrons is the valence band while the lowest unoccupied energy level is the conduction band.
In insulators electrons cannot cross the band gap or, if they do, they will be captured again by the huge attraction of the atomic nuclei, and electric current cannot flow. In doped semiconductors electrons build an additional narrow band that allows them to easily reach the conduction band and contribute to conductivity.
PRODUCING COMPUTER MICROCHIPS
To build microchips, semiconductors must be doped and tiny circuits built. The semiconductor is covered with a photo resistant varnish. A “mask” is then reproduced on the varnish using UV radiation or electron beams. A chemical bath dissolves the varnish in specified areas.
Acids then etch the circuits. This work must be done in clean rooms because if a dust particle settles upon the surface of a microchip circuit, the microchip will not work properly.
BASICS
AMONG CHEMICAL elements there are solid substances that conduct electricity (conductors) and those which do not (insulators).
However, with semiconductors, conductivity can be influenced by energy through heat, light, and an applied electrical current