MOSFET Calculator: Drain Current, gm, and Power Dissipation
Enter your MOSFET parameters to calculate drain current (I_D), transconductance (g_m), output conductance (g_ds), and power dissipation. The calculator automatically identifies the operating region (cut-off, triode, or saturation) and applies the correct square-law formula. Supports both N-channel and P-channel enhancement-mode devices, with optional channel length modulation for more accurate saturation-region results.
How a MOSFET works: the square-law model
A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a voltage-controlled current device. The gate voltage controls a thin inversion layer (channel) between the drain and source through a thin insulating oxide layer. Because almost no current flows into the gate, the input impedance is extremely high. The square-law model divides operation into three regions. In cut-off (V_GS below the threshold V_th), no channel forms and the drain current is effectively zero. In the triode (linear) region, the channel exists across the full length and the device behaves like a voltage-controlled resistor. In saturation, the channel pinches off at the drain end and the current becomes nearly constant, controlled mainly by the overdrive voltage V_ov = V_GS - V_th. The transconductance parameter K = (mu * Cox * W) / (2 * L) captures the device geometry and process technology. A wider channel (larger W), thinner oxide (larger Cox), or higher mobility (larger mu) all increase K and the resulting drain current. Shorter channel length L also increases K but can introduce short-channel effects that the simple square-law model does not capture.
Channel length modulation and the output resistance r_o
In an ideal square-law model, the saturation current is independent of V_DS. In real devices, increasing V_DS causes the pinch-off point to move slightly toward the source, effectively shortening the channel and increasing drain current. This channel length modulation is modeled by multiplying the saturation current by (1 + lambda * V_DS), where lambda (in V^-1) is the channel length modulation parameter. Channel length modulation creates a finite output resistance r_o = 1 / (lambda * I_D), which limits the voltage gain of MOSFET amplifiers. Longer-channel devices have smaller lambda and higher r_o, which is why analog designers prefer longer channels for current sources and cascode stages even though this reduces speed. Shorter-channel digital MOSFETs have larger lambda (lower r_o) but offer higher transconductance per unit width. Setting lambda = 0 in this calculator gives the ideal model result, which is useful for quick estimates. Use a non-zero lambda for more accurate saturation-region simulation, particularly in amplifier design.
N-channel vs P-channel enhancement MOSFETs
N-channel enhancement MOSFETs require a positive V_GS exceeding a positive V_th to turn on. Electrons are the majority carrier, and electron mobility in silicon (about 450 cm^2/V.s) is roughly 2.5 times higher than hole mobility, giving N-channel devices lower on-resistance for the same die area. N-channel MOSFETs are used as low-side switches and are the dominant type in CMOS digital logic (NMOS) and power converters. P-channel enhancement MOSFETs turn on when V_GS is more negative than V_th (which is itself negative, e.g., -2 V). Holes are the carrier with lower mobility, so P-channel devices are larger for the same on-resistance. However, a P-channel MOSFET can be driven directly from a logic output referenced to the supply rail without a bootstrap circuit, making it convenient as a high-side switch in simple power applications. In complementary CMOS (CMOS) logic, one NMOS and one PMOS transistor are paired so that only one conducts at a time, dramatically reducing static power dissipation compared to single-type circuits.
Transconductance, gain, and amplifier design
Transconductance g_m = dI_D / dV_GS is the most important small-signal parameter for amplifier design. In saturation, g_m = 2 * K * V_ov, so a larger overdrive voltage increases g_m and hence voltage gain. For a common-source amplifier with drain resistance R_D, the voltage gain is approximately -g_m * R_D (negative because increasing V_GS increases I_D, which drops more voltage across R_D and reduces V_DS). The output resistance r_o = 1 / (lambda * I_D) limits the maximum achievable gain to A_v(max) = g_m * r_o = 2 / (lambda * V_ov). This figure of merit, called the intrinsic gain, is the upper bound on gain from a single MOSFET stage. Reducing lambda (longer channel) or reducing V_ov (lower bias current, but also lower g_m) both increase intrinsic gain. In the triode region, the MOSFET on-resistance is approximately 1 / (2 * K * V_ov) for V_DS much smaller than V_ov. Maximizing V_ov (driving V_GS well above V_th) minimises on-resistance and conduction losses in switching applications.
