MOSFET Threshold Voltage Calculator

Created: 19/03/2026Updated: 13/06/2026By: Mert
MOSFET Threshold Voltage Calculator
Vth = VFB + 2φF + γ√(2φF)
MOSFET Threshold Voltage Calculator
VFB Flat-Band Voltage
2φF Surface Potential
γ Body Effect Parameter
VSB Source-Body Voltage
Enter values
MOSFET Threshold Voltage Calculator Results
Vth
—
V
Vth0 (no body)
—
V
Body Effect ΔVth
—
V
2φF
—
V
γ
—
V½
VFB
—
V
MOSFET Threshold Voltage Calculator Input Output

Figure 1: MOSFET Threshold Voltage Calculator — enter values to calculate the result with step-by-step workings.

Table of Contents
Fundamentals
  1. What Determines MOSFET Threshold Voltage
  2. The Body Effect
Worked Examples
  1. Standard NMOS (VSB = 0)
  2. NMOS with Body Effect (VSB = 2V)
Reference
  1. Frequently Asked Questions
  2. Related Calculators

What Determines MOSFET Threshold Voltage

Threshold voltage (Vth) is the minimum gate-source voltage needed to create a conducting channel. Below Vth, the MOSFET is off. Above it, current flows. Vth depends on the flat-band voltage (work function difference + oxide charges), the surface potential (2φF), and the body effect coefficient (γ).

Vth = VFB + 2φF + γ√(2φF + VSB)
Without body effect (VSB = 0): Vth0 = VFB + 2φF + γ√(2φF)

The MOSFET Calculator uses Vth as an input to determine the operating region and drain current. The MOSFET Gate Resistor Calculator ensures the gate voltage reaches well above Vth for full turn-on.

The Body Effect

When the source is not at the same potential as the body (substrate), the depletion region widens and Vth increases. This happens in stacked NMOS circuits, current mirrors, and any topology where VSB > 0. The increase is ΔVth = γ(√(2φF + VSB) − √(2φF)). The body effect coefficient γ depends on doping concentration and oxide thickness — typically 0.3–0.8 V½.

Example: Standard NMOS (No Body Effect)

VFB = 0.6 V, 2φF = 0.3 V, γ = 0.5 V½, VSB = 0

Vth0 = 0.6 + 0.3 + 0.5×√0.3 = 0.6 + 0.3 + 0.274 = 1.174 V

No body effect — source and body are at the same potential (grounded substrate).

Example: NMOS with Body Effect (VSB = 2V)

Same device, VSB = 2 V (source lifted above substrate)

ΔVth = 0.5×(√(0.3+2) − √0.3) = 0.5×(1.517 − 0.548) = 0.485 V

Vth = 1.174 + 0.485 = 1.659 V

The body effect raised Vth by 41%. This matters in analogue circuits where Vth shifts change the operating point. The Transistor Biasing Calculator handles the BJT equivalent where VBE variation with temperature shifts the Q-point.

Frequently Asked Questions

What determines threshold voltage?
Vth depends on flat-band voltage (work function + oxide charges), surface potential (2φF, set by doping), oxide thickness (via γ), and source-body voltage (body effect). Manufacturing process variation causes ±50–100 mV spread.
What is the body effect?
When VSB > 0, the depletion region widens and Vth increases by ΔVth = γ(√(2φF+VSB) − √(2φF)). This occurs in stacked transistors, current mirrors, and source followers.
What is a typical Vth?
Standard NMOS: 0.5–1.0 V. Low-Vth: 0.2–0.4 V. High-voltage discrete: 2–4 V. PMOS has negative Vth with similar magnitude. The Shockley Diode Calculator models a related threshold effect in diodes.
Why does Vth matter for switching?
VGS must exceed Vth by enough margin for low RDS(on). A logic-level MOSFET with Vth = 1 V needs VGS ≥ 4–5 V for full enhancement. The Transistor Base Resistor Calculator faces the analogous VBE requirement for BJT switches.
Can Vth be adjusted?
In IC design, Vth is set by ion implantation dose. Designers can create multiple Vth flavours on the same die. In discrete circuits, Vth is fixed — you choose a MOSFET with the right Vth for your application. The BJT Gain Calculator addresses a similar parameter (β) that is fixed by manufacturing but varies between devices.
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Last updated: March 2026