MOSFET Inversion Charge Calculator
Estimate the magnitude of the strong-inversion sheet charge from oxide capacitance and overdrive voltage using |Qinv| = Cox·Vov.
Description
Estimate strong-inversion sheet charge magnitude in µC/cm² from gate oxide capacitance per area and overdrive voltage.
In a first-order charge-sheet model, inversion charge magnitude is oxide capacitance per area times gate overdrive VGS − VT. This calculator reports a positive magnitude; the physical electron sheet charge in an n-channel MOSFET has negative sign.
When to use MOSFET Inversion Charge Calculator
- Estimate strong-inversion charge magnitude for a long-channel MOSFET
- Check charge scaling with Cox or gate overdrive
- Convert a Cox value in fF/µm² directly to µC/cm²
How the calculation works
The tool applies |Qinv| = Cox·Vov. Since 1 fF/µm² equals 10⁻⁷ F/cm² and 1 C equals 10⁶ µC, the combined conversion factor is 0.1.1
|Qinv| (µC/cm²) = 0.1 × Cox (fF/µm²) × Vov (V) Interpreting the result
The output is charge magnitude per gate area at a representative channel point. It is not the signed total channel charge and does not include gate width or length.
Assumptions and limitations
- The approximation assumes strong inversion and an ideal charge sheet.
- Body effect, drain-voltage variation along the channel, mobility degradation, short-channel effects, depletion charge, and quantum corrections are omitted.
- Overdrive must be positive; the tool does not model weak inversion or subthreshold charge.
References
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EE105 Lecture 7: MOSFET — University of California, Berkeley