Abstract: In this paper the expression of electron effective mass of semiconductors is deduced when the 3D kspace equienergy surfaces are not spherical by using multivariable calculus and tensor analysis. Then, it is discussed whether the conductivity effective mass tensors of Si and Ge are isotropic. The discussions can promote students′ understanding of energy band structures of semiconductors and its influence on conductivity.

Key words: semiconductor, effective mass, conductivity, isotropyq