Calculate MOSFET threshold voltage, drain current, operating region, transconductance, and overdrive voltage for NMOS and PMOS enhancement-mode transistors.
The **MOSFET Threshold Voltage Calculator** determines the operating region, drain current, transconductance, and power dissipation of enhancement-mode MOSFETs. Whether you''re designing analog amplifiers that must bias transistors in saturation or digital circuits that switch between cutoff and triode, understanding the threshold voltage and its impact on circuit behavior is essential.
This tool supports both **NMOS and PMOS enhancement-mode** transistors. Enter the threshold voltage (V_th), gate-source voltage (V_GS), drain-source voltage (V_DS), process transconductance parameter (k'), and W/L ratio to instantly see which operating region the MOSFET is in, along with detailed electrical parameters.
For circuit designers, analog IC engineers, and electronics students, this calculator provides immediate feedback on MOSFET behavior across different bias conditions. The included I_D vs V_GS table helps visualize the transfer characteristic, while the technology reference table compares parameters across process nodes from 180nm CMOS to SiC power devices.
Use the preset examples to load common values instantly, or type in custom inputs to see results in real time. The output updates as you type, making it practical to compare different scenarios without resetting the page.
Biasing a MOSFET correctly requires knowing its operating region, which depends entirely on the relationship between V_GS, V_th, and V_DS. This calculator instantly identifies the region and computes all key parameters — saving time that would otherwise be spent working through the piecewise equations by hand.
For analog design (amplifiers, current mirrors), the transistor must be in saturation. For digital logic (inverters, gates), transistors switch between cutoff and triode. This tool helps verify bias conditions, estimate power consumption, and optimize W/L ratios for both applications.
MOSFET Drain Current Equations: • Cutoff: V_GS < V_th → I_D = 0 • Triode: V_GS ≥ V_th AND V_DS < V_ov → I_D = K_eff × [V_ov × V_DS − ½V_DS²] • Saturation: V_GS ≥ V_th AND V_DS ≥ V_ov → I_D = ½ × K_eff × V_ov² Where V_ov = V_GS − V_th (overdrive voltage) K_eff = k' × (W/L), k' = µ_n × C_ox (process parameter) g_m = ∂I_D/∂V_GS — transconductance
Result: I_D = 122.5 mA in Saturation region
V_ov = 5 − 1.5 = 3.5V. Since V_DS (5V) ≥ V_ov (3.5V), the MOSFET is in saturation. I_D = ½ × (2×10) × 3.5² = 122.5 mA. Transconductance g_m = K_eff × V_ov = 20 × 3.5 = 70 mS.
The three operating regions of a MOSFET — **cutoff**, **triode**, and **saturation** — correspond to fundamentally different circuit behaviors. In cutoff (V_GS < V_th), the channel doesn't form and no current flows. In triode (V_DS < V_ov), the transistor behaves like a voltage-controlled resistor. In saturation (V_DS ≥ V_ov), the channel is pinched off and current depends primarily on V_GS, making the transistor act as a current source — the basis for all analog amplification.
V_th is not a fixed constant. It depends on **body effect** (V_SB raises V_th in bulk CMOS), **temperature** (V_th decreases ~1-2 mV/°C), and **short-channel effects** (drain-induced barrier lowering reduces V_th in short channels). Modern processes use techniques like high-k dielectrics, metal gates, and channel engineering to control V_th.
The W/L ratio is the primary design knob. For a given V_GS − V_th, doubling W/L doubles I_D and g_m. However, wider transistors have larger parasitic capacitances (C_gs, C_gd), which limit bandwidth. The gain-bandwidth product of a common-source amplifier is g_m / (2π × C_L), so optimizing W/L requires balancing gain, speed, power, and area constraints simultaneously.
The threshold voltage (V_th) is the minimum gate-source voltage needed to create a conducting channel between drain and source. Below V_th the MOSFET is off (cutoff); above it the transistor conducts current.
There are three regions: Cutoff (V_GS < V_th, no current), Triode/Linear (V_GS ≥ V_th and V_DS < V_GS − V_th, acts like a voltage-controlled resistor), and Saturation (V_GS ≥ V_th and V_DS ≥ V_GS − V_th, acts as a current source). Use this as a practical reminder before finalizing the result.
The overdrive voltage V_ov = V_GS − V_th is the "excess" voltage above threshold. It directly determines the drain current in saturation (I_D ∝ V_ov²) and the transconductance (g_m ∝ V_ov).
The width-to-length ratio scales the drain current linearly — doubling W/L doubles I_D. Wider transistors carry more current but consume more area and have higher gate capacitance. Longer channels reduce short-channel effects but lower speed.
NMOS has a positive V_th and conducts with positive V_GS above threshold. PMOS has a negative V_th and conducts when V_GS is sufficiently negative. NMOS typically has 2-3× higher mobility than PMOS, so NMOS transistors are faster for the same dimensions.
k' = µ × C_ox, where µ is carrier mobility and C_ox is gate oxide capacitance per unit area. It depends on the fabrication process — typical values range from 0.2 mA/V² (advanced CMOS) to 20+ mA/V² (power MOSFETs). The effective K = k' × (W/L).