SOLUTION

• The mobility is proportional to the carrier relaxation time and inversely proportional to the carrier effective mass.
• The electron mobility is often greater than hole mobility because quite often the electron effective mass is smaller than hole effective mass.
• The electron and hole effective masses in Si are 0.26 and 0. 38m_e, respectively, suggesting higher electron mobility than hole mobility in Si (in Si, µn ≈1400 cm²/V s and µ_p ≈ 500 cm²/V.
• The relaxation times are often of the same order of magnitude for electrons and holes, and therefore, they do not make too much difference.
• In order to increase the speed of a device, one has to choose materials with small electron and hole effective masses and long relaxation times, i.e. where the electrons and holes do not have to experience too much collision on crystal imperfections, impurities, etc.