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Fermi level position at 310 K

Short answer

For fixed donor concentration in extrinsic range, Ec – EF ∝ T. At 300 K it is 0.5 eV. At 310 K: Ec – EF = 0.5 × (310/300) ≈ 0.517 eV below the conduction band edge.

In an n-type semiconductor, the Fermi level lies 0.5eV below the conduction band at 300 K, if the temperature is increased to 310 K, find the new position of Fermi level.

In an n-type semiconductor (non-degenerate, all donors ionised)

n=Nc e−(Ec−EF)/kT  ⇒  Ec−EF=kTln⁡NcNdn = N_c\,e^{-(E_c-E_F)/kT} \;\Rightarrow\; E_c - E_F = kT\ln\frac{N_c}{N_d}

In the extrinsic range n=Ndn = N_d is fixed. If the small change of NcN_c with temperature is neglected, ln⁡(Nc/Nd)\ln(N_c/N_d) is constant, so Ec−EF∝TE_c-E_F \propto T.

Calculation

(Ec−EF)310(Ec−EF)300=310300(Ec−EF)310=0.5×310300=0.5167 eV\begin{aligned} \frac{(E_c-E_F)_{310}}{(E_c-E_F)_{300}} &= \frac{310}{300}\\ (E_c-E_F)_{310} &= 0.5\times\frac{310}{300} \\ &= 0.5167\ \text{eV} \end{aligned}

Answer: at 310 K the Fermi level lies about 0.517 eV below the conduction band edge, i.e. it moves down by about 0.017 eV, towards the middle of the gap.

More exact check (including Nc∝T3/2N_c \propto T^{3/2})

(Ec−EF)310=310300(0.5)+k(310)32ln⁡310300=0.5167+0.0013=0.518 eV\begin{aligned} (E_c-E_F)_{310} &= \frac{310}{300}(0.5) + k(310)\tfrac{3}{2}\ln\frac{310}{300}\\ &= 0.5167 + 0.0013 = 0.518\ \text{eV} \end{aligned}

The difference is only about 1 meV, so the simple answer is acceptable.

Physical meaning

As temperature rises, more electrons can be thermally excited across the gap and the material moves towards intrinsic behaviour, so the Fermi level drops from near EcE_c towards EFiE_{Fi} (mid-gap).

✓ Answer checked by Claude Opus 5.5 (high effort)

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