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MOS (Metal Oxide Semiconductor)-IC’s

As already mentioned in previous blog posts, ICs based upon the active devices are of two types viz. bipolar ICs using bipolar active devices like BJT and unipolar ICs using unipolar active devices such as FET. MOS ICs based on MOSFET structure are widely used in many applications especially in the digital field. This is because of its following advantages over bipolar ICs.

MOSFET IC

1. Reduction in Size.

The MOS IC typically occupies only 5% of the surface required by an expitaxial double-diffused transistor in a conventional IC. Also, a MOS resistor occupies less than 1% of the area of a conventional diffused resistor. Thus MOS ICs are well suited for LSI and VLSI.

2. Simple Fabrication Process.

MOS IC fabrication process is simple and cheaper than bipolar process. This is because in this process only one diffusion step is required to form both the source and drain regions while in the bipolar process two to four diffusion steps are required. In fabrication process of MOS IC two heavily doped N-type regions are diffused into a lightly doped P-type substrate to form the source and drain. An insulating layer of Si0₂ is grown, and holes are etched for the metal electrodes for the source and drain. The metal for these contacts, as well as for the gate electrode, is then evaporated at the same time to complete the device. The complete device (MOS IC) is illustrated in figure.

3. Crossovers and Isolation Islands.

The crossovers between components of MOS ICs are diffused simultaneously as the drain and source. The resistive effects of crossover- diffused regions are; quite small. This is because these regions are in series with large-value load resistors of the order of 100 K normally used with FETs.

Another important advantage is that there is no need of isolation regions between MOS transistors. This is because each source and drain region is isolated from each other by the P-N junctions formed within the P-type substrate.The main drawback of MOS ICs is that their operating speed is smaller than that of bipolar ICs and, therefore, they are not suitable for ultra high-speed applications. However, because of their advantages of low cost, low power consumption and high packing density, MOS ICs find wide applications in LSI and VLSI chips such as calculator chips, memory chips and microprocessors.