MOSFET operating-point calculator
Estimate the drain current and operating regime of an n-channel enhancement MOSFET with the long-channel square-law model.
Inputs
Gate-source voltage where the idealized channel begins to conduct.
Square-law conduction parameter. Must be zero or greater.
This n-channel model assumes a non-negative drain-source bias.
Compared with VT to determine whether an inversion channel is present.
Live results
The bias point is in the triode region, so ID depends on both Vgs and Vds.
Triode: ID = 2K(Vgs – Vds/2 – VT)Vds
Drain current 1.96875 A. Operating regime Triode. Overdrive voltage 6 V.
MOSFET structure parameters
These values reproduce K = ½(W/L)μNCox. Editing any structure field recalculates the K parameter when all four structure values are valid.
Positive channel width; converted internally from micrometres to metres.
Positive channel length; it appears in the denominator of W/L.
Positive gate-oxide capacitance per unit area.
Positive effective electron mobility used by the long-channel model.
Operating-point detail
| Quantity | Symbol | Current value | Interpretation |
|---|---|---|---|
| Drain current | ID | 1.96875 A | Idealized source-drain current at the selected bias point. |
| Overdrive voltage | VOV | 6 V | Vgs – VT; positive values indicate gate bias above threshold. |
| Saturation boundary | Vds,sat | 6 V | At this drain-source voltage, the square-law model reaches the saturation boundary. |
| K parameter | K | 0.05625 A/V² | Combines channel geometry, mobility, and oxide capacitance. |
How to use this MOSFET calculator
What this calculator does
This calculator estimates the drain current ID and classifies the operating point of an idealized n-channel enhancement MOSFET as cut-off, triode, or saturation. It uses the familiar long-channel square-law equations, so it is best for learning, first-pass bias calculations, and checking hand work. It does not replace a manufacturer SPICE model or a device datasheet when short-channel effects, temperature, body bias, channel-length modulation, parasitic resistance, or safe-operating-area limits matter.
When to use it
Use it when you want to check whether a chosen gate and drain bias places an enhancement NMOS device in the intended regime, estimate the ideal drain current from a known K parameter, compare how a change in threshold voltage affects a bias point, or derive K from simple structure parameters. For a deeper treatment of the long-channel assumptions and operating regions, see the MIT OpenCourseWare lecture on the long MOSFET.
How to calculate
- The calculator opens with a complete demonstration: VT = 2 V, K = 0.05625 A/V², Vds = 5 V, and Vgs = 8 V. A valid Excel workbook is ready immediately.
- Replace Threshold voltage (VT), K parameter, Drain-source voltage (Vds), and Gate-source voltage (Vgs) with the values for your operating point. Results update live.
- If you know device geometry instead of K, open MOSFET structure parameters and enter Channel width (W), Channel length (L), Oxide capacitance per area (Cox), and Electron mobility (μN). When all four are valid, the calculator recomputes K using ½(W/L)μNCox.
- Read Drain current (ID), Operating regime, Overdrive voltage (VOV), Saturation boundary, and Effective K, then use the detail table to see the same canonical values with their meanings.
- Select Download Excel to export the current validated state. Reset clears the demonstration and all calculated content; export is then disabled until a complete valid state is entered again.
Input guide
Threshold voltage (VT) is a required voltage in volts. Enter a standard decimal such as 2 or 1.3; scientific notation and decimal commas are rejected. A higher VT reduces overdrive for the same Vgs and therefore tends to reduce current. Do not confuse a datasheet minimum/maximum threshold specification with a single exact switching point in a real device.
K parameter is required, measured here in A/V², and must be zero or positive. The demonstration uses 0.05625 A/V². Increasing K increases drain current proportionally in both conducting regimes. If you directly edit K, the structure fields are cleared so there is only one active source for K.
Drain-source voltage (Vds) is required in volts and must be non-negative in this n-channel forward-bias model. The demonstration uses 5 V. In triode, raising Vds changes ID directly until Vds reaches the saturation boundary; in saturation, the ideal first-order equation no longer depends on Vds. A negative Vds needs a polarity-aware model and is intentionally rejected.
Gate-source voltage (Vgs) is required in volts. The demonstration uses 8 V. If Vgs is below VT, the calculator returns cut-off and zero ideal drain current. Above threshold, larger Vgs increases VOV and usually increases ID strongly because the saturation equation is quadratic in overdrive.
Channel width (W) and Channel length (L) are positive optional dimensions entered in micrometres. The demonstration uses 100 µm and 1 µm. A larger W/L ratio increases K. Oxide capacitance per area (Cox) is a positive optional value in F/m²; the sample is 0.00345 F/m². Electron mobility (μN) is a positive optional value in m²/(V·s); the sample is 0.3260869565. Structure values must be supplied as a complete four-field set so the derived K has an unambiguous meaning.
Output guide
Drain current (ID) is the model's primary estimate in amperes. Zero means the idealized device is cut off or exactly at a zero-current boundary. Large values are mathematical results, not permission to exceed a transistor's current, power, thermal, or safe-operating-area ratings. Operating regime is a classification: cut-off when Vgs < VT, triode when Vgs ≥ VT and Vds < Vgs – VT, and saturation at or above the boundary. Overdrive voltage (VOV) equals Vgs – VT. A negative value means the gate is below threshold. Saturation boundary is max(VOV, 0) in volts and marks the ideal transition from triode to saturation for forward Vds. Effective K is the K value actually used by the current calculation. The summary pills repeat regime, ID, and VOV for quick scanning, and the operating-point detail table repeats ID, VOV, boundary voltage, and K from the same model rather than recalculating them separately. The short sentence below the primary result interprets the selected regime, the active equation line identifies the formula currently in use, and the note below the table states the model effects that are intentionally excluded.
Worked example
With the startup values VT = 2 V, K = 0.05625 A/V², Vds = 5 V, and Vgs = 8 V, the overdrive is VOV = 8 – 2 = 6 V. Since 5 V is below the 6 V saturation boundary, the device is in triode. The calculator uses ID = 2K(Vgs – Vds/2 – VT)Vds = 2 × 0.05625 × (8 – 2.5 – 2) × 5 = 1.96875 A. The structure sample also yields K ≈ 0.05625 A/V² from ½(W/L)μNCox, so the direct and structure-based descriptions agree at first open.
Learn more
The square-law equations describe an idealized long-channel device. For a university-level derivation of MOSFET drain-current modeling, consult MIT OpenCourseWare's large-signal MOSFET model lecture.
How the model chooses a regime
The boundary equations meet continuously: when Vds = Vgs – VT, the triode expression reduces to K(Vgs – VT)², which is the same value as the saturation expression. Real devices depart from this ideal plateau because channel-length modulation and other second-order effects make current continue to vary with Vds. For measurement-oriented context on small-signal behavior, Tektronix explains how MOSFET transconductance is obtained from current-voltage measurements.
Practical interpretation and limitations
The K parameter compresses geometry and material properties into one coefficient, which makes the operating equations compact but also hides many real-device details. Datasheet curves are measured under specified temperatures, pulse widths, and bias conditions; threshold voltage itself varies from part to part. For switching applications, a device being above threshold does not mean it is fully enhanced at a useful low resistance. For analog work, the square-law model is best treated as a conceptual bias model rather than a precision prediction for modern short-channel transistors.
No chart is shown because a single current operating point does not provide a genuine multi-point measured series. Creating an I-V curve would require generating a sweep of additional Vds or Vgs values that the user did not enter. This calculator therefore keeps the current state honest with numeric results and a detail table instead of fabricating a quantitative visual from counterfactual points.