Semiconductors

Semiconductors

Semiconductors are a unique class of materials in which the number of charge carriers, the particles that move electric charge, is intermediate between those of insulators and those of conductors. A conductor is a material that contains a large number of free electrons that are able to move freely when a voltage is applied. An insulator is a material that has very few free electrons and hence is a poor conductor of electricity. There are two main types of semiconductors: intrinsic and extrinsic semiconductors. The conducting power of intrinsic semiconductors is greatly influenced by temperature and sometimes light making them particularly interesting.

Although there are some materials that are semiconductors in their pure state, they are virtually never used as such in electronic devices because the only charge carriers present are those produced by thermal breakdown of the covalent bonds within the material. Two of the most common pure semiconductor materials are germanium and silicon. Each of these materials has atoms with four valence electrons that are shared with the valence electrons of its four nearest neighbors in the solid state. As the temperature is raised the atoms start to vibrate and eventually some electrons have sufficient energy to break free of their neighboring atoms and move freely through the material. These free electrons participate in the conduction process if a voltage is applied across the material. For pure semiconductors as the temperature is increased the resistance decreases and so raising the temperature increases the conducting power of these materials. These types of semiconductor materials are called intrinsic semiconductors.

The other type of semiconductor is one in which the charge carriers are created by an impurity. These types of semiconductors are known as extrinsic semiconductors. The addition of impurities, such as arsenic or indium, has a significant influence on the conductive properties of semiconductors. The addition of such impurities is known as doping. Arsenic has five valence electrons and so when it is incorporated into silicon it shares electrons with four of its nearest neighboring silicon atoms but has one electron left unshared. This spare electron is loosely bound and so can move freely from the atom and participate in conduction. This type of impurity contributes an electron to the overall material and is known as a donor atom. A semiconductor of this type is known as an n-type semiconductor because most of the charge carriers are negative electrons. If the dopant is an atom with three valence electrons, such as indium, the three electrons form bonds with the three neighboring atoms. In this case there is a neighbor with an electron deficiency that is referred to as a hole. Electrons of neighboring bound atoms can jump over to the hole if an electric field is applied resulting in migration of the hole in effect. The impurity atoms added have in effect donated a hole and are called acceptors. The hole will migrate in the direction of the electric field as if it were a positive charge carrier. These types of semiconductor materials are known as p-type semiconductors since the charge carriers are positive holes.

Most devices incorporating semiconductors are fabricated using a combination of p-type and n-type semiconductor materials. The boundary between the two regions--the p-region and n-region--is called the p-n junction. In this region there are no free charge carriers since the ele lectrons and holes that move through it recombine in the area around the junction. In a diode of this type there is a delay in the decay to the normal value of reverse current because of the setup of the semiconductor. This delay is a serious disadvantage for operation at microwave frequencies or in high-speed switching. For high-speed applications a Schottky diode is used. This type of diode uses a metal-semiconductor contact instead of a p-n junction and so gives a superior reverse recovery.

Diodes that are composed of a combination of semiconductor materials, with p-regions and n-regions, are good conductors when a voltage is applied so that a current is driven in one direction but a poor conductor if the voltage is reversed. For example, if a battery is connected to such a device so that the positive end is connected to the p side of the device and the negative end is connected to the n side of the device, the situation is referred to as a forward-biased junction. The charges cross the p-n junction with ease, since there is an accelerating field setup, with electrons driven towards the positive end of the battery and positive holes driven towards the negative end of the battery, that allows them to overcome the opposing electric field. If the battery is connected in the opposite manner, with the polarity reversed, then the junction is said to be reverse-biased. The electric field at the junction is enhanced and very few charges are able to cross the junction because they combine with one another, resulting in a very small current. These types of combination semiconductor devices are very efficient at transferring electrical charge in one direction relative to the opposite direction and are, as such, widely used in commercial applications.

The transistor is a special semiconductor device discovered by John Bardeen, Walter Brattain (1902-1987), and William Shockley(1910-1989) in 1948. Their discovery revolutionized the world of electronics and the three were awarded the Nobel Prize in physics in 1956. There are two types of transistors: p-n-p transistors and n-p-n transistors. The p-n-p transistor consists of a semiconducting material with a very narrow n region sandwiched between two p regions. The n-p-n transistor consists of a p region sandwiched between two n regions. The operation of these two types of transistors is virtually identical. These types of devices are commonly used to amplify small, time dependent signals.

The most common semiconductor materials used today are combinations of group III (aluminum, gallium, indium) and group V (phosphorus, arsenic, antimony) elements. The most popular used in devices are gallium arsenide, indium phosphide, silicon carbide, and gallium nitride. The starting semiconductor materials are grown in boules (pear-shaped synthetically formed masses with the atomic structure of a single crystal) that are sliced and polished to form the thin substrates on which the electronic or photonic capabilities are etched or grown. Several novel techniques, such as photolithography, are employed to make the small chips. Eventually the individual devices are separated and packaged for electronic and photonic devices such as cell phones, lasers, computers, and LEDs. Compound semiconductor devices can perform highly specialized functions and are considered the wave of the future.