MOSFET Calculator — Drain Current & Operating Region
Determine the drain current I_D of an NMOS MOSFET in any operating region. Enter gate-source voltage V_GS, threshold voltage V_th, drain-source voltage V_DS and the transconductance parameter k_n to get I_D, power dissipation and the I_D–V_DS output characteristic curve.
V
V
V
mA/V²
Operating region: Saturation
- 1
Overdrive voltage V_OV
V_GS − V_th = 2.5 − 1 = 1.5 - 2
Saturation region
V_DS (2) ≥ V_OV (1.5)Channel pinches off at drain; I_D depends only on V_GS. - 3
Drain current I_D
(k_n ÷ 2) × V_OV² = (2 ÷ 2) × 1.5² = 2.2500
How does this calculator work?
NMOS drain current: I_D = 0 (cutoff, V_GS ≤ V_th); I_D = k_n[(V_GS−V_th)V_DS − V_DS²/2] (triode); I_D = (k_n/2)(V_GS−V_th)² (saturation, V_DS ≥ V_GS−V_th). k_n = μ_n × C_ox × W/L is the process transconductance × aspect ratio. The I_D–V_DS curve rises parabolically then flattens at the saturation current.
Formula
How this is calculated
An NMOS MOSFET has three DC operating regions depending on the gate overdrive voltage V_OV = V_GS − V_th and the drain-source voltage V_DS. In cutoff (V_GS ≤ V_th) no channel forms and I_D ≈ 0 — the device is off. When V_GS exceeds V_th a conducting n-channel forms between drain and source.
In the triode (linear) region (V_DS < V_OV), the channel is intact from drain to source and the device behaves roughly like a voltage-controlled resistor: I_D = k_n[(V_OV)V_DS − V_DS²/2]. In the saturation region (V_DS ≥ V_OV) the channel pinches off at the drain end and I_D is nearly independent of V_DS: I_D = (k_n/2)(V_OV)², where k_n = μ_n × C_ox × W/L combines carrier mobility μ_n, oxide capacitance per area C_ox and the transistor width-to-length ratio W/L.
This model uses the long-channel square-law approximation — it is accurate for large feature-size devices but does not capture short-channel effects such as velocity saturation, channel-length modulation (λ), or threshold-voltage roll-off found in modern sub-micron transistors.
Frequently asked questions
k_n = μ_n × C_ox × W/L, where μ_n is the electron mobility in the channel (~500 cm²/V·s for silicon), C_ox is the gate-oxide capacitance per unit area, and W/L is the gate width-to-length ratio. The SPICE parameter K is half this value (k_n/2 = K). Typical values range from tenths of mA/V² for small devices to tens of mA/V² for wide transistors.
When V_DS reaches V_OV the conducting channel "pinches off" at the drain end — the voltage drop across the channel is fixed at V_OV regardless of how large V_DS grows. As a result, I_D is set by V_GS alone (I_D = k_n/2 × V_OV²) and is ideally independent of V_DS. In practice, a small slope (channel-length modulation) exists and is modelled by multiplying by (1 + λV_DS).
A PMOS uses holes as carriers. Voltages are referenced to the source and the relevant quantities are V_SG (source-gate) and V_SD. The threshold voltage V_tp is negative. The same formula applies with magnitudes: I_D = (k_p/2)|V_OV|² in saturation.
Also known as
TG we-Calculate Editorial Team. (2026). MOSFET Calculator — Drain Current & Operating Region [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/mosfet-calculator
TG we-Calculate Editorial Team. "MOSFET Calculator — Drain Current & Operating Region." TG we-Calculate. 2026. https://we-calculate.com/calculator/mosfet-calculator.
TG we-Calculate Editorial Team, "MOSFET Calculator — Drain Current & Operating Region," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/mosfet-calculator
@misc{wecalculate_mosfet_calculator, title = {MOSFET Calculator — Drain Current & Operating Region}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/mosfet-calculator}}, year = {2026}, note = {TG we-Calculate} }
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