MOSFET operating regions
| Region | Condition | I_D formula | Typical use |
|---|---|---|---|
| Cut-off | V_GS < V_th (V_ov <= 0) | I_D = 0 | Digital OFF state |
| Triode (Linear) | V_GS >= V_th and V_DS <= V_GS - V_th | I_D = K[2*V_ov*V_DS - V_DS^2] | Analog switch, on-resistance |
| Saturation | V_GS >= V_th and V_DS > V_GS - V_th | I_D = K*V_ov^2*(1+lambda*V_DS) | Amplifier, current source |
Conditions and formulas for each region of MOSFET operation (enhancement-mode, N-channel). For P-channel, replace all voltages with their negatives.
Frequently asked questions
What is the threshold voltage V_th of a MOSFET?
The threshold voltage V_th is the minimum gate-source voltage required to create an inversion layer (conducting channel) between the drain and source. Below V_th, the device is off and no significant current flows. V_th depends on the oxide thickness, doping concentration, and manufacturing process. Typical values range from 0.3 V to 0.7 V for low-voltage CMOS logic MOSFETs to 2 V to 5 V for power MOSFETs. V_th decreases slightly with temperature, which can cause thermal runaway in parallel devices if not managed.
What is the difference between the triode and saturation region?
In the triode (linear) region, V_DS is small relative to the overdrive voltage V_ov = V_GS - V_th, so the channel exists continuously from source to drain and the MOSFET acts like a resistor whose value is controlled by V_GS. In the saturation region, V_DS exceeds V_ov and the channel pinches off at the drain end. The drain current becomes nearly constant (set by V_GS, not V_DS), making the device useful as a voltage-controlled current source for amplifiers. For power switching, designers use the triode region (deep saturation for digital: low V_DS on-state) to minimize conduction losses.
How do I calculate the K parameter from datasheet values?
The transconductance parameter K = (mu * Cox * W) / (2 * L). If you know the process parameters, enter them in the geometry section of this calculator. Alternatively, K can be extracted from a datasheet using the saturation equation: rearranging I_D = K * (V_GS - V_th)^2 gives K = I_D / (V_GS - V_th)^2. Use a datasheet operating point in saturation (where V_DS > V_GS - V_th) to extract K directly.
Why does the drain current still increase slightly in saturation?
In an ideal MOSFET, saturation current is independent of V_DS. In practice, increasing V_DS shortens the effective channel length (channel length modulation), which increases current. This is modeled by the lambda term: I_D = K * V_ov^2 * (1 + lambda * V_DS). The lambda parameter is typically small (0.01 to 0.1 V^-1) but becomes significant at high V_DS or for very short-channel devices. It creates a finite output resistance r_o = 1 / (lambda * I_D) that limits amplifier gain.
How does temperature affect MOSFET parameters?
Temperature has two main effects on MOSFETs. Carrier mobility decreases with temperature (roughly proportional to T^-1.5), which reduces K and hence drain current at a given bias - this is the dominant effect at normal operating temperatures. Threshold voltage also decreases by about 2 to 4 mV per degree Celsius. For power MOSFETs, the positive temperature coefficient of on-resistance (from reduced mobility) is actually beneficial for parallel operation: a device running hotter than its neighbors draws less current, providing natural current sharing. However, if drain current is set by a fixed V_GS (not via feedback), the negative V_th coefficient can cause more current at higher temperature, which in some bias conditions leads to thermal runaway.
What is transconductance and why does it matter?
Transconductance g_m = dI_D / dV_GS is the ratio of a small change in drain current to the small change in gate voltage that caused it. It is the key amplifier parameter: the voltage gain of a common-source stage equals g_m times the load resistance. In saturation, g_m = 2 * K * (V_GS - V_th), so a higher overdrive voltage or a larger K (wider/shorter channel) increases g_m. For high-frequency applications, a large g_m per unit current (g_m / I_D) is desirable to achieve gain without high power consumption.