"bi-directional" mosfet

Thread Starter

mos_6502

Joined Dec 11, 2017
28
Hi to all!
There is a "bi-directional" mosfet?
In some circuit, I noticed that the Mosfet are drawn without any arrow; why? Are this different mosfet?
 

WBahn

Joined Mar 31, 2012
24,974
Hi to all!
There is a "bi-directional" mosfet?
In some circuit, I noticed that the Mosfet are drawn without any arrow; why? Are this different mosfet?
In general, the MOSFET is a four terminal device and the source and drain are interchangeable. In integrated circuit designs, most of the FETs will be symmetric and there is truly no distinction between the source and the drain. But there IS a difference between N-channel and P-channel FETs, so the symbol needs to indicate which type of FET it is. There are several common ways of doing that.
 

Wolframore

Joined Jan 21, 2019
1,481
there are some ugly FET symbols out there even ones where the arrows are reversed. Look on the gate if its a straight line its N Type... if it's a "o" or part of one it's a P Type.
 

Bordodynov

Joined May 20, 2015
2,455
I myself once engaged in the development of dynamic memory. The fourth pin of all transistors of the same type is connected to the same node of the circuit. Drawing an arrow is not rational, as there will be a blockage of the scheme. When they draw the topology of the transistor, they draw only the drain, the source and the gate. Pin substrate is obtained automatically.
 

WBahn

Joined Mar 31, 2012
24,974
I myself once engaged in the development of dynamic memory. The fourth pin of all transistors of the same type is connected to the same node of the circuit. Drawing an arrow is not rational, as there will be a blockage of the scheme. When they draw the topology of the transistor, they draw only the drain, the source and the gate. Pin substrate is obtained automatically.
While that is likely the case with DRAM, it is most certainly NOT the case with CMOS in general.

Most CMOS processes are N-well processes, which means that while the NFETs are built directly in the substrate -- and therefore share the same bulk connection -- PFETs are built within an N-well and the designer gets to decide what the well, which is the bulk connection for all of the transistors within it, is connected to. In most digital circuits all of the N-wells are connected to the positive power rail, but in analog circuits this is often not the case and it is not uncommon to see each PFET transistor in it's own well specifically so that the well can be tied to the source of that transistor to minimize bulk-bias modulation. For high performance analog circuits, a twin-well process is sometimes used specifically so that the same thing can be done with the NFETs. There are other reasons, of course, for using a twin-well process, such as radiation hardening.
 
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