Mosfet Substrate on CMOS

[bioroids]

Ok, another dumb question:

What is the effect of connecting or not connecting the substrate on a Mosfet transistor?

I've seen on CMOS chips datasheets, sometimes they indicate "All P substrates are connected to Vdd" or "All N substrates are connected to VSS". Is this standard practice on all CMOS devices, or varies depending on which IC you have?

I think it varies, at least according to the simbols used on the datasheets (some indicate the substrate -arrow- floating and some indicate it connected, normally to the source), but I'm really not sure how much these can be trusted...

Can you damage an IC by not connecting the substrates (for example using it without connecting Vdd or Vss)?

Thanks and luck!

Miguel

[Sir H C]

In general CMOS devices are really four terminals, Gate, Source, Drain, and Body (substrate for NMOS usually). This 4th terminal is usually connected to the source or Vdd for PMOS devices, and usually is only possible to be the substrate for NMOS devices.  If you do not properly tie the substrate/body terminals, you get diodes from the substrate (P doped) or well (N doped) and the source/drain terminals of the devices (these are doped the other way).