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Exemplarily drawn dispersion relation \( W(k) \) (energy as a function of wavenumber) with completely filled valence band and partially filled conduction band separated by a band gap \(\Delta W\).

The effective mass \(m^*\) of an electron in the conduction band or the effective mass of a hole (missing electron) in the valence band depends reciprocally on the magnitude of the band curvature \( \frac{\partial^2 W}{\partial k^2} \).

- At the conduction band minimum (blue dot), the band curvature is
*large*(and*positive*), that is, the effective electron mass is*small*. - At the valence band maximum (red dot), the band curvature is
*large*(and*negative*), that is, the effective hole mass is*small*. - At an inflection point (green), the band curvature is
*small*, that is, the effective mass is*large*.