Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES
Lecture 32 : Covalent Bond
  The effective mass $ m^\ast$ is given by
$\displaystyle \frac{1}{m^\ast} = \frac{1}{\hbar^2}\frac{d^2E}{dk^2}$
  For a band with anisotropy, (i.e., where $ E-k$ relationship is not spherically symmetric), the effective mass is direction dependent
 
$\displaystyle \frac{1}{m^\ast_{ij}} = \frac{1}{\hbar^2}\frac{\partial^2E}{\partial k_i \partial k_j}$
  Note that unlike real mass, the effective mass can even be negative as it is proportional to the curvature of the band.
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