Transistors, semiconductor chips

Because of the unique property of some of its compounds, gallium is able to translate a mechanical motion into electrical impulses. This makes it invaluable for manufacturing transistors, computer chips, semiconductors, and rectifiers. 

The elements Si and Ge of group 14 act as semiconductors. A semiconductor is an element that can, to some extent, conduct electricity and heat, meaning it has the properties of both metal and nonmetals. The abihty of semiconductors to transmit variable electrical currents can be enhanced by controlling the type and amount of impurities. This is what makes them act as on-ofF circuits to control electrical impulses. This property is valuable in the electronics industry for the production of transistors, computer chips, integrated circuits, and so on. In other words, how well a semiconductor conducts electricity is not entirely dependent on the pure element itself, but also depends on the degree of its impurities and how they are controlled. 

The silicon used for making solid-state semiconductor devices such as transistors, computer chips, and solar cells must be ultrapure, with impurities at a level of less than 10 7% (1 ppb). For electronic applications, silicon is purified by converting it to SiCl4, a volatile liquid (bp 58°C) that can be separated from impurities by fractional distillation and then converted back to elemental silicon by reduction with hydrogen ... 

In 1960, the construction of the functional laser by American physicist and Nobel laureate Arthur Schawlow began the next phase in the development of semiconductor electronics, as the assembly of transistors on silicon substrates was still a tedious endeavor that greatly limited the size of transistor structures that could be constructed. As lasers became more powerful and more easily controlled, they were applied to the task of surface etching, an advance that has produced ever smaller transistor structures. This development has required ever more refined methods of producing silicon crystals from which thin wafers can be cut for the production of silicon semiconductor chips, the primary effort of electronic materials production (though by no means the most important). 

It would not be an overstatement to claim that virtually every manufacturing company, every service organization, and most homes in the United States use electronic equipment containing transistor-loaded semiconductor chips. Research in nanoscience and nanotechnology will continue to create a culture and a society even more heavily dependent on the availability of advanced electronics. 

The size of active transistors on semiconductor chips has become progressively smaller with time scaling according to Moore s Law, which states that the number of transistors/unit area on a chip will roughly double every 18 months. Devices with 130 nm minimum dimensions are already in production while odiers with critical dimensions below 100 nm (e.g., 90 nm) have been produced in a manu cturing environment Figure 1 shows a visuid conq)arison of these dimensions with those of some common familiar objects. 

Integrated circuits (IC s) are circuits in which bipolar transistors, field-effect transistors (FET), resistors, capacitors, and their required connections are combined on a single chip of semiconductor material which is usually made of single-crystal silicon. 

Silicon s atomic structure makes it an extremely important semiconductor. Highly purified silicon, doped with such elements as boron, phosphorus, and arsenic, is the basic material used in computer chips, transistors, sUicon diodes, and various other electronic circuits and electrical-current switching devices. Silicon of lesser purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, and bronze. 

A small amount of boron is added as a dope to silicon transistor chips to facilitate or impede the flow of current over the chip. Boron has just three valence electrons sihcon atoms have four. This dearth of one electron in boron s outer shell allows it to act as a positive hole in the silicon chip that can be filled or left vacant, thus acting as a type of switch in transistors. Many of today s electronic devices depend on these types of doped-sihcon semiconductors and transistors. 

Once germanium is recovered and formed into blocks, it is further refined by the manufacturer of semiconductors. It is melted, and the small amounts of impurities such as arsenic, gallium, or antimony, are added. They act as either electron donors or acceptors that are infused (doped) into the mix. Then small amounts of the molten material are removed and used to grow crystals of germanium that are formed into semiconducting transistors on a germanium chip. The device can now carry variable amounts of electricity because it can act as both an insulator and a conductor of electrons, which is the basis of modern computers. 

By far, the most common use for germanium is in the semiconductor and electronics industries. As a semiconductor, germanium can be used to make transistors, diodes, and numerous types of computer chips. It was the first element that could be designed to act as different types of semiconductors for a variety of applications just by adding variable amounts of impurities (doping) to the germanium crystals. 

The first solid-state transistor was made not from silicon but from the element below it in the Periodic Table germanium. This substance is also a semiconductor, and can be doped in the same way. William Shockley, Walter Brattain, and John Bardeen devised the germanium transistor at Bell Telephone Laboratories in New Jersey in 1947. It was a crude and clunky device (Fig. VJa) - bigger than a single one of today s silicon chips, which can house millions of miniaturized transistors, diodes, and other components (Fig. Vjb). The three inventors shared the Nobel Prize in physics in 1956. 

The first prototype transistor (a point-contact semiconductor amplifier ), built by Bardeen and Brattain at Bell Laboratories in 1947 (a), is a far cry from today s silicon chips, packed with miniaturized semiconductor components (6)... 

If we place n- and p-type semiconducting crystals in contact (a p-n junction), we create a device that conducts electricity preferentially in one direction this is the basis of action of the semiconductor diodes used in the electronics industry, although specially refined silicon (Section 17.8.2) is usually employed rather than Ge. Transistors and electronic chips are designed using similar basic principles—typically with n-p-n or p-n-p junctions. We consider chemical aspects of electronic devices in more detail in Chapter 19. 

Doped semiconductors are essential components in the modern solid-state electronic devices found in radios, television sets, pocket calculators, and computers. Devices such as transistors, which control electrical signals in these products, are made from M-type and p-type semiconductors. In modern integrated circuits, an amazing number of extremely small devices can be packed into a small space, thus decreasing the size and increasing the speed of electrical equipment. For example, computer microprocessors now contain up to 42 million transistors on a silicon chip with a surface area of about 2 cm2 and are able to execute as many as 1.5 billion instructions per second. 

Semiconductor (transistor) biosensors are widely used. They possess a several process advantages over the others small size, good reproducibility and high sensitivity, multipurpose chip design, accessibility and low price. 

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