INTEGRATED CIRCUIT
The circuit in which various active and passive components such as transistors, diodes, capacitors, resistors etc. are the part of a small piece of semiconductor (silicon) chip is called integrated circuit.
An integrated circuit comprises of various components like resistors, diodes, transistors, capacitors etc., and their inter-connections. All the components and their inter connections are formed on a very small piece of semiconductor wafer called chip.
On the surface of this small chip, it is possible to produce circuits containing many transistors, diodes, resistors and capacitors. Thus, the components of ICs are very small and one needs a microscope to see the interconnections of these components.
The following points are worth noting about integrated circuits :
(i) No component of an integrated circuit can be removed or replaced since all the components are the integral part of a small semiconductor chip.
(ii) No component of an integrated circuit is projected above the surface of the chip since they are formed within the chip.
(iii) The size of an IC is extremely small and one needs a microscope to see the connections between the components.
CLASSIFICATIONS OF ICs
The integrated circuits may be classified as :
1. According to their construction: From construction (manufacturing) point of view the integrated circuits may be classified as ;
(i)Monolithic ICs : In this type, all the components are formed as part of a single p-type wafer.
(ii) Thin film ICs : In this type, the substrate is ceramic or glass (i.e., an insulator). All the components required for the circuit are evaporated on this insulating platform.
(iii) Thick film ICs : In this type, resistors (R) and capacitors (C) are formed on the substrate, but the transistors are added as discrete.
(iv) Hybrid ICs : In this type, the monolithic and thin film units are combined on a single ceramic platform. Discrete transistors may also be included where high power rating is needed.
2. According to their function: In terms of their function, IC chips are broadly classified as:
(i) Digital ICs : These ICs process digital signals and are basically pulse circuits. A digital signal is that signal in which only two discrete voltage levels are used, these levels are sometimes referred to as logical 0 and sometimes as logical 1. The digital IC units may be further classified according to the complexity of their circuitry such as :
(a) VLSI Chips : A very-large-scale integration (VLSI) chip is that chip which contains circuitry equal to 1000 or more logic gates or integrated components.
(b) LSI Chips : The large-scale integration (LSI) chip is that chip which contains circuitry equal to 100 to 1000 logic gates or integrated components.
(c) MSI Chips : The medium-scale integration (MSI) chip is that chip which contains circuitry equal to 12 to 100 logic gates or integrated components.
(d) SSI Chips : The small-scale integration (SSI) chip is that chip which contains circuitry equal to less than 12 logic gates or integrated components.
In all the above cases, an extension of silicon monolithic IC which places a large number of identical circuits on a single silicon chip and interconnects circuits on the same chip so that the total entity performs a system or a subsystem function.
(ii) Linear ICs : These ICs process analog signals. An analog signal is that signal which varies continuously in proportion to its input. The linear IC chips generally contain several amplifier circuits for either audio or RF signals. The output of a linear amplifier varies in direct proportion to its input.
BASIC STRUCTURE OF A MONOLITHIC IC
The word monolithic is taken from the Greek language, where mono, meaning 'single' and lithos, meaning 'stone'. Thus monolithic means single stone or one stone. This word suits because in monolithie ICs all tbe components are fabricated and interconnected on the same (single) wafer.
Thus, a monolithie IC is one in which all circuit components are fabricated and interconnected on a single thin wafer called 'substrate'.
Although our main interest is in using ICs in various electronic circuits, yet it is profitable to know some thing about the basic structure of these ICs which is discussed below :
p-Substrate
The first step in making a monolithic IC is to prepare a wafer of p-type semiconductor. For this a cylindrical p-type silicon crystal is grown having typical dimensions-5 cm long 2.5 cm diameter.
The crystal is then cut by a diamond saw into many slices called wafer. Each wafer has a typical thickness of 200 µm. These wafers are lopped and polished to a mirror finish on one side for succeeding operations. This lighly doped p-type wafer provides the base on which the transistors and other components are built and is called substrate.
