Semiconductor
In 1833, Michael Faraday reported that the resistance of silver sulfide specimens decreases when they are heated. This observation contradicted the behavior of metallic substances like copper, which lose conductivity as temperature rises. The modern understanding of this phenomenon relies on quantum physics to explain how charge carriers move through a crystal lattice. Electrical conductivity arises from electrons occupying states that extend through the material, yet these states must be partially filled to allow transport. A state is inert if it is always occupied with an electron, blocking the passage of others via that specific path. Metals possess many partially filled states near their Fermi level, allowing high conductivity. Insulators have few such states, and their Fermi levels sit within band gaps containing no energy states to occupy. An intrinsic semiconductor has a band gap smaller than that of an insulator, allowing significant numbers of electrons to cross at room temperature.
A pure semiconductor lacks practical utility because it conducts neither well nor poorly enough for most applications. Scientists discovered that adding controlled impurities could increase electrical conductivity by factors of thousands or millions. In a one cubic centimeter sample of pure germanium at twenty degrees Celsius, there were only about ten billion free electrons and holes combined. Adding just 0.001 percent of arsenic donated an extra one hundred quadrillion free electrons in the same volume. Group III elements contain three valence electrons and function as acceptors when used to dope silicon. When an acceptor atom replaces a silicon atom in the crystal, it creates a vacant state known as an electron hole. This hole can move around the lattice and function as a charge carrier. Group V elements have five valence electrons and act as donors, creating an extra free electron when substituted for silicon. A silicon crystal doped with boron results in a p-type material, while one doped with phosphorus becomes n-type.
The history of understanding semiconductors began with experiments on the electrical properties of materials in the early nineteenth century. Willoughby Smith observed in 1873 that selenium resistors exhibit decreasing resistance when light falls upon them. Karl Ferdinand Braun developed the first semiconductor device, the crystal detector, in 1874. This device utilized conduction and rectification in metallic sulfides to process signals. In 1904, Jagadish Chandra Bose created point-contact microwave detector rectifiers made of lead sulfide. These devices became common in radio receivers but were unpredictable in operation and required manual adjustment. Alexander Graham Bell used the light-sensitive property of selenium to transmit sound over a beam of light in 1880. Charles Fritts constructed a working solar cell in 1883 using a metal plate coated with selenium and a thin layer of gold. This device became commercially useful in photographic light meters during the 1930s.
Detector and power rectifiers could not amplify a signal until researchers developed solid-state amplifiers. The first working transistor was a point-contact transistor invented by John Bardeen, Walter Houser Brattain, and William Shockley at Bell Labs in 1947. They managed to amplify signals by twenty decibels or more. Russell Ohl observed the first p-n junction in silicon around 1941 when he found a specimen that was light-sensitive. A slice cut from this specimen at the boundary between p-type impurity and n-type developed a voltage when exposed to light. Morris Tanenbaum fabricated the first silicon junction transistor at Bell Labs in 1954. Early junction transistors were relatively bulky devices that were difficult to manufacture on a mass-production basis. Herbert Mataré had observed amplification between adjacent point contacts on a germanium base during the war in France. His group announced their Transistron amplifier only shortly after Bell Labs announced the transistor.
Almost all modern electronic technology involves the use of semiconductors for integrated circuits found in desktop computers and smartphones. To create an ideal semiconducting material, chemical purity is paramount because any small imperfection can have a drastic effect on behavior. Current mass production processes use crystal ingots grown as cylinders and sliced into wafers. The Czochralski method produces these single-crystal ingots which are then cut into round wafers. Silicon wafers were first introduced in the 1940s. Thermal oxidation forms silicon dioxide on the surface of the silicon to serve as a gate insulator. Photomasks and photolithography create patterns on the circuit using ultraviolet light and a photoresist layer. Plasma etching uses chlorofluorocarbon gas pumped into a low-pressure chamber to remove uncovered parts of the silicon wafer. A high radio-frequency voltage creates plasma that hits the positively charged ions released from the cathode. This process etches the silicon anisotropically before diffusion embeds impure atoms at one thousand one hundred degrees Celsius.
The most common semiconducting materials are crystalline solids, though amorphous and liquid varieties exist. Certain pure elements found in group fourteen of the periodic table include silicon and germanium. These elements have four valence electrons allowing them to gain or lose electrons equally. Binary compounds often form between elements in groups thirteen and fifteen, such as gallium arsenide. After silicon, gallium arsenide is the second-most common semiconductor used in laser diodes and solar cells. It also appears in microwave-frequency integrated circuits and other specialized applications. High thermal conductivity semiconductors play a crucial role in electric vehicles and power modules. Semiconductors with large thermoelectric power factors make them useful in thermoelectric generators. They serve as substrates for HEMT devices and can even be treated as wide-gap semiconductors for insulating purposes. Organic semiconductors made of organic compounds and semiconducting metal-organic frameworks expand the range of available materials.
Common questions
When did Michael Faraday report that silver sulfide resistance decreases when heated?
Michael Faraday reported in 1833 that the resistance of silver sulfide specimens decreases when they are heated. This observation contradicted the behavior of metallic substances like copper which lose conductivity as temperature rises.
What year was the first working transistor invented and by whom?
The first working point-contact transistor was invented by John Bardeen, Walter Houser Brattain, and William Shockley at Bell Labs in 1947. They managed to amplify signals by twenty decibels or more using this device.
How does adding arsenic affect electrical conductivity in germanium samples?
Adding just 0.001 percent of arsenic to a one cubic centimeter sample of pure germanium at twenty degrees Celsius donates an extra one hundred quadrillion free electrons. This increases electrical conductivity by factors of thousands or millions compared to the original ten billion free electrons and holes combined.
Which elements form p-type and n-type silicon materials through doping?
A silicon crystal doped with boron results in a p-type material while one doped with phosphorus becomes n-type. Group III elements function as acceptors creating electron holes whereas Group V elements act as donors creating extra free electrons.
When did Charles Fritts construct the first working solar cell?
Charles Fritts constructed a working solar cell in 1883 using a metal plate coated with selenium and a thin layer of gold. This device became commercially useful in photographic light meters during the 1930s.