We have already explained in integrated circuits, Hybrid IC’s about the classification of IC’s as bipolar IC’s and Unipolar IC’s. In bipolar IC’s, devices like BJT’s are used. In unipolar IC’s, devices like Field Effect Transistors are used.
Metal oxide semiconductor IC (MOS IC) based on MOSFET structure have found wide applications in the digital field. Here are some comparisons and advantages over bipolar IC’s.
1. Reduction in size
MOS IC’s are more suitable than bipolar IC’s I applications like large scale integration (LSI), and very large scale integration (VLSI). They are also used in memory chips and microprocessors. This is because MOS IC’s does not occupy as much surface when compared to a bipolar epitaxial double diffused transistor IC. The size can be reduced to almost 5%. The same concept goes much better for a MOS resistor with a reduction in area to almost 1% when compared to a conventional diffused resistor.
2. Simple Fabrication Process
When compared to a bipolar IC, the fabrication process of a MOS IC is much more simple and cost effective. As the entire process constitutes only one diffusion step to form the source and drain regions. But in a bipolar IC process almost 4 steps are required. The fabrication process of MOS IC consists of two heavily doped N-type regions that are diffused into a less doped P-type substrate to form the source and drain.
Holes are etched for the metal electrodes for the source and drain after the layer of silicon dioxide. The device fabrication is completed by evaporating the metal for the contacts and for the gate electrode at the same time. The detailed configuration is shown in the figure below.
3. Crossover and Isolation Islands
By diffusing the drain and source of the MOS IC, the crossover between the components are diffused simultaneously. The resistive effects of crossover diffused regions are quite small. The reason for this is that these regions are in series with large value load resistors of the order of 100K normally used with FET’s.
Since, in a MOS IC each source and drain is isolated from each other by the P-N junctions formed within the P-type substrate, there is no need for isolation between the MOS transistors. This is a huge advantage when compared to a bipolar IC. Some other advantages include low buying cost, low manufacturing cost, low power consumption and high packing density.
The only drawback of MOS IC is that the operating speed is less when compared to bipolar IC’s. Thus they are not used for very high speed applications.