Layout_Examples

216 the combination of high poly sheet resistance and

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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.

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