Semiconductors and Transistors Fundamentals

Semiconductors Overview

Semiconductors are materials with electrical properties somewhere between those of metals and insulators—they can act like either depending on how they're treated. A semiconductor is typically made from silicon, germanium, gallium arsenide, indium phosphide, or another material that has some electrical conductivity when exposed to light, heat, pressure, magnetic fields, mechanical forces, or electric fields. However, unlike most good conductors, these materials have a very low electrical conductivity unless something else is added to them.

Band Gap

The behavior of semiconductors depends heavily on their band gaps. This refers to the energy difference between two levels of electronic states within the solid crystal lattice. Below this energy level, electrons don't move around much; above it, they can move easily through the substance, carrying electricity. At room temperature, only one out of every million trillion atoms will have enough extra energy to jump over its band gap into the conduction region where it moves freely. Normal conductive materials like copper and aluminum never have any such jumps because their band gaps are too small.

Conductivity

In pure semiconductors, there aren't many charge carriers available in the upper valence bands. They need to be put into the lower energy regions by adding a little bit of impurity. So you might think of a pure semiconductor as insulating, but with just a tiny amount of impurity, it becomes highly conducting.

Doping

Adding carefully chosen amounts of certain other elements, known as 'doping', creates more charge carriers inside the semiconductor, so they become better conductors. For example, adding boron makes silicon conductive, while phosphorus does the opposite thing. Each atom shares its four outermost electrons equally with its neighbors, so the outer shells of each set of four share eight electrons altogether, making the lattice neutral overall. But if we introduce one kind of single atom of one type, instead of sharing all six, it leaves a hole behind itself, which acts like a positive charge carrier. These holes were first found in mechanically stressed silicon crystals in 1947. When you make a piece of pure silicon slightly negatively charged, these positive holes move towards it—and if the charge goes away, the positive charges are left behind, creating a current across the whole body.

Doped silicon looks almost identical to undoped silicon under normal magnification, except under strong electron microscopes, you see that the donor atoms form clusters of their own. Silicon is heavily used, especially as silicon dioxide in computer chips because it doesn't oxidize well. In contrast, aluminum oxides form thick layers of alkali metal salts within themselves due to diffusion, interfering with high voltage operation. Aluminum gets used mainly in power cables and transformers nowadays.

Transistors

A transistor uses three different kinds of semiconductor junctions to control the flow of electric current, allowing amplified signals to pass along wires without being distorted. One type of transistor, called a bipolar junction transistor, consists of a p–n–p sandwich in which the middle layer is thin enough so that the p layer controls the n layer, the base, and determines whether the collector layer, connected to the output terminal, carries any current or none at all. Another type, called a field effect transistor, operates similarly but with an additional gate electrode in series with the usual input wire, so that the input signal current can turn the switch off completely, breaking the circuit completely, whereas the collector still receives no current even though the emitter sends a constant stream of charge carriers up to the base layer. There is also a third type of transistor, called a unijunction transistor or trigger, in which there are only two types of cells: one that always accepts current flowing downward and another that always accepts current flowing upward. It was discovered accidentally during World War II when scientists noticed that germanium could pass larger quantities of radiation through it than ordinary glass.