diagramintegrated-circuitmosfet
I found a circuit diagram, which I do not understand. In the diagram below, I bounded a peculiar terminal with a red box. The bulk of m3 and m4 MOSFETs are connected to this negative terminal.
My question is, what does this mean? Does this mean that the bulk of m4 and m3 is connected to some terminal that has a potential lower than VSS?
I appreciate any help.
The body terminal is there whether we like it or not because of the construction of a MOSFET:
The gate voltage modifies the channel width. But the gate potential is measured relative to the substrate potential, and the channel is formed in the substrate material. Therefore the MOSFET behavior depends strongly on the substrate (body) potential.
If we did not make a contact to the body, it would be free to float, and we could not control the behavior of the transistor. We connect to the body to take control of this behavior. Normally we tie the body to either a most-positive or most-negative potential, like you say, because that gives the lowest |Vgs| relative to the appropriate supply and prevents forward biasing the source-body junction. If we did not contact the body, it would still be there, but we would not have control over its effect.
(MOSFET image by Wikimedia user CyrilB licensed under CC-BY-SA)
That is a mosfet used to make a very small capacitor. This is often done so that cmos process variation affect the active transistor and the "capacitor" equally. It also takes up fewer metal layers. What they are using the capacitor for is beyond me.
Best Answer
The fact that it gives the Width to Length ratio indicates it is a design internal to an IC, not one using discrete components.
With devices inside an IC the body is usually connected to the substrate (which in turn is usually the most negative or the most positive voltage available, depending upon the doping, N or P-type).