MOSFET Calculator — Drain Current, Triode & Saturation Regions

Our MOSFET calculator will teach you everything you need to know about this ubiquitous electronic component, from the basics to the calculations of the MOSFET operations. Discover the component that, since the 60s, revolutionized the world.

Here you will learn:

• What are MOSFETs?
• What is the structure of a MOSFET?
• How does a MOSFET work?
• The operating regimes of a MOSFET.
• The triode and saturation MOSFET current equations.
• How to use our MOSFET calculator.

What are MOSFETs?

MOSFET stands for "metal-oxide-semiconductor field-effect transistor": a name that fills one's mouth for sure. Let's learn what it means.

• Metal-oxide-semiconductor is a reference to the structure of the device. We will shortly analyze these in detail.
• Field-effect transistor means that a MOSFET is a device able to control an electric current using an electric field (in contrast with other types of transistors, where the current is controlled by a smaller current).

🔎 The older bipolar junction transistors operate similarly to MOSFETs but with substantial differences.

As for other transistors, the MOSFET has three terminals:

• Source;
• Drain; and
• Gate.

In a few words, a change in the electric field on the gate electrode causes a variation in the electric conductance of a channel between source and drain. Imposing a potential difference between source and drain, we can control the resulting current I_D through a variation in V_g, the gate voltage, until the device reaches a saturation state where it behaves, substantially, like a shorted circuit.

The structure of a MOSFET

MOSFETs rely on semiconductor technologies and a pinch of solid-state physics. The first element we meet is a semiconductor bulk called substrate. The substrate is lightly doped with impurities, which defines the type of MOSFET we are working on:

• p-doped substrate, used in n-type MOSFETs; and
• n-doped substrate, used for p-type MOSFETs.

In the substrate, we create two wells of doped semiconductor, either n+ or p+, depending on the type of MOSFET. The wells become the source and drain electrodes of the device. A stretch of semiconducting material called a channel separates the two terminals.

The surface of the semiconductor is oxidated, creating a layer of dielectric, which insulates the device. Above the channel, we can find a metal electrode (the gate), which we use to tune the electric field above the channel. The electrodes and the semiconductor make contact by employing openings in the oxide obtained through etching. The gate is where we finally see the metal-oxide-semiconductor structure of a MOSFET!

There are two types of MOSFETs:

• n-type MOSFET — electrons are the charge carriers in the channel (a channel for electrons opens in a large p-doped well).
• p-type MOSFET — holes are the charge carriers (a channel for holes opens in a large n-doped well).

🙋 MOSFETs are symmetric. Reversing the polarity of the bias only reverses the direction of the current flowing in the device: this is opposite to the behavior of other types of transistors (such as the bipolar junction transistor), where the component can work only in a specified direction.

How does a MOSFET work?

In an n-type MOSFET, applying a positive gate voltage V_GS creates an electric field through the oxide layer. This field repels holes from the p-type substrate directly beneath the gate, creating a depletion region. As V_GS increases, a thin layer of electrons — an inversion layer — forms at the oxide–semiconductor interface. This inversion layer is the channel that allows current to flow between source and drain.

The minimum gate voltage required to form the channel is the threshold voltage, V_th. The quantity V_ov = V_GS − V_th is called the overdrive voltage: it controls how strongly the channel conducts.

The regimes of operation of a MOSFET: cut-off, triode, and saturation

Depending on the applied voltages, an n-type MOSFET operates in one of three regimes:

• Cut-off region (V_GS < V_th): No channel forms, so the device is OFF and ideally I_D = 0. The MOSFET behaves like an open switch.
• Triode (linear/ohmic) region (V_GS > V_th and V_DS < V_GS − V_th): A continuous channel connects source and drain, and the MOSFET behaves like a voltage-controlled resistor. The drain current is:

  I_D = k · [(V_GS − V_th)·V_DS − V_DS²/2]

• Saturation (active) region (V_GS > V_th and V_DS ≥ V_GS − V_th): The channel "pinches off" near the drain, and the current becomes almost independent of V_DS. This is the region used for amplification:

  I_D = ½ · k · (V_GS − V_th)² · (1 + λ·V_DS)

The conduction parameter k (also called the transconductance parameter) collects the device's physical and geometrical properties:

k = μ · C_ox · (W / L),   C_ox = ε_ox / t_ox

Where:

• μ — carrier mobility (electrons for NMOS, holes for PMOS) in cm²/V·s;
• C_ox — gate-oxide capacitance per unit area;
• ε_ox — permittivity of the SiO₂ gate oxide (3.9 × ε₀);
• t_ox — gate-oxide thickness;
• W / L — the aspect ratio of the channel (width over length);
• λ — channel-length-modulation parameter (the "Early effect" for MOSFETs).

For a p-type MOSFET, the same equations hold using the magnitudes of the voltages: the device conducts when |V_GS| > |V_th| (i.e., V_GS and V_th are negative), and the current flows from source to drain.

How to use our MOSFET calculator

1. Select the MOSFET type — N-channel (NMOS) or P-channel (PMOS).
2. Choose a unit system — Metric (µm for W and L, nm for oxide thickness) or American (mils for W and L, Ångström for oxide thickness). Voltages and current are the same in both systems.
3. Enter the carrier mobility μ in cm²/V·s. Typical: ~600 for electrons, ~250 for holes in silicon.
4. Enter the channel width W and length L. Only their ratio W/L matters for the current.
5. Enter the gate oxide thickness t_ox.
6. Enter the threshold voltage V_th (use a negative value for a PMOS).
7. Enter the gate-source voltage V_GS and the drain-source voltage V_DS.
8. Optionally enter λ (channel-length modulation) to include the slight current increase with V_DS in saturation. Leave it at 0 for the ideal model.
9. The calculator detects the operating regime automatically and returns the drain current I_D, the conduction parameter k, the transconductance g_m, and more.

Frequently Asked Questions

What does a MOSFET do?

A MOSFET uses a voltage on its gate terminal to control the current flowing between its source and drain. It works as an electronically controlled switch (in digital logic) or as an amplifier (in analog circuits).

How do I know which regime my MOSFET is in?

Compute the overdrive voltage V_ov = V_GS − V_th. If V_ov ≤ 0, the device is in cut-off. If V_ov > 0 and V_DS < V_ov, it is in the triode region. If V_DS ≥ V_ov, it is in saturation.

What is the conduction parameter k?

k = μ·C_ox·(W/L) bundles together the carrier mobility, the oxide capacitance, and the channel aspect ratio. It sets how much current the transistor delivers for a given overdrive voltage.

Why does the saturation current keep rising slightly with V_DS?

Because of channel-length modulation, modeled by the λ term: the effective channel length shrinks slightly as V_DS grows, raising the current. The factor (1 + λ·V_DS) captures this.

What is the difference between a MOSFET and a BJT?

A MOSFET is controlled by a gate voltage and draws essentially no gate current, making it highly efficient and easy to drive. A bipolar junction transistor (BJT) is controlled by a base current. MOSFETs dominate digital electronics; BJTs are still common in certain analog and high-frequency applications.