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Occupation probability 3kT above Fermi level

Short answer

Fermi-Dirac distribution f(E)=1/(1+exp((E−E_F)/kT)). With E−E_F=3kT, f=1/(1+e³)=1/21.09=0.0474, i.e., 4.74%.

Evaluate the probability that an energy state 3KT above the Fermi level will be occupied by an electron.

The probability that a state of energy EE is occupied by an electron is given by the Fermi–Dirac distribution function:

f(E)=11+exp⁡(E−EFkT)f(E) = \frac{1}{1 + \exp\left(\dfrac{E - E_F}{kT}\right)}

Given: the state is 3kT3kT above the Fermi level, so E−EF=3kTE - E_F = 3kT.

f(E)=11+e3kT/kT=11+e3=11+20.09=121.09=0.0474\begin{aligned} f(E) &= \frac{1}{1 + e^{3kT/kT}} = \frac{1}{1 + e^{3}} \\ &= \frac{1}{1 + 20.09} = \frac{1}{21.09} = 0.0474 \end{aligned}

Answer: f(E)=0.0474f(E) = 0.0474, i.e. about 4.74 %.

The result does not depend on temperature, because the energy is given in units of kTkT. A state only 3kT3kT above EFE_F (about 0.078 eV at 300 K) is already rarely occupied; the probability that it is empty is 1−0.0474=0.95261 - 0.0474 = 0.9526.

✓ Answer checked by Claude Opus 5.5 (high effort)

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