Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide. Semiconductor devices have replaced thermionic devices in most applications. They use electronic conduction in the solid state as opposed to the vacuum state or gaseous state. Semiconductor devices are available as discrete units (such as those sold in electronics stores) or can be integrated along with a large number — often millions — of similar devices onto a single chip, called an integrated circuit (IC).
Semiconductor device fundamentals
If a semiconductor is pure and if it is unexcited by an input such as an electric field it allows only very small values of electric current to exist within itself, and it is practically an insulator. The main reason why semiconductors are so useful is that the conductivity of semiconductors can be manipulated by addition of impurities (doping), by introduction of an electric field, by exposure to light, or by other means. CCDs, for example, the primary unit of digital cameras, rely on the fact that semiconductor conductivity increases with exposure to light. Transistor operation, discussed below, depends on the fact that semiconductor conductivity can be increased by the presence of an electric field.
Current conduction in a semiconductor occurs via movable or free electrons and holes. Holes aren't real particles; in a sense which requires some knowledge of semiconductor physics to understand: a hole is the absence of an electron. Nevertheless, this absence, or hole, can be treated as a positively-charged counterpart to the negatively-charged electron. Indeed, the precise meaning of "free electrons" also requires a background in semiconductor physics to understand. For descriptive ease, "free electrons" are often simply denoted "electrons", but it should be understood that the majority of electrons in a solid, which aren't free, do not contribute to conductivity.
If a semiconductor crystal is perfectly pure, with no impurities, and it is held at a temperature near absolute zero with no excitations (e.g. electric fields or light), it will contain no free electrons and no holes, and thus will be a perfect insulator. At room temperature, thermal excitations produce some free electrons and holes in pairs, but most semiconductors at room temperature are insulators for practical purposes.
Doping a semiconductor such as silicon with impurity atoms, such as boron and phosphorus, creates unequal numbers of free electrons and holes. Very low levels of doping cause a semiconductor to conduct, yet because the number of carriers is small, the cloud of carriers can easily be swept from the volume of the semiconductor with e-fields, producing depletion zones. When a doped semiconductor contains excess holes it is called "p-type", and when it contains excess free electrons it is known as "n-type". The semiconducting material in devices is almost always carefully doped for engineering purposes. In fact, junctions between n-type and p-type semiconductors, called p-n junctions, are the fundamental elements of many semiconductor devices, such as the p-n diode and the bipolar junction transistor.
An electric field can also create a unequal number of free electrons and holes in a semiconductor. This is the basis for "field effect transistors" like the MOSFET. Exposure to light generally creates electron/hole pairs in a semiconductor, which change its conductivity and allows the light to be sensed, as with components called photocells.
Semiconductor materials
By far, silicon (Si) is the most widely used semiconductor material as of 2004. Its combination of low raw material cost, reasonable speed, relatively simple processing, and a useful temperature range make it currently the best compromise among the various competing materials. Silicon is currently fabricated into boules that are large enough to allow the production of 300 mm (12 in) wafers.
Germanium (Ge) was a widely used early semiconductor material but its lower melting point makes it less useful than silicon. Today, germanium is often alloyed with silicon for use in very-high-speed SiGe devices; IBM is a major producer of such devices.
Gallium arsenide (GaAs) is also widely used in high-speed devices but so far, it has been difficult to form large-diameter boules of this material, limiting the wafer diameter to a few hundred millimeters and making mass production of GaAs devices significantly more expensive than silicon.
Other less common materials are also in use or under investigation:
Silicon carbide (SiC) has found some application as the raw material for blue light emitting diodes (LED's) and is being investigated for use in semiconductor devices that could withstand very high operating temperatures and environments with the presence of significant levels of ionizing radiation. IMPATT diodes have also been fabricated from SiC.
Various indium compounds (indium arsenide, indium antimonide, and indium phosphide) are also being investigated as is selenium sulfide.
Source: CDX Global & Wikipedia - en.wikipedia.org
