Mosfet Calculator - Drain Current and Operating Regime

A MOSFET calculator solves the drain current of a metal-oxide-semiconductor field-effect transistor in any of its three operating regimes, given the gate-source voltage Vgs, the drain-source voltage Vds, the threshold voltage Vt, and the transconductance parameter K. The same equations cover logic-level switching, power-converter commutation, and small-signal amplification, so one tool fits hobby, lab, and coursework needs.

• • Sizing a low-side switch with a BS170: Pick the gate drive voltage and the on-resistance target, then verify the drain current.
• • Designing a logic-level MOSFET interface: Confirm that a 3.3 V GPIO drives a 2.5 V threshold MOSFET into saturation.
• • Working an electronics homework problem: Check hand calculations of I_D in triode and saturation against the calculator.
• • Comparing n-channel and p-channel behaviour: Reuse the same K, Vt, Vgs, and Vds to compare saturation scaling.

The three operating regimes are cut-off (off), triode (linear), and saturation. Each regime uses a different drain current equation, and the boundary between them is set by the overdrive voltage Vov = Vgs - Vt. The calculator evaluates those conditions automatically and returns the regime label, so the user never has to remember which formula applies at a given bias point.

Use the Ohm's Law Calculator to confirm the I = V / R form that the triode equation collapses to when V_ds is small.

The calculator applies the standard long-channel MOSFET equations. The same equations describe p-channel devices after reversing the bias polarity. The overdrive voltage Vov = Vgs - Vt separates the off state from conduction, and the Vds-to-Vov ratio separates triode from saturation.

• V_gs: Gate-source voltage (V) that controls the channel.
• V_ds: Drain-source voltage (V) across the channel.
• V_T: Threshold voltage (V) from the datasheet.
• K: Transconductance parameter in 1/(V*ohm), equal to (1/2) (W/L) mu C_ox, often back-calculated from the datasheet saturation current.

If V_gs is below V_T the MOSFET is in cut-off, and the calculator returns 0 A. If V_gs is at or above V_T and V_ds is below Vov, the device sits in the triode (linear) region. If V_ds is at or above Vov, the channel pinches off near the drain and the current follows the saturation equation.

The K parameter is a constant for a given device, so the calculator accepts it as a single input. If the datasheet only lists the typical saturation current at a given Vgs, compute K = 2 I_D / (V_gs - V_T)^2 once and reuse it across bias points.

According to the ON Semiconductor 2N7000 Datasheet, an n-channel enhancement-mode MOSFET in the triode (linear) regime has drain current I_D = 2 K (V_gs - V_ds/2 - V_T) V_ds, while in saturation the same device follows I_D = K (V_gs - V_T)^2. The 2N7000 datasheet reports a typical static drain-source on-resistance of 5 Ω at V_GS = 10 V and I_D = 500 mA, which is a clean triode-region point for back-calculating K.

When the MOSFET in a switching converter must sink the inductor current, the Boost Converter describes the larger power-conversion stage the device is part of.

Four concepts describe the biasing, the regimes, and the geometry of a MOSFET, and they all show up in the calculator inputs and outputs.

Once the device is in saturation, the drain current is independent of V_ds at first order, so the same K value predicts the current for any Vds above the pinch-off point.

According to All About Circuits, an n-channel MOSFET is in cut-off when V_gs is below V_T, in triode when V_gs > V_T and V_ds < V_gs - V_T, and in saturation when V_gs > V_T and V_ds >= V_gs - V_T.

Saturation-mode operation of a power MOSFET is what the high-side switch in the Buck Converter relies on to deliver a regulated output.

Follow these six steps to bias a BS170 with this MOSFET calculator using a K value derived from the datasheet.

1. 1 Read the threshold voltage from the datasheet: Look up V_T in the 'Electrical Characteristics' table. For the BS170 this is 2.0 V typical, and for the BSS138 it is 1.3 V typical.
2. 2 Compute or read the K parameter: Back-calculate K from a datasheet on-resistance point as K ≈ 1 / (2 * R_DS(on) * (V_GS - V_T)) and enter it directly. For the BS170 with R_DS(on) = 1.8 Ω at V_GS = 10 V and V_T = 2 V, that gives K ≈ 0.035 1/(V*ohm).
3. 3 Enter the gate-source voltage V_gs: Type the actual drive voltage you will apply to the gate. For a 3.3 V logic GPIO driving a BSS138, use 3.3 V; for a 10 V gate drive on a BS170, use 10 V.
4. 4 Enter the drain-source voltage V_ds: Type the supply rail the load is connected to. The calculator uses V_ds to decide between the triode and saturation regions.
5. 5 Read the operating regime and drain current: The calculator labels the bias as cut-off, triode, or saturation and shows I_D in amperes. If the device is in cut-off, raise V_gs above V_T; if it is in triode when you expected saturation, raise V_ds above V_ov.
6. 6 Use the result to size the gate drive or load: For switching applications, confirm that the on-state current is well above the load current so the device is fully enhanced. For amplifier applications, confirm that the bias point lands in saturation.

For a BS170 low-side switch at Vgs = 8.0 V, Vds = 5.0 V, Vt = 2.0 V, K = 0.035 1/(V*ohm), the calculator returns I_D = 1.225 A in triode, useful for thermal checks on a 1.0 A load.

Five reasons students, hobbyists, and practising engineers prefer this tool over solving the MOSFET equations by hand each time.

• • Removes regime guesswork: Returns the operating regime (cut-off, triode, saturation) alongside the drain current.
• • Accepts the K parameter directly from the datasheet: One K value already folds in channel width-to-length ratio, oxide capacitance, and mobility.
• • Speeds up bias-point design: Returns I_D and overdrive voltage for any Vgs, Vds, Vt, and K combination, shortening the gate-drive trial loop.
• • Cross-validates hand calculations: Confirms handwritten solutions against a second source before lab work or exams.
• • Supports common small-signal MOSFETs: Works for the BS170, BSS138, 2N7000, and similar n-channel enhancement-mode parts.

Treat the result as a starting point. Real MOSFETs also have channel-length modulation, body effect, and temperature dependence, so bench measurement may differ by 10 to 30 percent.

The same V-I relationships used here are needed when a power MOSFET follows a Bridge Rectifier on the AC side of a power supply.

Four inputs drive the drain current returned by the calculator, and a fifth real-world caveat explains why the bench measurement can differ.

• • The model is the long-channel textbook model. Channel-length modulation, body effect, subthreshold conduction, and temperature drift are not included, so bench measurements may differ by 10 to 30 percent.
• • K is treated as constant for a given device. In practice K depends on the operating point because carrier mobility mu varies with the vertical electric field.

According to the ON Semiconductor BS170 Datasheet, the typical threshold voltage is 2.0 V and the typical static drain-source on-resistance is 1.8 Ω at V_GS = 10 V and I_D = 200 mA. Back-calculating K from that on-resistance point gives K ≈ 0.035 1/(V*ohm) for the BS170, so the device sits well into triode as a low-side switch.

Channel-length modulation matters when the MOSFET in a Flyback Converter operates in saturation, so the simplified K model used here can be a starting point only.