Explain Stacking effect in MOS transistors and its effect on

Explain Stacking effect in MOS transistors and its effect on subthreshold leakage.

Solution

Stacking Effect in MOS transistors and its effect on subthreshold leakage:

    Subthreshold current depends exponentially on VT, VDS and VGS. Therefore it is a function of the terminal voltages, VD, VB, VS and VG. This means that to know subthreshold leakage of a device the biasing condition should be known or by controlling the terminal voltages the subthreshold leakage can be controlled.

    Input pattern of each gate affects the subthreshold as well as gate leakage current. The leakage of transistors in a stack is a function of no. of transistors and input pattern.

    Source biasing is the general term for several techniques that change the voltage at the source of a transistor. The goal isto reduce VGS, which has the effect of exponentially reducing the subthreshold current. Another result of raising the source is that it also reduces VBS, resulting in a slightly higher threshold voltage due to the body effect. Circuits that directly manipulate the source voltage are rare, and those that exist usually use switched source impedance or a self-reversed biasing technique. Probably the simplest example of source biasing occurs when “off” transistors are stacked in series. Conceptually, the source voltage of the upper transistor will be a little higher than the source voltage of the lower transistors in the stack. Hence VGS of upper transistor is negative, VBS is negative resulting in increase in threshold voltage and VDS is also lower. Due to this, the leakage of upper transistor reduces. This reduction is called stack effect. But this reduction in leakage comes at an increase in delay performance. The reduction in leakage due to stack effect can lead to increase in delay and hence can be used in situations where this delay can be tolerated or by using gates with natural stack.