What is a Subthreshold swing?

We usually refer to subthreshold swing when discussing the performance of MOSFETs in weak inversion. It's the slope of the current vs. gate voltage curve in the subthreshold region. A smaller subthreshold swing means the transistor can turn on more quickly, which is crucial for low-power applications.

You might find it interesting that subthreshold swing determines the amount of gate voltage needed to increase the current by a decade. In simpler terms, it's a measure of how sensitive the MOSFET is to changes in gate voltage in the subthreshold region. Lower values are better because they mean the device switches more efficiently.

In my experience, subthreshold swing is critical for understanding MOSFET behavior in weak inversion. It represents how rapidly the current increases with gate voltage when the transistor is near its off state. A lower subthreshold swing is desirable because it means the transistor can switch states with less voltage change, which is especially important as devices get smaller and supply voltages decrease.

I think of subthreshold swing as a key indicator of a MOSFET's switching efficiency. When a MOSFET operates in the subthreshold region, the subthreshold swing tells you how much the gate voltage needs to change to significantly increase the current. For modern transistors, keeping this swing low is vital for high performance and low power consumption.