Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium and gallium arsenide. This class of device has replaced thermionic devices in most applications. They utilize electric conduction in the solid state, as opposed to the vacuum state or gaseous state.
Conduction in a semiconductor device relies on impurities in the crystal structure of a semiconductor material to create either an excess of free electrons, or a deficiency in free electrons. The missing electrons create so called holes in the lattice structure. These holes react to changes in the electrical field in the same way a free electron with a positive charge would.
Negative-type or N-type semiconductors have an excess of free electrons, and positive-type or P-type semiconductors are materials containing holes.
Conduction is usually via "majority carriers" (electrons in N material, and holes in P material), but many devices such as transistors rely also on "minority carriers" (holes in N material or electrons in P material) in order to operate.
Most semiconductor devices use crystalline silicon as the semiconductor material. Other semiconductor materials in use include germanium, gallium-arsenide (GaAs), and gallium-arsenide-phospide (GaAsP). The semiconductor effect of rectification (passing current in one direction better than in the other direction) was originally discovered in galena crystals; such crystals, embedded in a lead carrier, were used for early radio receivers in conjunction with a fine wire called a "cat's whisker" which barely contacted the surface of the galena crystal.
Some of the great many devices in current usage that utilize the solid-state include: diodes (rectifiers), transistors (originally an abbreviation of "transit resistor"), field-effect transistors (FET), light-emitting diodes (LED), photodiodes, silicon controlled rectifiers (SCR), triacs, diacs and unijunction transistors (UJT).
Semiconductor devices are available as discrete units, or can be integrated along with a large number of similar devices onto a single die, called an integrated circuit (IC).
Integrated circuits incorporate many small semiconductor devices on one small chip of material.