What is Thermal Voltage? Definition, Formula

Definition: Thermal voltage is the voltage generated within a semiconductor junction due to temperature variation. The voltage is generated in a semiconductor material due to changes in the behavior of electrons and holes when exposed to varying temperatures. This voltage is represented by the letter VT.

How does Thermal Voltage Produce?

Thermal voltage is generated in a semiconductor material such as a diode, transistor, MOSFET, IGBT, etc, due to the thermal energy given to the charge carriers, i.e., electrons and holes. As temperature rises, more charge carriers cross the junction, creating a voltage. This phenomenon is related to the diffusion of charge carriers and is described by the Boltzmann factor, also known as Boltzmann voltage.

Thermal Voltage Formula

The equation for calculating the thermal voltage VT is as follows.


Thermal Voltage at Room Temperature

At room temperature, which is typically around 300 Kelvin (K), you can calculate the voltage VT​ using the following equation:


The voltage VT at room temperature is approximately 25.86 millivolts (mV).

Thermal Voltage and Diode Current Equation

The voltage VT​ has a significant role in the diode current equation, which describes the current-voltage relationship in a semiconductor diode. The diode equation is given by:


The term enVT / V​ in the diode equation represents the exponential relationship between the voltage across the diode (V) and the thermal voltage (VT​). The voltage influences the diode’s behavior at different temperatures.

Solved Example

Calculate the voltage (VT) of a semiconductor diode at two different temperatures, 50 and 60 degrees Celsius.

VT at 500C
T = 50 +273=323K

VT at 600C
T = 60 +273=333K

From the above, it is clear that the voltage VT increases with the increase in temperature.


The thermal voltage, denoted as VT, is an important concept in semiconductor physics. It refers to the voltage that exists across a p-n junction or other similar semiconductor devices due to temperature. This voltage is crucial in determining the behaviors of these devices, including the threshold voltage in transistors and the diode equation in a semiconductor diode.

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