Definition: Thermal voltage (VT) 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. Its value depends on temperature and, at room temperature (300K), VT is approximately 25.86 mV.
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.
From above formula it is clear that the value of thermal voltage (VT) depends on absolute temperature. At 300K (room temperature), the calculated value is about 25.86 mV, which is commonly used in semiconductor analysis.
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:
Thus, the voltage VT at room temperature is approximately 25.86 millivolts (mV).
This is also referred to as the thermal voltage at 300K, and the value remains close to 25–26 mV depending on precision.
Thermal Voltage of Diode
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.
Hence, the thermal voltage of a diode is a fundamental parameter in this equation, typically around 25.86 mV at 300K.
Solved Example
Calculate the thermal 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.
Importance of Thermal Voltage in Semiconductors
Thermal voltage plays a vital role in:
- Calculating current in diodes using the exponential law.
- Setting threshold conditions in BJTs and MOSFETs.
- Explaining temperature dependency of electronic devices.
Main Advantage of Thermal Voltage
The main advantage of understanding and applying thermal voltage is that it helps engineers predict and control the behavior of semiconductor devices under different temperatures. This knowledge is widely used in the design of temperature sensors, biasing circuits, and temperature-dependent electronic switches. It ensures reliable performance of diodes, transistors, and MOSFETs in varying operating conditions.
Conclusion
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.
FAQs on Thermal Voltage (VT)
VT=kT/q, where k = Boltzmann constant, T = temperature in Kelvin, q = charge of electron.
About 25.86 mV.
Approximately 25.86 mV at 27°C (300K).
It is the junction voltage caused by thermal energy, ≈ 25 mV at room temperature.
It determines current flow in semiconductors and is crucial in analyzing diodes, BJTs, and MOSFETs.
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