MOS (Metal Oxide Semiconductor) IC’s

jojo March 31, 2014 1 Comment

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.

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) IC
Metal Oxide Semiconductor Field Effect Transistor (MOSFET) IC

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.

  • January 10, 2013

    You all remember the Commodore 1351 MOUSE
    I wish there was a way to create this chip the same way, since these Mice are quite rare,
    because the controller chip is rare an obsolete.

    The 1351 IC MOS part number:
    S506015P 8637

    If someone knew of the innerworkings and the way this controller chip functions, could this be
    coded with like: C code or 8-bit ML compiled code?
    Myself Im definitely not a good programmer but have tinkered with the basics
    with 8-bit ML, but this is a very complex language to learn (to me anyway).

    So I was wondering if there is anyone that could possibly build these again, maybe on a small scale, or
    just create the code and burn that into a small eprom.
    The problem with that is the code is probably larger than what would fit into a small eprom the same size as the original chip, as to keep the chip compatible with the 8-bit circuit also.

    Anybody interested?
    Thank you.

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