What is the formula for Thermal Voltage?

The Thermal Voltage formula is VT = k × T / q (≈ 25.85 mV at 300 K), where VT—Thermal voltage (V); T—Absolute temperature (K); k—Boltzmann constant 1.381×10⁻²³ J/K; q—Elementary charge 1.602×10⁻¹⁹ C.

Thermal Voltage

Q: How do you calculate Thermal Voltage?

Use the formula VT = k × T / q (≈ 25.85 mV at 300 K). The thermal voltage is the natural voltage scale of a semiconductor junction, set by temperature and the fundamental constants. Enter the values you know in the calculator above and it solves for the remaining variable.

VT = k · T / q

Calculator

Thermal Voltage

Temperature

Result

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Formula

VT = k × T / q (≈ 25.85 mV at 300 K)

Description

The thermal voltage is the natural voltage scale of a semiconductor junction, set by temperature and the fundamental constants. It appears in the diode equation, transistor transconductance, and small-signal emitter resistance. At room temperature it is about 26 mV, and it rises directly with absolute temperature, which is why junction behaviour drifts with temperature.

Variables

VT — Thermal voltage (V)
T — Absolute temperature (K)
k — Boltzmann constant 1.381×10⁻²³ J/K
q — Elementary charge 1.602×10⁻¹⁹ C

Practical Notes

VT ≈ T/11600 volts. It sets gm = Ic/VT and re = VT/Ie, so amplifier gain has an inherent temperature dependence that bias and feedback must compensate.

Related Concepts

All Discrete Amplifiers formulas →

Thermal Voltage Small-Signal Emitter Resistance Transistor Transconductance Diode Dynamic Resistance Common-Emitter Voltage Gain BJT Alpha–Beta Relationship Darlington Pair Current Gain Emitter-Follower Input Impedance

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