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Unformatted text preview: such signals to switch very slowly. However, it is
common to use the same poly line to drive complementary transistors (such as in an inverter). In
this case, connect the driving signal to the poly (that forms the two gates) between the two
transistors. Obviously, the impact of driving one transistor through another is more pronounced
with wider transistors. Therefore, if routing is significantly simplified by going against this
recommendation in a specific case, consideration should be given to the widths of the transistors
involved. Fig.2.16 Transistor Chain [BTT]. Compact Layout
Most ICs are designed using very small circuits, which are easier to handle, and to understand
the designs. These little circuits are then wired together to create a larger, more complex circuit.
This approach to circuit design makes layout much easier. Instead of trying to determine how to
wire twelve million transistors in one go, it is better to start with a circuit that has twelve and
creep up on the larger problem. The goal is to compact layouts. A rectangular circuit layout is
much easier to use in conjunction with one million other rectangular circuit blocks than
irregularly shaped lumpy circuit layouts. 10 Next, sharing diffusion regions of interconnected transistors minimizes both area and load
capacitance, which it thus improves switching speed. There are a number of techniques that
optimize diffusion sharing. If the particular junction being shared is connected to VDD or ground,
it is acceptable to widen the diffusion region (increase the intra-gate distance) to allow one
additional metal line to cross the diffusion beside the connection to the supply as shown in
Fig.2.17. The extra diffusion capacitance is acceptable since the supply node is always at the
same potential Since one of the transistors will see an increased resistance to the supply rail,
however, the widening should be limited to accommodate a single extra line. Fig.2.17 Routing over diffusion [BTT]. One of the largest design rules in many proces...
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This document was uploaded on 01/14/2014.
- Spring '13