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Effective minority-carrier mobility in heavily doped silicon defined by trapping and energy-gap narrowing

A simple model for effective minority-carrier mobility in heavily doped silicon, lower than the extended-state (∼ majority-carrier) mobility, is developed based on the assumption that some of the carriers are trapped in band-tail (or shallow bound) states. The model demonstrates for the first time a correlation between measured mobility and energy-gap narrowing, which is also influenced by band tails. Numerical simulations of temperature-dependent hole transport in the emitters of n+-p-n transistors, which accurately predict the measured (base) currents when the effective hole mobility model and the energy-gap narrowing it implies are used, are discussed.