MOSFET Calculator for Drain Current and Power

A metal-oxide-semiconductor field-effect transistor controls drain current with an electric field produced by its gate voltage. The gate is insulated from the channel, so an ideal MOSFET draws essentially no steady-state gate current. This combination of voltage control, high input resistance, and scalable construction makes MOSFETs fundamental to digital logic, analog amplifiers, switching power supplies, motor drives, and countless embedded systems. This calculator applies the long-channel square-law model to an enhancement-mode n-channel MOSFET. It first computes overdrive voltage as gate-source voltage minus threshold voltage. If overdrive is zero or negative, the channel is treated as off and the device is in cutoff. When drain-source voltage is positive but below overdrive, the device operates in the triode or linear region. At a drain-source voltage equal to or greater than overdrive, the channel pinches off near the drain and the model uses the saturation equation. The transconductance parameter k combines carrier mobility, oxide capacitance, and channel width-to-length ratio. Here it is entered in milliamperes per volt squared, so calculated current is in milliamperes and transconductance is in millisiemens. Channel-length modulation lambda accounts for the modest increase of saturation current as drain voltage rises. Set lambda to zero for the ideal square-law model, or use a positive value fitted from a datasheet output curve. Power dissipation is drain current multiplied by drain-source voltage. The calculator converts milliamperes to amperes before displaying watts. This is only the static channel contribution; practical thermal design must also consider switching losses, gate-drive loss, diode conduction, package thermal resistance, ambient conditions, and transient loading. Always compare the operating point with the device's safe operating area rather than relying on its headline current rating alone. Real power MOSFETs and short-channel integrated devices can depart substantially from the simple equations. Temperature shifts threshold voltage and on-resistance, body effect changes threshold when source and bulk differ, and mobility falls at high field. Datasheet transfer curves, SPICE models, and measured component variation provide better final verification. The square-law result remains valuable for education, first-pass bias design, and understanding how gate overdrive and drain voltage determine operating region.