MOSFET Devices - Subthreshold Swing (STS) Detailed Explanation

MOSFET Devices - Subthreshold Swing (STS) Detailed Explanation

The subthreshold swing is a performance indicator that measures the rate of transition between the on and off states of a transistor. It represents the amount of change in gate voltage required for the source drain current to change tenfold, also known as the S factor. The smaller the S, the faster the on/off rate.

Subthreshold Swing refers to the change in source drain current IDS by one order of magnitude VGS.

Based on the definition of subthreshold swing and the above equation, we can have the following thoughts:,

1) We hope that the smaller the sub threshold swing, the better; When the gate voltage is less than the threshold voltage in the sub domain region, the device is completely turned off and the source drain current is zero. As soon as the threshold voltage is reached, the transistor quickly turns on; So it is best if the current is very sensitive to voltage changes, that is, a small change in gate voltage can cause a change of one order of magnitude in current. Therefore, S is small, reflecting better gate control capability and smaller subthreshold leakage current.

Factors affecting subthreshold swing:

1) Temperature, as the temperature rises, the sub threshold swing increases

2) The capacitance of the gate oxide layer increases, and the sub threshold swing decreases; By using a high-k dielectric and reducing the thickness of the gate oxide layer, the sub threshold swing can be reduced.

3) The depletion layer capacitance of Si decreases, and the subthreshold swing decreases; Factors that increase the width of the depletion layer, such as a decrease in substrate concentration Na and an increase in substrate bias voltage, will result in a decrease in subthreshold swing.

4) There may be some interface defects between the gate oxide layer and the substrate silicon interface, which can store charges. The increase of these defects is equivalent to adding a capacitor, which will increase the subthreshold swing.

5) A smaller channel length will weaken the gate control capability and increase the subthreshold swing.

6) As the gate voltage increases, with the enhancement of surface inversion, the control ability of the gate over the channel becomes weaker, and the subthreshold swing increases.