In Semiconductors > Basic Principles > Conduction the following expressions are given: - m_e v_e/\tau_e = -eE and - m_h v_h/\tau_h = eE . I think these should be: m_e v_e/\tau_e = -eE and m_h v_h/\tau_h = eE (which would be consistent with lecture 3). Otherwise you end up with \sigma=-(n_ee\mu_e+n_he\mu_h).