THE INTEGRATED CIRCUIT

abstract-electronics-digital-technology-blue-background_5205-32An integrated circuit ( IC , from the English integrated circuit ), in digital electronics, is a miniaturized electronic circuit where the various transistors have all been formed at the same time thanks to a single physical-chemical process. A chip (lit. “piece”) is the electronic component made up of a tiny silicon wafer plate ( die ), from which the integrated circuit is built; in practice, the chip is the support that contains the elements (active or passive) that make up the circuit. Sometimes the term chip is used to indicate the integrated circuit as a whole. The integrated circuit is used, in the form of a digital or analog logic network, for processing or processing functions of inputs expressed in the form of electrical signals, in order to obtain output data. The design of the integrated circuit is due to Jack St. Clair Kilby , who in 1958 built the first example made up of about ten elementary components, for which he won the Nobel Prize for Physics in 2000.

TYPE

Integrated circuits are mainly divided into two broad categories: analog and digital. There are types of circuits that do not fall into these two: they have particular functions, of less widespread use, such as, for example, Active Filters or sample and hold . Manufacturers group them into specialized subcategories. The analog ones are designed to process analog signals (that is, which can vary continuously over time in an arbitrary way), while the digital ones are created to deal with binary digital signals, which can only assume two different “legitimate” values. An example of a generic analog IC is the operational amplifier, while examples of digital ICs are logic gates, multiplexers and counters. Historically the first integrated circuits were digital, developed for the first computers. These ICs adopted internal type circuit diagrams RTL (from resistor Transistor Logic ), i.e. they integrated a series of resistors on the semiconductor for the internal polarizations: subsequently the resistors were replaced with diodes, obtaining schemes DTL (D iode Transistor Logic ), and about thirty years ago also the diodes were replaced with transistors, and today most of the digital integrated circuits on the market are TTL ( Transistor Transistor Logic ). There is a family called ECL ( E mitter Coupled Logic ) whose operating principle was created in 1956 in the IBM laboratories; of less widespread use than the others but still used today, it is characterized by an extremely rapid switching speed, however at the expense of the very high current consumption. Depending on the type of transistor used, the integrated circuits are then further divided into Bipolar if they use classic bipolar transistors or CMOS ( Complementary Metal Oxide Semiconductor ) if they use MOSFET transistors. In the 90s, Intel developed a new hybrid technology for its microprocessors, called BiCMOS , which allows you to use both types of transistors on the same chip.

LADDER OF INTEGRATION

The integration scale of an integrated circuit gives an indication of its complexity, roughly indicating how many transistors it contains. Based on the scale of integration, circuits can be classified into:

  • Small Scale Integration ( SSI ): Less than 10 transistors.
  • MSI ( Medium Scale Integration ): 10 to 100 transistors.
  • LSI ( Large Scale Integration ): 100 to 10000 transistors.
  • VLSI ( Very Large Scale integration ): from 10000 to 100000 transistors.
  • ULSI ( Ultra Large Scale Integration ) (not widely used): Up to 10 million transistors.

For higher numbers of transistors present, the integration is defined as WSI ( Wafer Scale Integration ), being able to contain an entire computer. Related to the scale of integration is the processing capacity of the integrated circuit, which increases with the number of transistors in accordance with Moore’s law.

REALIZATION

The starting material is a circular wafer of semiconductor, called substrate , this material, generally already lightly doped, is further doped by ion implantation or by thermal diffusion to create the active zones of the various devices (eg. p and n zones in transistors); then a series of thin layers of different materials are deposited, grown by epitaxy or thermally:

  • Layers of polycrystalline semiconductor (defined as electronic grade EGS, Electronic G rade Silicon , i.e. silicon with less than one impurity every billion atoms and therefore very pure);
  • Thin insulating layers;
  • Insulating oxide layers, much thicker;
  • Metallic layers (silicides or metals such as aluminum, tungsten or more rarely copper) for the electrical connections.

The geometry of the areas which must receive the doping and that of the various layers is imprinted on the substrate with a photolithography process: every time the integrated circuit under construction must receive a new layer or a new dopant implantation, it is covered with a thin film photosensitive, which is impressed through a very high definition photographic negative (called “mask” or “layout”). The illuminated areas of the film become soluble and are removed by washing, leaving the underlying chip uncovered, ready for the next processing step, selective removal or doping of the areas without the photosensitive film. Once the creation of the chips on the substrate is finished, they are tested, the substrate is cut and the chips encapsulated in the packages with which they will be mounted on the printed circuits through connections or pins called pins.

COMPONENTS THAT CAN BE INTEGRATED

Transistors and diodes can be easily integrated in an integrated circuit: it is also possible to create small resistors and capacitors in the semiconductor substrate, but generally these last components take up a lot of space on the chip and we tend to avoid their use, replacing them when possible with networks of transistor. It is also possible to integrate inductors, but the value of the obtainable inductances is very small (in the order of nano henries (nH)): their use is very limited due to the enormous occupation of the area that they also require, even if only for make inductors of very small value. Furthermore, the manufacturing technology of the integrated circuits (not dedicated to very high frequencies) and therefore the considerable parasitic effects significantly limit their performance, above all when compared to the classic inductors not on integrated circuits. Such integrated inductors are usually employed in radio frequency integrated circuits (LNAs , mixer, etc.), for example at frequencies around gigahertz (GHz) . Medium and large capacity capacitors cannot be integrated at all. On the other hand, there are various types of integrated circuits having the relay function, i.e. devices equipped with logic inputs, by means of which to interrupt or divert even multiple analog signals.