MOSFET Calculator

Analysis Scope

Channel Type

Gate-to-Source Voltage (|V_GS| in Volts)

Drain-to-Source Voltage (|V_DS| in Volts)

Threshold Voltage (|V_th| in Volts)

Transconductance Parameter (k = μ · Cox · W/L in mA/V²)

Channel-Length Modulation Parameter (λ in 1/V)

Drain Current RMS / On-State (I_D in Amperes)

On-Resistance at 25°C (R_DS(on) in mΩ)

DC Bus / Drain Supply Voltage (V_DS in Volts)

Switching Frequency (f_sw in kHz)

Total Switching Transition Time (t_r + t_f in ns)

Ambient Operating Temperature (T_a in °C)

Thermal Resistance Junction-to-Ambient (R_θJA in °C/W)

MOSFET Analysis Results

Drain Current (I_D) 0.00 mA

Operating Region & Overdrive Voltage —

Small-Signal Parameters & Resistances —

Thermal Limits & Engineering Guidance —

DC calculations use the standard gradual-channel approximation (Level 1 Shichman-Hodges model). Power loss estimations account for resistive conduction heating with positive thermal coefficients (α ≈ 0.5%/°C) alongside linear hard-switching dynamic turn-on and turn-off cross-over losses.

What Is a MOSFET Calculator?

A MOSFET calculator estimates transistor operating behavior or power dissipation from user-entered electrical parameters. In DC analysis mode, it calculates drain current, operating region, transconductance, and output resistance. In power analysis mode, it estimates conduction and switching losses, total power dissipation, and junction temperature using current, resistance, voltage, switching, and thermal inputs.

A MOSFET is a voltage-controlled semiconductor device commonly used for switching and amplification. Its gate-to-source voltage controls the conductive channel between the drain and source terminals.

The calculator offers two analysis modes. The DC mode uses a simplified Level 1 Shichman-Hodges transistor model to estimate electrical behavior. The power mode uses temperature-dependent on-resistance and a simplified hard-switching loss model to estimate heating.

These are mathematical estimates based on the entered values. Actual MOSFET behavior also depends on device specifications, circuit conditions, and characteristics not fully represented by the models.

How the MOSFET Calculator Formulas Work

The calculator uses different equations depending on the selected analysis scope. DC analysis determines the transistor’s operating region and associated electrical parameters. Power analysis estimates losses and junction temperature.

1. MOSFET Overdrive Voltage

In DC mode, the calculator first determines the overdrive voltage. This is the amount by which the gate-to-source voltage magnitude exceeds the threshold voltage magnitude.

V_{ov}=|V_{GS}|-|V_{th}|

Where:

Vov = Overdrive voltage in volts
VGS = Gate-to-source voltage in volts
Vth = MOSFET threshold voltage in volts

The calculator uses the absolute values of the entered voltages for both NMOS and PMOS selections. It also calculates the drain-to-source saturation boundary:

V_{DS,sat}=\max(0,V_{ov})

The overdrive voltage and drain-to-source voltage magnitude determine which operating-region formula applies.

2. Drain Current and MOSFET Operating Regions

The calculator classifies the MOSFET into three operating regions.

Cutoff Region: When the overdrive voltage is zero or negative, the model sets the drain current to zero.

I_D=0\quad\text{when }V_{ov}\leq0

Triode or Linear Region: When overdrive voltage is positive and the drain-to-source voltage magnitude is below the saturation boundary, the calculator uses:

I_D=k\left(V_{ov}|V_{DS}|-\frac{|V_{DS}|^2}{2}\right)(1+\lambda|V_{DS}|)

Saturation or Active Region: When overdrive voltage is positive and the drain-to-source voltage magnitude reaches or exceeds the saturation boundary, the equation becomes:

I_D=\frac{k}{2}V_{ov}^2(1+\lambda|V_{DS}|)

In these equations:

ID = Drain current, calculated in milliamperes (mA)
k = Transconductance parameter in mA/V²
VDS = Drain-to-source voltage in volts
λ = Channel-length modulation parameter in inverse volts (1/V)

Channel-length modulation accounts for a simplified dependence of drain current on drain-to-source voltage. When λ is zero, the model omits this effect.

