Body Effect

1. Threshold Voltage

Body effect refers to the change in the threshold voltage of the device when there is a difference between substrate(body) and source voltages. Body bias is usually the lowest voltage in the chip(in case of of p-substrate).

Let’s look at the NMOS given below.

Here source(Vs) is connected to VSS and the substrate is biased with voltage Vbulk. There are two pn-junctions formed due to drain-sustrate and source-substrate. There will form a depletion region at these junctions. As mentioned earlier, A gate bias is required which is greater than Vth to inverse the channel.

However, if we were to connect Vbult to a voltage lower than VSS (Source voltage), there is an increased flow of carriers between these source-bulk junstion thereby increasing the width of the depletion region. This in turns increases the minimum gate voltage needed to achieve channel inversion.