doubts in merging of different transistor

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mursri

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hi fnds,

can we merge the transistor in analog part layout like current mirror and driver circuits ? please fnds suggest me . if n't it ,what is the reason ..
 

What do you mean about merge transistors?

You can connect them together to a Darlington pair t.ex. And yes, under certain circumstances two analog transistors can be used as current mirrors.
 

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You can merge drains/sources but if matching is important you need to make sure that matched devices are laid out in an identical manner.

Keith
 

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But some fnds told me that the Reason of STI ( shallow trench isolation ) we can't merge that source and drain of series transistor in analog part except matching consideration .In digital part of transistor we can do right ? ( rail to rail ). please fnds suggest me ...
 

sorry mr.keith,just i called friends (shortly fnds )
Actually analog part transistors got some effect during STI . but if not merged ,the source and drain cap has increasing for every single device ..that s why i need to know whether i need we can merge or not ?
 

Diffusion sharing is perfectly fine. Of course, you don't expect a NMOS to share a diffusion with a PMOS.
 

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mursri said:
... that the Reason of STI ( shallow trench isolation ) we can't merge that source and drain of series transistor ...
Click to expand...
Do you think of an STI separation problem similar like that described below?
(Here a possible STI separation between source & bulk tap is considered).

In this case you just have to care that no STI is between the active areas to be merged by appropriate layout, i.e. using a common active area for both drain and source of the stacked transistors.

Both transistors, however, own the same bulk tap, i.e. their threshold voltages will differ.
 

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