An integrated circuit (IC) consists of several
interconnected transistors, resistors, capacitors etc., all contained in one
small package with external connecting terminals. The circuit may be entirely
self-contained, requiring only input and output connections and supply voltage
to function. Alternatively, a few external components may have to be connected
to make the circuit operative.
On the basis of fabrication techniques used, the ICs can be divided into following three classes.
1.Monolithic IC’s
The word ‘monolithic’
is derived from the Greek monos, meaning ’single’ and lithos, meaning
‘stone’.Thus monolithic circuit is built into a single stone or single
crystal i.e. in monolithic ICs, all circuit components, (both active and
passive) and their interconnections are formed into or on the top of a
single chip of silicon. This type of technology is ideal for manufacturing
identical ICs in large quantities and, therefore, provides lowest per unit
cost and highest order of reliability. Monolithic ICs are by far the most
common type of ICs used in practice, because of mass production , lower cost
and higher reliability.
Monolithic IC
Since their invention, manufacturers have been manufacturing
monolithic ICs to carry out all types of functions. Commercially available ICs
of this type can be used as ampli¬fiers, voltage regulators, crowbars, AM
receivers, TV circuits, and computer circuits. However, the monolithic circuits
have the following limitations or drawbacks:
• Low power rating. Since monolithic ICs are of about the size
of a discrete small-signal transistor, they typically have a maximum power
rating of less than 1 watt. This limits their use to low-power applications.
• Poorer isolation between components.
• No possibility of fabrication of inductors.
• Small range of values of passive components used in the ICs.
• Lack of flexibility in circuit design as for making any
variation in the circuit, a new set of masks is required.
2.Thin and Thick Film ICs.
These devices are
larger than monolithic ICs but smaller than discrete circuits. These ICs
can be used when power requirement is compara¬tively higher. With a thin-or
thick-film IC, the passive components like resistors and capacitors are
integrated, but the transistors and diodes are connected as discrete compo¬nents
to form a complete circuit. Therefore, commercially available thin- and
thick-film circuits are combination of integrated and discrete
components. The essential difference between the thin- and thick-film ICs
is not their relative thickness but the method of deposition of film. Both have
similar appearance, properties and general characteristics.
Thin-film ICs
They are fabricated by depositing films of conducting material
on the surface of a glass or ceramic base. By controlling the width and
thickness of the films, and by using different materials selected for their
resistivity, resistors and conductors are fabricated. Capacitors are produced
by sandwiching a film of insulating oxide between two conducting films.
Inductors are made by depositing a spiral formation of film. Transistors and
diodes can be produced by thin-film technology; but usually tiny discrete
components are con¬nected into the circuit.
One method used for producing thin films is vacuum evaporation
in which vaporized material is deposited on a substrate contained in a vacuum.
In another method, called cathode sputtering, atoms from a cathode made of the
desired film material are deposited on a substrate located between a cathode
and an anode.
Thick-film
ICs
They are sometimes referred to as printed thin-film circuits. In
their manu-facturing process silk-screen printing techniques are used to create
the desired circuit pattern on a ceramic substrate. The screens are actually
made of fine stainless steel wire mesh, and the inks are pastes having
conductive, resistive, or dielectric properties. After printing, the circuits
are high temperature-fired in a furnace to fuse the films to the substrate.
Thick-film passive components are fabricated in the same way as those in
thin-film circuits. As with thin-film circuits, active components are added as
separate devices. A portion of thick-film circuit is given in figure.
ICs produced by thin-or thick film techniques have the
advantages of forming passive components with wider range and better tolerances,
better isolation between their com¬ponents, greater flexibility in circuit
design and of providing better high-frequency per¬formance than monolithic ICs.
However, such ICs suffer from the drawbacks of larger physical
size, comparatively higher cost and incapability of fabrication of active
components.
3.Hybrid or Multi-chip ICs.
As
the name implies, the circuit is fabricated by interconnecting a number of
individual chips. The active components are diffused transistors or diodes. The
passive components may be group of diffused resistors or capacitors on a
single chip, or they may be thin-film components. Wiring or a metalized
pattern provides con-nections between chips. Hybrids ICs are widely used for
high power audio amplifier appli-cations from 5 W to more than 50
W. The structure of a hybrid or multi-chip IC is shown in figure. Like
thin- and thick-film ICs, hybrid ICs usually have better performance than
mono¬lithic ICs. Although the process is too expensive for mass production,
multi-chip techniques are quite economical for small quantity production and
are more often used as prototypes for monolithic ICs.
Based
upon the active devices employed the ICs can be classified as bipolar ICs using
bipolar active devices (BJT) and unipolar ICs using unipolar active devices
like FET.
ICs can
also be classified on the basis of their chip size as given below :
1.
Small scale integration (SSI)—3 to 30 gates/chip.
2.
Medium scale integration (MSI)—30 to 300 gates/chip.
3.
Large scale integration (LSI)—300 to 3,000 gates/chip.
4.
Very large scale integration (VLSI)—more than 3,000 gates/chip.
On the basis of applications ICs are of two types namely, linear
ICs and digital ICs.
When
the input and output relationship of a circuit is linear, linear
ICs are used. An
important application of linear IC is operational amplifier commonly referred
to as op-amp. As it was originally designed for performing mathematical
operations such as summation, subtraction, multiplication, differentiation,
integration, sign changing etc. so it was named OP-AMP. Though now-a-days it
has numerous usages (such as scale changing, analog computer operations, in
instrumentation and control systems and in various phase-shift and oscillator
circuits) but still it is known by old popular name op-amp.
When
the circuit is either in on-state or off-state and not in between the two, the
circuit is called the digital circuit. ICs used in such circuits are called the digital ICs. They find
wide applications in computers and logic circuits.