final sign fix

docs/14_doping_and_devices_solutions.md

@@ -131,7 +131,7 @@ g_h = \frac{m_h^{\ast}}{\pi\hbar^2}
 $L$ can be found here using previous questions, by setting:
 
 $$
-E_e - E_h - E_c + E_v = 1 eV = \frac{\hbar^2}{2}(\frac{\pi n}{L}^2+k_x^2+k_y^2)
+E_e + E_h - E_c + E_v = 1 eV = \frac{\hbar^2}{2}(\frac{\pi n}{L}^2+k_x^2+k_y^2)
 (\frac{1}{m_e^{\ast}}+\frac{1}{m_h^{\ast}})
 $$