Device physicsMVP
Subthreshold swing explorer
The subthreshold swing S = n·(kT/q)·ln10 — how many millivolts of gate voltage change the off-state current by 10×.
Inputs
Educational classic-MOSFET limit
S = n·(kT/q)·ln10 is the thermal limit on how sharply a conventional MOSFET turns off. The ideal (n = 1) value is ≈ 60 mV/decade at 300 K; it cannot be beaten by classic MOSFETs (steep-slope devices use different physics).
n ≥ 1; set by the depletion/oxide capacitance divider.
Subthreshold swing
59.56 mV/decade
Interpretation
In the subthreshold region the drain current is exponential in gate voltage, so on a log scale it falls as a straight line — one decade for every S volts. A smaller S means the transistor switches off more sharply, which lowers leakage and allows a lower supply voltage. The n·(kT/q)·ln10 form shows why this “60 mV/decade” limit is fundamentally thermal for classic MOSFETs.
Formula
S = n · (kT/q) · ln(10)
Ideal n=1 → ≈ 60 mV/dec at 300 K.
Variables
| Symbol | Variable | Unit |
|---|---|---|
| S | Subthreshold swing | V/decade (shown as mV/dec) |
| n | Body/ideality factor | dimensionless (≥ 1) |
| kT/q | Thermal voltage | V |
Assumptions & validity
This tool assumes:
- Classic MOSFET subthreshold conduction (diffusion current).
- The log-Id line is a first-order picture; real curves bend near and above threshold.
Worked example
n = 1, 27 °C.
S = 1 · 0.0259 · ln10 ≈ 60 mV/decade