fix E_F equation

src/13_semiconductors.md

@@ -227,7 +227,7 @@ $$n_h \approx N_V e^{-E_F/kT} = N_C e^{-(E_G-E_F)/kT} \approx n_e$$
 
 Solving for $E_F$:
 
-$$E_F = \frac{E_G}{2} - \frac{3}{4}kT\log(m_e/m_h)$$
+$$E_F = \frac{E_C + E_V}{2} - \frac{3}{4}kT\log(m_e/m_h)$$
 
 An extra observation: regardless of where $E_F$ is located, $n_e n_h = N_C N_V e^{-E_G/kT} \equiv n_i^2$.