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techniques for transistor layout matching 4


14. Keep all junctions of deep diffusions far away
from active gate area.

The minimum spacing between a drawn well boundary and a precisely matched MOS transistor should equal at least twice the well junction depth. Moderately and minimally matched transistors need only obey the applicable layout rules. Similar considerations apply to deep-N+ sinkers and other deep diffusions.

15.Place precisely matched transistors on axes of symmetry of the die.

Arrays of precisely matched transistors should be placed so that the axis of symmetry of the array aligns with one of the two axes of symmetry of the die. If the design contains large numbers of matched transistors, then reserve the optimal locations for the most critical devices.

16. Connect gate fingers using metal straps.

Connect the gate fingers of moderately and precisely matched transistors using metal rather than poly. Minimally matched transistors can use a poly comb structure to simplify the connection of the gate electrodes.

17. Use thin-oxide devices in preference to thick-oxide devices.

Some processes offer multiple thicknesses of gate oxides. The transistors with thinner gate oxides generally exhibit better matching characteristics than those with thick gate oxides. Whenever circuit considerations allow, consider using thin-oxide transistors in preference to thick-oxide transistors.





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