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Unformatted text preview: the maximum width of a transistor to ensure
effective current conduction at the end opposite the driving signal. When larger transistors are
required (e.g. in buffers to drive cross-chip or off-chip signals), many smaller transistors can be
connected in parallel to achieve the same effect. In reality, the resistance and capacitance are evenly distributed along the width of the gate. The
gate voltage will rise and fall according to the RC time constant. These parasitic elements
prevent the device from operating optimally. Since the transistor gate width defines the speed at
which the transistor switches, the gate width cannot be changed. Thus, the effective gate width
should be maintained. A long transistor can be split into several smaller transistors that are hooked up in parallel. This
is the same size transistor with the same effective gate width, but with less parasitic resistance. In
Fig.2.14, each individual transistor has a Gate finger that is one fourth of the width of the
original device. This means that the gate finger has a resistance that is one fourth of the
resistance of the original device. In addition, the gate fingers are wired in parallel. Four equal
resistors in parallel yield an overall resistance of one fourth of the individual resistance value. 7 The overall effect of this splitting technique gives a parasitic resistance that is one sixteenth of
the original. Next, leg transistors can optimize routing and achieve diffusion sharing. Legging refers to the
process of splitting a very wide transistor into a number of narrower parallel transistors with the
equivalent overall drive strength. For example if a 32μm transistor is split into four parallel 8μm
transistors, the entire structure is referred to as a four-leg transistor. Fig.2.14 shows an example
of a four-legged transistor. Besides being necessary for wide transistors, legging can also aid
routing and layout density. By choosing to leg a transistor, its aspect ratio may be better suited to...
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This document was uploaded on 01/14/2014.
- Spring '13