@@ -376,7 +376,7 @@ where $a$ is the lattice constant and $\varepsilon$ and $t$ are tight binding pa
3. Show that the sum of the electron and hole concentration in this band is constant as a function of the temperature.
### Exercise 4: The Hall coefficient when both electrons and holes are present in the system
### Exercise 4*: The Hall coefficient when both electrons and holes are present in the system
1. Recall from the [Drude model](/3_drude_model/) that electrons give rise to a negative Hall coefficient. Explain why the Hall coefficient is positive if holes are the charge carriers in a material.
2. Recall that the Hall coefficient $R_H$ describes the change of the transverse resistivity $\rho_{xy}$ due to a change in $B$: $\rho_{xy} = \frac{E_y}{J_x}=R_H B$. When both electrons and holes are present in a system, the Hall coefficient is given by