The calculator displays drain current in milliamperes when it is below 1,000 mA. At 1,000 mA or more, it converts the result to amperes. Both formats use three decimal places.

3. Transconductance and Output Resistance

Transconductance, represented by gm, describes how strongly drain current responds to changes in gate-to-source voltage.

For the triode region, the calculator uses:

g_m=k|V_{DS}|(1+\lambda|V_{DS}|)

For the saturation region:

g_m=kV_{ov}(1+\lambda|V_{DS}|)

In cutoff, gm is zero. The calculator displays transconductance in millisiemens (mS), equivalent to milliamperes per volt.

In the saturation region, when λ is greater than zero, the calculator estimates small-signal output resistance using:

r_o=\frac{1+\lambda|V_{DS}|}{\lambda I_D}

With drain current expressed in milliamperes, the formula gives output resistance in kilohms (kΩ). The calculator displays infinite output resistance when λ is zero or when the transistor is in cutoff or triode operation.

For the triode region, it additionally reports an approximate small-signal on-resistance:

R_{on}\approx\frac{1000}{kV_{ov}}

This approximation is displayed in ohms (Ω) and uses the entered k value in mA/V².

4. MOSFET Conduction Loss

In power analysis mode, conduction loss represents the estimated power dissipated through the MOSFET’s on-state resistance.

P_{cond}=I_{D,rms}^2R_{DS(on),hot}

Where ID,rms is the entered RMS/on-state drain current in amperes and RDS(on),hot is the estimated on-resistance at the calculated junction temperature, expressed in ohms.

The calculator adjusts the entered 25°C on-resistance using a fixed temperature coefficient of 0.005 per degree Celsius:

R_{DS(on),hot}=R_{DS(on),25}\left[1+0.005\max(0,T_j-25)\right]

Here, Tj is junction temperature in degrees Celsius. The formula increases resistance when junction temperature exceeds 25°C. It does not reduce resistance below the entered 25°C value when the estimated junction temperature is lower.

5. MOSFET Switching Loss

Switching loss estimates energy dissipated while the MOSFET transitions between conducting and nonconducting states.

P_{sw}=\frac{1}{2}V_{bus}I_{D,rms}(t_r+t_f)f_{sw}

Where Vbus is the DC bus voltage, tr + tf is the entered total switching transition time in seconds, and fsw is switching frequency in hertz.

The calculator converts the entered switching frequency from kilohertz to hertz and switching transition time from nanoseconds to seconds.

For a positive switching frequency, switching energy per cycle is:

E_{sw}=\frac{P_{sw}}{f_{sw}}

The result is displayed in microjoules (µJ). If switching frequency is zero, the calculator reports zero switching power and zero switching energy.

6. Total Power Dissipation and Junction Temperature

The main result in power mode is total estimated power dissipation:

P_{total}=P_{cond}+P_{sw}

The calculator then estimates junction temperature using ambient temperature and thermal resistance:

T_j=T_a+P_{total}R_{\theta JA}

Where Ta is ambient temperature in °C and RθJA is junction-to-ambient thermal resistance in °C/W.

Because conduction resistance changes with junction temperature, the calculator solves these relationships iteratively. It begins with a temperature estimate 20°C above ambient and repeatedly updates resistance, power loss, and temperature. It allows up to 20 iterations, with a convergence check of 0.05°C, and flags calculations that predict a temperature above 350°C during an iteration.

The resulting total power dissipation is shown in watts, together with the conduction and switching loss amounts and their percentage contributions.

Worked Example: DC MOSFET Drain Current

Consider these hypothetical DC analysis inputs:

Channel type: N-Channel MOSFET
Gate-to-source voltage: 5 V
Drain-to-source voltage: 4 V
Threshold voltage: 1.5 V
Transconductance parameter: 2.5 mA/V²
Channel-length modulation parameter: 0.02 1/V

First, calculate the overdrive voltage:

V_{ov}=5-1.5=3.5\text{ V}

Since the drain-to-source voltage of 4 V exceeds the saturation boundary of 3.5 V, the calculator selects the saturation-region equation:

I_D=\frac{2.5}{2}(3.5)^2(1+0.02\times4)

I_D=16.5375\text{ mA}\approx16.538\text{ mA}

The calculator also reports transconductance of 9.450 mS, output resistance of 3.27 kΩ, overdrive voltage of 3.500 V, and a drain-to-source saturation boundary of 3.500 V. The displayed operating state is Saturation / Active (Pinch-off) Region.