Epitaxial n-layer
An n-type epitaxial layer, typically 20 µm thick, is grown onto the p-type substrate. For growing epitaxial layer, the wafers are put in a diffusion furnace and a gas mixture of silicon atoms and pentavalent atoms is passed over the wafers which forms a thin layer of n-type semiconductor on the heated surface of substrate. The resistivity of this n-type layer varies from 0.1 to 0.5 ohm-cm. All the active and passive components are fabricated within this layer using a number of diffusion steps. In the fabrication of all the components like resistor, diode, transistor etc., it is necessary to diffuse impurities in certain precisely defined regions within this layer.
Insulating layer
A thin layer (about 1 µm) of silicon dioxide SiO₂ (insulating material) is deposited over the entire surface of epitaxial layer. This is achieved by passing pure oxygen over the epitaxial layer which when combines with silicon atoms foms a layer of silicon dioxide (SiO₂). This thin glass insulating layer protects the silicon surface against contamination. This also acts as a barrier in the selective diffusion of impurities in epitaxial layer and protects portions of the wafer against impurity penetration.
Etching
In the selected regions where diffusion is o take place, the silicon dioxide (SiO₂) layer is removed windows are provided leaving the rest of the wafer protected against diffusion. The process of removal of SiO₂ layer is called etching and is achieved by photochemical techniques.
Producing components
Atter etching the SiO₂ layer, by diffusion various junctions or components like diodes, transistors, re sistors and capacitors are formed on the wafer.
Metallic paths
Metallic paths are needed to interconnect the various fabricated components of the IC chip. These are essentially aluminium printed wires, These paths are terminated at the edges of the chip. Where tiny wires are bonded for connection to the external leads or the IC pins.
Chips
An IC chip is bounded to its mounting and connections are made between the printed paths of an IC and the pins. Actually an IC Chip is just a small part of the wafer. The wafer is divided into a large number of areas (625) and each area is an IC chip. The manufacturer produces hundreds of alike ICs on the wafers over each area. Then all the ICs chips) are separated by the process similar to glass cutting. This simultaneous mass production is the ra6son for the low cost of integrated circuits.
ADVANTAGES AND LIMITATIONS OF INTERGRATED CIRCUITS
The advantages and limitations of integrated circuits in comparison to discrete circuits are given below :
Advantages :
(i) Extremely small size, since various circuit components are fabricated on a single chip.
(ii) High reliability because the various components are fabricated and interconnected on the same chip and there are no soldered joints.
(iii) Low cost because large number of chips are processed simutaneously.
(iv) Lesser weight due to miniaturised circuit.
(v) Ability to operate at extreme values of temperature.
(vi) Low power consumption.
(vii) Improved performance; because of tbe low cost, more complex circuitry may be used to obtain better functional characterstics.
(viii) Close matching of components is possible which further improves the performance.
(ix) There is no interconnection error.
Limitations :
(i) In case, any component in an IC goes out of order, the whole IC chip has to be replaced by the new one.
(ii) The maximum value of intergrated capacitors is limited to 30 pF. It is neither convenient nor economical to fabricate capacitors exceeding this value. Therefore, in the circuits where capacitors of higher capacitance are required, they are connected exterior to IC chip.
(iii) The maximum value of resistors is also limited because of the size of semiconductor chip.
(iv) The values of resistances and capacitances are not fixed, they often depend upon voltage.
(v) It is not possible to fabricate inductors and transformers on the surface of chip. Therefore, in the circuit where these components are required, they are connected exterior to the semiconductor chip.
(vi) It is not possible to produce high power (more than 10 watts) ICs.
(vii) Power dissipation is limited.
APPLICATIONS OF ICs
Integrated circuits (Cs) have large number of application. However, some of the very important applications are given below:
(i) In solid-state transistor radio sets as audio and r-f transistor amplifiers.
(ii) In solid-state television sets as audio and r-f amplifiers.
(iii) In digital electronic devices like clocks, calculators, measuring instruments, video games etc.
(iv) In computers.
(v) In control circuits etc.
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