Transistor is one direction

[spicer]

I had seen from a book that"In layout, all transistors should be placed in one direction". could somebody tell me the reason? Thank you in advance.

 

[road_0506]

the reason why the xsistors need to be in one direction is simple. diff directions causes mismatches among xsistor... this is due to the diff in what they call "implant angle" and even mask alignment errors...

 

[franck]

Hi,

The main reason for placing devices in the same direction is masks shift during processing.

Franck.

 

[cmos_dude]

Hi,

The main reason for placing devices in the same direction is masks shift during processing.

Franck.

Then how do you manage the processes where 45 deg. poly over difussion is allowed ???

 

[franck]

Hi,

The main reason you need to think about mask shift is for matching.
45 degree angle for gates is used in digital layout.
You don't really need to take care of matching in digital.
And by the way I don't think a mask shift would be big enough to not process poly over the desired active area. That will just result in an unexpected source and drain area or a bad overlap of poly on diff.
Also I want to add something important. You CAN place transistors in different direction (I mean even those who need to match) as long as you have found a way of spliting your devices (and of course routing) equally. This can be done for exanple in a VCO core, a detector or a JK Latch.

Franck.