Worked Example: MOSFET Power Dissipation

For a separate power-mode example, use a 2 A drain current, 20 mΩ on-resistance at 25°C, 24 V bus voltage, 100 kHz switching frequency, 40 ns total transition time, 25°C ambient temperature, and 50°C/W junction-to-ambient thermal resistance.

The switching loss is:

P_{sw}=0.5\times24\times2\times(40\times10^{-9})\times100000=0.096\text{ W}

After the calculator’s temperature-dependent conduction-loss iterations, the displayed results are approximately:

Power Analysis Output	Result
Total Power Dissipation	0.18 W
Conduction Loss	0.08 W (46.5%)
Switching Loss	0.10 W (53.5%)
Effective Hot On-Resistance	20.90 mΩ
Switching Energy per Cycle	0.96 µJ
Estimated Junction Temperature	34.0°C
Margin Below 150°C	116.0°C
Displayed Thermal Status	SAFE OPERATION

The SAFE OPERATION label reflects the calculator’s fixed temperature limits. It does not verify every safe operating condition for an actual MOSFET.

How to Use the MOSFET Calculator

Start by choosing the analysis scope. The available inputs change depending on the selected mode.

DC Operating Region and Drain Current Mode

Select Analysis Scope: Choose DC Operating Region & Drain Current (ID, gm, ro). This is the default mode.
Choose Channel Type: Select N-Channel MOSFET (NMOS) or P-Channel MOSFET (PMOS).
Enter Gate-to-Source Voltage: Provide VGS in volts.
Enter Drain-to-Source Voltage: Provide VDS in volts.
Enter Threshold Voltage: Provide Vth in volts.
Enter Transconductance Parameter: Specify k in mA/V². It must be positive.
Enter Channel-Length Modulation: Provide λ in 1/V. Leaving it blank or entering a negative value makes the calculator use zero.
Click Calculate: Read the drain current, operating region, overdrive voltage, saturation boundary, transconductance, output resistance, and any region-specific explanation.

The DC calculation uses voltage magnitudes, so negative entered voltage values are converted to their absolute values. The selected NMOS or PMOS type changes the displayed channel identification, but both use the same magnitude-based equations.

Power Dissipation and Thermal Sizing Mode

Select Analysis Scope: Choose Power Dissipation & Thermal Sizing (Pcond, Psw, Tj).
Enter Drain Current: Provide RMS/on-state drain current in amperes.
Enter On-Resistance: Specify RDS(on) at 25°C in milliohms.
Enter DC Bus Voltage: Provide the drain supply voltage in volts.
Enter Switching Frequency: Specify switching frequency in kilohertz.
Enter Total Switching Transition Time: Provide the sum of rise and fall times in nanoseconds.
Enter Ambient Temperature: Provide the operating temperature in °C. A blank or nonnumeric entry uses 25°C.
Enter Thermal Resistance: Provide junction-to-ambient thermal resistance in °C/W. A blank, invalid, zero, or negative entry uses 50°C/W.
Click Calculate: Review the total power dissipation, loss breakdown, hot on-resistance, switching energy, junction temperature, and thermal status.

Current, on-resistance, and bus voltage must be positive. Switching frequency and transition time may be zero but cannot be negative. The calculator displays an alert for invalid required inputs.

Click Reset to clear the numeric inputs, return to the default DC analysis and NMOS channel selections, and hide the results.

Understanding Your MOSFET Calculator Results

DC Operating Region Results

The operating region describes how the transistor behaves within the calculator’s simplified model.

