In the VN2222 MOSFET below (diagram is from PSPICE), what is the terminal with the arrow pointing in? The one to the left is the gate, above and to the right is the drain and at the bottom is the source. What's the terminal in the middle, though? Under what circumstances should it be grounded?
Thanks!
The middle terminal is the substrate (edit: also body or bulk as Null says).
In almost all discrete MOSFETs (including the VN2222) it is effectively connected internally to the source. It behaves a bit like a second gate (though not an insulated one) if you had access to it. You should connect it to the source if you're trying to simulate a real VN2222.
An important application that requires the substrate not be connected to the source is the analog switch:
That's the body of the MOSFET. It's also sometimes called the substrate. The VN2222 datasheet from Microchip shows that the body is tied to the source internally, which is standard practice for discrete MOSFETs. Your simulation should do the same. I'm not sure why PSPICE thinks there's a fourth terminal on the package, unless it's using a generic model.
In integrated circuits, the substrate is tied to VSS (for NMOS) or VDD (for PMOS) due to the construction of the chip. Sometimes this is shown in schematics for CMOS IC circuits, but often it's just implied.
