# What is thermal voltage?

1. Mar 22, 2012

### Godwin Kessy

Hallow!! For real it's not clear to me on how the thermal voltage in a semi-conductor is established... Can any one help?

2. Mar 22, 2012

### DragonPetter

Do you know the equation for it?

The constant used in it should give you a lot of hints in understanding it.

3. Mar 24, 2012

### Godwin Kessy

Yeah! It's (KT/q) but i don't get the physical meaning, and where is it established?, i haven't found any explanation whether using electronic theory, or band theory.... that explains it....?

4. Mar 24, 2012

### DragonPetter

This isn't really a band theory concept, although I'm sure it can be related since electrons can gain enough thermal energy to jump to the conduction band energy level in a semiconductor (hence they have a positive thermal coefficient). Its more of a thermodynamics concept.

The thermal voltage equation uses Boltzmann's constant, and K describes the average energy of many particles moving randomly at a given temperature. Because these displacing particles have charge, they create an electric field as they're moving around.

Think of it this way, assume there is a thermal voltage Vt, and then try to think of a way to describe this voltage. Voltage can be expressed as Energy/coulomb, so if we are assuming this energy per coulomb is only from thermal energy, then we know that the charge in this potential has a certain amount of energy directly proportional to temperature.

Now, with all of this known, we also know the relation of energy of a particle at a given temperature by using boltzmann's constant, and we can describe the voltage by this much energy per a charge.

This is in the absence of all other electric fields.

Last edited: Mar 24, 2012
5. Apr 2, 2012

### Godwin Kessy

Okay... thanks a lot... But yet i don't see it directly as it is spoken... Thermal voltage... I would like to see how it is created just as barrier voltage has been explained, and how is it influenced by temperature as the formula says... What do you say?

6. May 27, 2013

### Jose21

Thermal Voltage

At 0 K, electrons in a semiconductor are in rest..or you can say 0 energy state. As we increase the temperature, electron starts getting energy proportional to the temperature and this constant of proportionality is k, the Boltzmann constant. kT/q is the voltage corresponding to this energy. Since the cause of this voltage is temperature, it is so called Thermal Voltage. It is an average value. For individual electrons, it can vary a little, but on an average, it will be kT/q. At a given temperature, an electron can have energies as multiples of kT.

I think this will clear your doubt.

Have a good day..

7. Jul 31, 2014

### tasnim rahman

I suddenly came across this thread from a long time ago, while searching on the same topic on the Internet. As I have problem with this topic too, I am posting my question here, instead of on a new thread. I did not quite grasp the explanation above very well. And, I was wondering if it had anything to do with this. That is, in semiconductors, at absolute zero, the electrons in the bands have zero or no energy, and no electron can escape to the conduction band. At any temperature above absolute, a proportion of electrons, according to the Boltzmann distribution, escape to the conduction band, leaving an equal number of holes in the valence band. This creates a potential difference between the valence band and conduction band, which is the thermal voltage, VT. Also, it is dependent on temperature, as expected, as the greater the temperature, the greater the number of conduction electrons and holes produced, and thus the greater the voltage, between them. Am I right?? Someone please clarify. :P

8. Jul 31, 2014

### analogdesign

You're on the right track but not quite. The potential difference between the conduction and valence band is the bandgap voltage, not the thermal voltage.

The thermal voltage is kind of a subtle concept relating how current and potential are related. Read up on Boltzmann's constant.

9. Aug 8, 2014

### tasnim rahman

Thank you for the answer. But I am not quite familiar with bandgap voltage. If you could give a more elaborate description of it or direct me to me a similar link or book, then it would be really helpful. Also, I was looking for the physical significance of the thermal voltage, but didn't actually find it anywhere. If there is any, that is if it has a physical meaning. Doesn't it? And I read about its relation to the Boltzmann constant in some places but more often the maths is too complex and often beyond my knowledge, so I can't quite understand it. :P