Integrated Circuits - IC

INTRODUCTION

The separately manufactured components like resistor, capacitor, diode, and transistor are joined by wires or by printed circuit board (PCB) to form circuits. These circuits are called discrete circuits and they have following disadvantages:

1. In a large electronic circuit, there may be very large number of components and as a result the discrete assembly will occupy a very large space.
2. They are formed by soldering which causes a problem of reliability.

To overcome these problems of space conservation and reliability, the integrated circuits were developed.
Integrated Circuit (IC).

An IC comprises a number of circuit components like resistors, transistor etc. They are interconnected in a single small package to perform the desired electronic function. These components are formed and connected within a small chip of semiconductor material. In IC the following features are observed.

1. In IC, the various components are integral part of a small semiconductor chip and the individual components cannot be removed for repair and replacement as in discrete circuits.
2. It combines both active elements like diodes and transistors with passive components like resistors and capacitors in a monolithic structure, so the complete unit in a monolithic circuit. Their size is very small. To see connections between their various components, a microscope is needed.
3. All the components are formed within the chip and no components is seen projected above the surface of the chip.

Scale Of Integration

The number of components fitted into a standard size IC represents its integration scale, in other words it’s a density of components. It is classified as follows:

1. SSI – Small Scale Integration

   It have less than 100 components (about 10 gates).

2. MSI – Medium Scale Integration

   It contains less than 500 components or have more than 10 but less than 100 gates.

3. LSI – Large Scale Integration

   Here number of components is between 500 and 300000 or have more than 100 gates.

4. VLSI – Very Large Scale Integration

   It contains more than 300000 components per chip

5. VVLSI - Very Very Large Scale Integration

   It contains more than 1500000 components per chip.

Comparison of IC and Discrete Circuits

Integrated circuits and discrete circuits can be compare as follows:

Integrated Circuits (IC’s)

1. I.C can work on low voltages.
2. They can handle limited amount of power.
3. They are very small in size
4. They are cheap
5. Complex circuitry on a chip may be used to obtain improved performance characteristics.

Discrete Circuits

1. Discrete circuits require comparatively more voltage.
2. Discrete circuits can handle much more power than IC.
3. Circuit with discrete components acquires large space.
4. Discrete circuits are costly than IC’s.
5. The performance is not good.

Fabrication Of Components

Usually electronics components such as diodes, resistors and capacitors are made on monolithic integrated circuit (IC). In order to fabricate these IC components impurities are added or diffused in specific place in the semiconductor wafer (i.e substrate) so PN junction can be made Figure (a) shows the cross section area of the basic monolithic components.

All the four components are made inside the P-type substrate or wafer. N-type and P-type portion are made from N-type and P-type materials inside the P-type substrate. However this is done by diffusion process. In this process P-type and N-type materials (in the form of gas) are added in semiconductor wafer at high temperature. Wafer is placed in a high temperature, furnace (of about 100^o C)

At first a thin layer of silicon diode SiO₂ layer is made at particular areas of N-type layer which are subject to diffusion. The N-type material is diffused in to substrate. Now the first and large N-type potion is diffused inside the substrate.

Again a thin layer of SiO₂ is grown over the other new place is selected to diffuse P-type material inside the N-type material. This same process is repeated to diffuse last potion of N-type material.