Cutoff: The model reports zero drain current and zero transconductance, representing a nonconducting switch. It does not calculate real subthreshold leakage.
Triode / Linear: The device behaves approximately like a voltage-controlled resistance. The calculator also reports an approximate small-signal on-resistance.
Saturation / Active: The model represents the MOSFET as a voltage-controlled current source and calculates output resistance when channel-length modulation is positive.

These operating-region names are based on the Level 1 transistor model. The saturation region in this context is different from the low-voltage, fully enhanced on-state commonly sought in switching applications.

Thermal Status and Temperature Limits

In power mode, the calculator compares estimated junction temperature against a fixed maximum junction limit of 150°C. Its thermal status follows these conditions:

Condition	Displayed Thermal Status
Estimated junction temperature at or below 125°C, without an excessive-temperature flag	SAFE OPERATION
Above 125°C and at or below 150°C	WARNING: High Thermal Stress
Above 150°C, or an iteration triggers the 350°C excessive-temperature condition	EXCEEDED MAX JUNCTION LIMIT!

For the first two conditions, the calculator displays the temperature margin below 150°C. If the maximum limit is exceeded, it displays guidance concerning cooling, heatsinking, or lowering on-resistance.

The 150°C limit is a fixed value in this tool, not a device-specific rating. Real MOSFETs have different maximum junction temperatures and safe operating limits. A displayed SAFE OPERATION result should not be treated as a complete safety assessment.

Important Calculation Assumptions

The DC analysis uses a simplified transistor model rather than measured device curves. It does not individually model subthreshold leakage, body effect, temperature-dependent threshold shifts, or short-channel behavior.

The power analysis uses a linear temperature coefficient for on-resistance and a simplified switching overlap-loss equation. It does not separately calculate gate-drive loss, reverse-recovery loss, output-capacitance loss, or transient thermal impedance.

The thermal calculation also assumes the entered junction-to-ambient resistance can represent the evaluated cooling conditions. Different packages, circuit boards, and cooling arrangements can produce different real temperatures.

Use the results as preliminary engineering estimates and compare them with the selected MOSFET’s datasheet and expected operating conditions.

Frequently Asked Questions

How do I calculate MOSFET drain current?

MOSFET drain current depends on gate-to-source voltage, drain-to-source voltage, threshold voltage, and transistor parameters. This calculator first determines overdrive voltage and operating region. It then applies the appropriate cutoff, triode, or saturation equation and displays the resulting drain current in milliamperes or amperes.

What is the difference between the triode and saturation regions?

In the triode region, a MOSFET’s drain current depends strongly on drain-to-source voltage, and the transistor can behave like a controlled resistance. In saturation, the simplified model behaves more like a controlled current source. The calculator distinguishes these regions by comparing drain-to-source voltage magnitude with the calculated overdrive voltage.

Can I calculate both NMOS and PMOS drain current?

Yes. The DC analysis mode includes NMOS and PMOS selections. The calculator applies the same equations to the absolute values of the entered voltages and identifies the selected channel type in the result. It does not implement separate signed-voltage current equations for NMOS and PMOS.

How do I calculate MOSFET power dissipation?

The calculator adds estimated conduction loss and switching loss. Conduction loss depends on RMS drain current and temperature-adjusted on-resistance. Switching loss depends on bus voltage, drain current, total transition time, and switching frequency. The combined result is displayed as total power dissipation in watts.

Why does MOSFET on-resistance increase with temperature?

On-resistance in many power MOSFETs increases as junction temperature rises because temperature affects carrier mobility. The calculator models this behavior with a fixed positive coefficient of 0.5% per °C above 25°C. Actual resistance changes depend on the device and may not follow this simple linear relationship exactly.

What happens when switching frequency is zero?

In power analysis mode, zero switching frequency is allowed. The calculator then determines zero switching power and displays zero switching energy per cycle. Conduction loss is still calculated using drain current and on-resistance, and the resulting power contributes to the estimated junction temperature.

How accurate is the MOSFET junction temperature estimate?

The junction temperature is an estimate derived from the calculated power loss, entered ambient temperature, and thermal resistance. Accuracy depends on these assumptions and the simplified loss model. Real temperatures can differ because of packaging, PCB layout, cooling conditions, switching behavior, and device-specific electrical characteristics.

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