MOSFET Calculator

Find a MOSFET's operating region, drain current, overdrive voltage, and transconductance from the gate-source voltage, threshold voltage, drain-source voltage, and process parameters using the standard square-law model.

Quick Facts

Cutoff region
I_D = 0
No channel forms and no current flows while V_GS ≤ V_th.
Triode (linear) region
I_D = k_n[(V_GS−V_th)V_DS − V_DS²/2]
Applies when 0 < V_DS < V_GS − V_th; the MOSFET acts like a voltage-controlled resistor.
Saturation region
I_D = (k_n/2)(V_GS−V_th)²(1+λV_DS)
Applies when V_DS ≥ V_GS − V_th; the device behaves as a current source.
Overdrive voltage
V_OV = V_GS − V_th
Sets the triode/saturation boundary and scales I_D in saturation.

Your Results

Calculated
Operating Region
-
Cutoff, triode, or saturation
Drain Current (I_D)
-
From the square-law MOSFET model
Overdrive Voltage (V_OV)
-
V_OV = V_GS − V_th
Transconductance (g_m)
-
∂I_D / ∂V_GS at this bias point

Ready

Enter V_GS, V_th, V_DS, k_n, and λ, then press Calculate.

How the MOSFET Drain Current Calculator Works

A MOSFET (metal-oxide-semiconductor field-effect transistor) starts to conduct between drain and source once the gate-source voltage V_GS exceeds the threshold voltage V_th. This calculator uses the standard long-channel "square-law" model for an enhancement-mode transistor to determine which of the three operating regions — cutoff, triode (linear), or saturation — the device sits in for your inputs, then computes the resulting drain current I_D and small-signal transconductance g_m.

Understanding the formula

Everything starts from the overdrive voltage V_OV = V_GS − V_th. If V_OV ≤ 0, the channel never forms and I_D = 0 (cutoff). Otherwise the calculator compares V_DS to V_OV: when V_DS < V_OV, the device is in the triode region and I_D = k_n[(V_GS−V_th)V_DS − V_DS²/2]; when V_DS ≥ V_OV, it is in saturation and I_D = (k_n/2)(V_GS−V_th)²(1+λV_DS). Here k_n = μ_n·C_ox·(W/L) is the process transconductance parameter — it bundles the electron mobility, the gate oxide capacitance per unit area, and the transistor's width-to-length ratio into a single number. Transconductance g_m (how much I_D changes per volt of V_GS) is k_n·V_DS in triode and approximately k_n·V_OV in saturation.

Working with units

  • k_n is commonly specified in mA/V² for discrete power MOSFETs or µA/V² for small-signal IC transistors — pick whichever unit matches your datasheet or SPICE model.
  • Keep all voltages (V_GS, V_th, V_DS) in volts; the calculator returns I_D in µA or mA depending on magnitude, and g_m in the matching per-volt unit.
  • Channel-length modulation λ is in inverse volts (V⁻¹); typical long-channel values run about 0.005–0.05 V⁻¹, and setting λ = 0 gives the idealized flat-saturation approximation.

Knowing the limits

This calculator uses the simplified long-channel square-law model taught in introductory electronics — it does not capture short-channel effects such as velocity saturation or mobility degradation that show up in modern deep-submicron MOSFETs. It also assumes an NMOS device with the source at the reference node (V_SB = 0, no body effect); for a PMOS device, apply the same equations to the magnitudes of V_SG, V_th, and V_SD. For hardware design work, verify results against the manufacturer's datasheet or a SPICE simulation.

Frequently Asked Questions

What is the difference between the triode and saturation regions?
In the triode (linear) region, the MOSFET behaves like a voltage-controlled resistor and drain current depends on both V_GS and V_DS; this happens when V_DS is less than the overdrive voltage (V_GS − V_th). In saturation, which occurs once V_DS reaches or exceeds that overdrive voltage, drain current depends mainly on V_GS and the device acts as a current source — the region used for amplification.
What is the overdrive voltage?
The overdrive voltage is V_OV = V_GS − V_th, how far the gate-source voltage exceeds the threshold voltage. It marks the boundary between the triode and saturation regions (at V_DS = V_OV) and, in saturation, drain current is proportional to V_OV squared.
What does channel-length modulation (λ) do?
λ models the small increase in drain current with V_DS in saturation, caused by the effective channel length shrinking as the drain depletion region widens. Set λ to 0 for the ideal flat-saturation approximation, or use your device's datasheet or SPICE value (often 0.01–0.05 V⁻¹) for a more realistic slope.
How do I find k_n for my transistor?
k_n = μ_n·C_ox·(W/L) combines the electron mobility, the gate oxide capacitance per unit area, and the transistor's width-to-length ratio. You can read it from a SPICE model's KP parameter multiplied by W/L, or estimate it from two points on the device's I_D vs. V_GS saturation curve using k_n = 2·I_D / V_OV².

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