 >   >  >   >  >  >  The Thermal voltage equation is Vt=kT/q
 >  >  > 
 >  >  >
 >  >
 >

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 >  >  > 
 >  >  >  Does anyone know how to derive the Thermal voltage equation?
 >  >  > 
 >  >  >  Thanks,
 >  >  >  Paul
 >  >  >
 >  >  > Read the first line:
 >  >  > In semiconductors, the relationship between the flow of
electrical
 >  > current
 >  >  > and the electrostatic potential across a pn junction depends on
a
 >  >  > characteristic voltage called the thermal voltage.
 >  >  >
 >  >  > Simplistically, Ohm's law will tell you that there must be a
 >  > characteristic
 >  >  > resisitance since
 >  >  > you have a voltage difference (electrostatic potential) and a
 > current
 >  >  > through the junction,
 >  >  > and hence it is not a perfect switch but a resistance and any
 > resistor
 >  > will
 >  >  > radiate energy as heat.
 >  >  > Being a semiconductor, this "resistance" is variable but it is
quite
 >  > real
 >  >  > and your
 >  >  > processor, having a lot of them, needs a fan to cool it. It is
a
 >  >  > "characteristic"
 >  >  > thermal output because of the switching taking place, the
voltage
 > across
 >  > the
 >  >  > junction being greater when less current is passed as with any
 > normal
 >  >  > switch.
 >  >  > In the perfect switch the resistance is zero and the voltage is
 > across
 >  > it
 >  >  > zero when
 >  >  > closed, the current having an undefined value controlled
elsewhere,
 > and
 >  > when
 >  >  > open the current is zero and the voltage undefined.
Semiconductors
 > are
 >  > not
 >  >  > perfect switches.
 >  >  > Hope that helps.
 >  > 
 >  > 
 >  >  I've read dozens of technical books on diode and semiconductor
 >  >  science, but have yet to find any details. Obviously the thermal
 >  >  voltage, which is ~ 25mV at 300K, is related to thermal
fluctuations.
 >  >  Thermal voltage may be similar to kTC noise, if not directly
related.
 >  >  kTC noise is Vn=sqrt(kT/C). Resistance term is irrelevant in kTC
noise
 >  >  since the bandwidth is 1/(4RC), and therefore R cancels out in the
 >  >  Johnson noise voltage equation. As mentioned, the thermal voltage
 >  >  equation is Vt=kT/q, or in terms of kTC noise we would convert q
to
 >  >  capacitance, which is ~ 5.6aF. If thermal voltage is kTC noise
then
 >  >  why would the capacitance remain constant (~ 5.6aF @300K) for a
given
 >  >  temperature regardless of the diode dimensions and size?
 >  > 
 >  >  Regards,
 >  >  Paul
 >  >
 >  > Capacitance is a function of plate surface area and separation.
Those
 >  > remain constant. You seem to be struggling with DC superimposed
 >  > on AC. The Kirchhoff model here is a capacitor in parallel with a
 > resistor.
 >  >
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 > 
 > 
 >  I understand basic concepts such as Kirchhoff model of plates,
 >  capacitance being a function of area and separation, and actually
 >  there are other parameters and hence it is not always a constant;
 >  e.g., diode capacitance. Also I know how to superimpose DC over AC.
 >  Perhaps my question was a bit too technical here.
 >
 > Perhaps your understanding of basic principles is a bit less technical
here.


 Sorry, I'm here to discuss science.
Science is the observation, investigation and explanaton of natural
phenomena; there is no way a semiconductor can be considered
natural. Perhaps your misunderstanding of science is what the rest
of the world calls technology.
 Understandably some people are
 here so time will fly by at their jobs.
Perhaps some people are retired.
 I'm not here to pick fights.
Then don't start one when others are trying to help you understand
the question you asked.
 If you want attempt to derive the Thermal Voltage equation then great.
I have no great desire to compute direct current * resistance or the heat
radiated from a reactive capacitor.
 Anyhow, one consideration is the limitation of thermal noise
 bandwidth, which begins to fall significantly near 1THz.
I'd be interested to see a semiconductor operating at that frequency.