Many materials conduct electricity while others resist its flow. Silver, copper, and aluminum are famous conductors, but in fact, all metals conduct electricity and heat to a degree and most elements are metals. If a charge goes in one end of something metal, it generally pushes along through. Another group of materials are insulators in most circumstances. These include paper and glass as well as the plastics that wrap wires. They do not allow the flow of electricity.
Conductivity varies though. Silver conducts better than copper, which conducts better than aluminum. The ability to insulate varies too. The plastic coating on home wiring would be inadequate on a high-line pole. Conductivity also varies with circumstance.
The conductivity of semiconductors is variable in a valuable way. These materials have conductivity that is precisely controllable.
Semiconductors are materials, not necessarily elements, though the metalloid element silicon is commonly used in semiconductors. Semiconductor materials besides silicon include germanium, gallium arsenide, and silicon germanium, among many others.
Their conductivity is made selectively variable by adding or subtracting electrons to areas of the base material. Doping, impregnating materials with ions or sometimes diffusing impurities into regions of the semiconductor, is a way of adding or subtracting electrons to or from areas marked off with photolithography. This creates variable conductivity that is exploited to form the basis of modern electronic devices.
The variable properties of semiconductors produce transistors, diodes, photovoltaic cells, and integrated circuits.
A transistor is a switch, and/or an amplifier. Within its structure is a sort of gate at which a small amount of current can control a larger amount of current by permitting or impeding its flow. A transistor is small, efficient, fast, cheap, and hard to break. It is the basis of modern electronics, and possibly the greatest scientific discovery of the twentieth century.
A diode made of semiconductor material permits electricity to pass in only one direction. It can convert alternating current, in which electrons alter their direction, to direct current, in which they don’t. Diodes can also perform more complicated functions like tuning, regulating voltage, and producing light in a light emitting diode.
A photovoltaic cell changes the energy of sunlight to electrical energy. According to an article on photovoltaics by Gil Knier at NASA Science, these cells are made of silicon or similar materials. In his example, a semiconductor wafer is doped to have a positive charge on one side and a negative charge on the other. When photons, light energy, strike the cell, they knock electrons loose. If the wafer is connected in a complete circuit, the electricity created by the loose electrons can be harnessed to do work.
An integrated circuit is an electric circuit, much like one in a house, which has been miniaturized to an extraordinary degree and integrated into a small bit of semiconductor material. Like a circuit in a house, it connects parts with different functions, just as the circuit in a house may connect a toaster, a television, and a computer.
Modern phones, solar chargers, computers and even automobiles now depend upon semiconductors. Yet the exploration of the properties of semiconductors is only beginning. Scientists and engineers are working on organic semiconductors, and on semiconductor devices that are ever smaller, faster, and more cool.
Principles of Semiconductor Devices, Bart Van Zeghbroeck
Electrical Engineering Training Series: Semiconductor Diodes
NASA: How do Photovoltaics Work? By Gil Knier