The effective mass of a carrier (like an electron or hole) in a semiconductor depends on several factors, primarily doping and temperature, as well as the energy and direction of the carrier within the crystal lattice.
Factors Influencing Effective Mass
- Doping: Higher doping levels increase the carrier concentration. As carrier concentration increases, so does the carrier energy. The effective mass tends to increase with higher carrier energy because the band curvature of the energy bands decreases.
- Temperature: Similar to doping, temperature affects the carrier concentration and energy, thus influencing the effective mass.
- Carrier Energy: As mentioned above, the effective mass is related to the energy of the carrier within the band structure. Higher energy levels within a band can correspond to different curvatures, leading to variations in effective mass.
- Crystal Direction (Anisotropy): In many semiconductor materials, the valence and conduction bands exhibit anisotropy, meaning their properties vary depending on the direction within the crystal lattice. This anisotropy directly affects the effective mass of holes (and sometimes electrons), leading to different effective mass values for different crystal directions. This is particularly relevant in valence bands.
In summary, the effective mass is not a fixed property but rather a function of the semiconductor's conditions and the carrier's state.
