MOSFET Calculator
Calculate MOSFET drain current in cut-off, triode, and saturation regions. Free online semiconductor calculator with I-V characteristic charts for engineers and students.
About This Calculator
The MOSFET Calculator computes the drain current and operating region of a metal-oxide-semiconductor field-effect transistor (MOSFET) based on its threshold voltage, transconductance parameter, and bias voltages. This tool is essential for electronics engineers, semiconductor students, hobbyists, and circuit designers working with MOSFET-based amplifiers, switches, and power electronics.
Using the standard MOSFET current equations, the calculator determines whether the device operates in cut-off (off state), triode/linear region (voltage-controlled resistor), or saturation region (constant current source). The triode current follows ID = 2K(VGS − VDS/2 − VT)VDS, and the saturation current follows ID = K(VGS − VT)². The K parameter (transconductance parameter) can be found in the MOSFET datasheet or computed from K = ½(W/L)μₙCₒₓ, where W/L is the channel aspect ratio, μₙ is the carrier mobility, and Cₒₓ is the oxide capacitance.
Operating Regions
- Cut-off: VGS < VT — no conductive channel, ID ≈ 0 (transistor is off)
- Triode (Linear): VGS ≥ VT and VDS < VGS − VT — channel acts as a variable resistor
- Saturation: VGS ≥ VT and VDS ≥ VGS − VT — channel pinched off, ID constant, ideal for amplifiers
The interactive I-V characteristic chart displays drain current versus drain-source voltage for multiple gate-source voltage levels, helping you visualize the transition between operating regions and select the appropriate bias point for your application.
Popular MOSFETs include the BS170 (VT ≈ 2.0 V, used in low-power switching), BSS138 (VT ≈ 1.3 V, logic-level MOSFET), IRF520 (power MOSFET), and 2N7000 (general-purpose switching). Always verify the rated maximum VDS and ID from the datasheet before designing your circuit.
Frequently Asked Questions
What is a MOSFET and how does it work?
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled semiconductor device that regulates current between source and drain terminals using an electric field applied at the gate electrode. It operates in three regions: cut-off (off), triode/linear (variable resistor), and saturation (constant current source), depending on the gate-source voltage relative to the threshold voltage and the drain-source voltage.
How do you calculate the drain current of a MOSFET?
The drain current ID depends on the operating region. In cut-off (VGS < VT), ID = 0. In the triode region (VGS ≥ VT and VDS < VGS − VT), ID = 2 × K × (VGS − VDS/2 − VT) × VDS. In saturation (VGS ≥ VT and VDS ≥ VGS − VT), ID = K × (VGS − VT)², where K is the transconductance parameter in A/V², VT is the threshold voltage, VGS is the gate-source voltage, and VDS is the drain-source voltage.
What is the K parameter in a MOSFET?
The K parameter, also called the transconductance parameter, is K = ½ × (W/L) × μₙ × Cₒₓ, where W/L is the channel width-to-length ratio, μₙ is the electron mobility, and Cₒₓ is the oxide capacitance per unit area. Typical values range from 0.01 to 1 A/V² for power MOSFETs. You can find K in the MOSFET datasheet or compute it from the saturation region I-V characteristics.
What is the difference between triode and saturation regions?
In the triode (linear) region, the MOSFET behaves like a voltage-controlled resistor where drain current depends strongly on both VGS and VDS. In saturation, the channel is pinched off near the drain, and the drain current becomes independent of VDS — the transistor acts as a constant current source controlled by VGS. The boundary is VDS = VGS − VT.
Can I use this calculator for p-channel MOSFETs?
Yes, the same equations apply to p-channel MOSFETs if you use absolute voltage values. For a p-channel (PMOS) device, VT is negative, VGS must be more negative than VT to turn on, and the current polarity reverses. Enter the threshold voltage as a positive value and use the magnitude of the gate-source and drain-source voltages.
What is transconductance (gm) in a MOSFET?
Transconductance gm = dID/dVGS is a measure of how effectively the gate voltage controls the drain current. In saturation, gm = 2 × K × (VGS − VT) = 2 × √(K × ID). Higher gm means greater amplification capability, which is why MOSFETs with larger K parameters and higher overdrive voltages (VGS − VT) are preferred in analog amplifier designs.
How do I find the threshold voltage of my MOSFET?
The threshold voltage VT is specified in the MOSFET datasheet's electrical characteristics section. For example, the BS170 has a typical VT of 2.0 V, while the BSS138 has VT ≈ 1.3 V. You can also use our dedicated MOSFET Threshold Voltage Calculator at calculy.in/physics/mosfet-threshold-voltage to compute VT from oxide capacitance, doping concentration, and surface potential.
What happens when VDS exceeds the breakdown voltage?
When VDS exceeds the drain-source breakdown voltage (BVDS), the MOSFET enters avalanche breakdown where the drain current increases rapidly regardless of gate voltage. This can destroy the device if not current-limited. This calculator assumes normal operating conditions below breakdown — always check the maximum VDS rating in the datasheet.