How does the body effect change the drain current in a MOSFET?

Posted in general
at 2019.12.17


The body effect comes into play when the body and source potentials of the mosfet are unequal, that is to say that the body is not linked to the source. this causes a modification of the threshold voltage of the mosfet and affects the drain current in the following ways: –

running point continuous: a positive [math] v_ {sb} [/ math] will increase the threshold voltage and give all the other factors remain the same, the drain current will decrease.

small signal: a new transconductance term appears in the small signal model, namely [math] g_ {mb} [/ math] and [math] g_ {mb} v_ {bs} [/ math] is its contribution to small signal drain the current.

Of course, this is only a very short explanation. I encourage you to consult a manual if you want to know more and get information.

by affecting the threshold voltage. depending on the polarization polarity of the body with respect to the source, the effect may be an increase or decrease of the threshold voltage. therefore, it also affects the drain current indirectly. lower it or raise it.

Whereas it was previously considered a detrimental effect, the effect on the body is often used by the designers of integrated circuits. an increasing threshold voltage is used to reduce leakage below the threshold, which is a significant power loss in large systems on chips (Socs). a decreasing threshold voltage is used to accelerate the low voltage logic circuits when the threshold voltage is in the operating voltage range. These two uses can be applied simultaneously in the same integrated circuit by modulating the local polarization of the body.

in a nutshell, here is the underlying mechanism. the presence of a current-conducting inversion layer in an enhancement type mos transistor depends on the surface potential at the source end of the transistor. Here are the source region, the gate oxide and the mass (body region). While people tend to focus on grid-to-source voltage when the generation of the inversion layer is discussed, it is clear that the potential of the three regions will affect the potential at this stage. for simplicity, the two voltages are separated into vgs and vbs (gate-source, body-source) and treated as control variables. you can sometimes read the term back door polarization, which refers to the source-body voltage. Conceptually, you can imagine two devices in parallel, one controlled by the gate-source voltage, the other by the body-source voltage (back gate).