P-Channel MOSFET: Implications of Bulk Connected to +Vdd

[seang]

What are the implications of connecting the bulk of a p-channel mosfet to a +Vdd. Up until now, we've only had examples where the bulk has been shorted to the source of the mosfet, or shorted to ground. I don't know how a bulk voltage changes the source voltage or the drain voltage.

Thanks for your help

 

[es1]

Long story short, changing the builk voltage changes the threshold value of the transistor.

The threshold value is in both the linear and quadratic models of the FET so this has implications to Ids as well but they are usually minor unless you are dealing with low power electronics or operating in weak inversion.

The value that really matters is the difference between the source and the bulk. Tying bulk to the source shorts this so the threshold never changes.

This link has more details and the math:
http://ece-www.colorado.edu/~bart/book/book/chapter7/ch7_4.htm#7_4_2

However, in practice people usually tie the bulk to ground (or to vdd for pfets) to avoid biasing the body diodes. The drain and the source form pn junctions (aka a diode) with the substraight (aka bulk). Connecting the bulk to the lowest voltage in your system for nfets (or highest for pfets) insures these diodes never get biased and thus you get to ignore them which is nice when one is starting out with electronics.

 

[ravenprp]

!

Connecting the bulk of a p-channel MOSFET to +Vdd can have several implications on its operation. Firstly, it can affect the threshold voltage of the MOSFET. The threshold voltage is the minimum voltage required at the gate to turn on the MOSFET. By connecting the bulk to +Vdd, the threshold voltage can increase, making it harder for the MOSFET to turn on. This can result in slower switching speeds and higher power consumption.

Additionally, connecting the bulk to +Vdd can also affect the body effect of the MOSFET. The body effect is the change in threshold voltage due to the voltage applied to the bulk. With the bulk connected to +Vdd, the body effect can be reduced, leading to a more stable threshold voltage and improved performance.

Moreover, connecting the bulk to +Vdd can also impact the source and drain voltages of the MOSFET. As the bulk is now at a higher voltage, it can create a potential barrier between the source and the bulk, making it more difficult for the source to inject carriers into the channel. This can result in reduced current flow and higher resistance in the channel, leading to decreased performance.

Overall, connecting the bulk of a p-channel MOSFET to +Vdd can have significant implications on its operation, including changes in threshold voltage, body effect, and source/drain voltages. It is important to carefully consider these implications when designing circuits using p-channel MOSFETs and to choose the appropriate bulk connection based on the desired performance.